BEACON GPS guide-maps for hill-walkers and climbers

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Mountain Navigation – Compass & GPS Use

Contents of mountain navigation

·       Pre-trip Planning   Naismith’s rule   Emergency “escape” routes

·       Smartphone & GPS or Paper Map & Compass?

·       Map Appreciation – contour lines

·      Compass Use    Taking Bearings   Magnetic Variation & the Three Norths

·       Walking Bearings

·       Estimating distances

·       Route Planning – use of line features – aiming off

·       Getting lost

·       Use of GPS    Grid References   Selecting a GPS   Sarloc system

·       Next steps


Pre-trip Planning

Every trip into the hills or mountains needs to be planned in advance to some extent. How much planning is required naturally depends on a number of factors, for example, the difficulty and length of the route, the time of year, the weather conditions expected and prior knowledge of the route. Normally pre-trip planning will require close study of the map and selecting a route bearing in mind the level of experience, aspirations and fitness of the party.

A key element in all pre-trip planning is estimating the time required to complete the route allowing for rest stops, food, conversation, photography etc. See Naismith’s Estimating Rule in a later section. Consider potential difficulties in route finding, and in particular, possible difficulties in finding the descent route at the end of the day when the party is tired. Think about the amount of daylight available, and allow a good safety margin for unexpected delays.

Always leave behind details of your intended route and when you expect to be back. A Route Card is an ideal way of noting the key details to leave with a friend, family member, or at the hotel / hostel etc, even leaving details in your car is better than having no record at all of where you might be. Remember to report your safe return and let your contact know if you change your plans.

Another vital element in the planning of all but the most simple routes is giving thought to emergency “escape routes.” That is, what alternative options are available for shortening the route and returning to safe ground and shelter as soon as possible if the weather makes continuing impossible or unwise.  Think about what would need to happen if a member of the party becomes unwell or injured for example. Ask the question “what would I do if something goes wrong when I/we are here, and what is the quickest way back to safety?”

If you are leading a group always monitor what effect the route and the conditions are having on the other members of the group (especially the weakest) and be prepared to change plans before any problem becomes serious.

Bad weather and high wind speeds can sometimes mean that diverting via a more sheltered route to safety is the most sensible option, rather than continuing with the planned route.  If you do this remember to inform your route plan contact as soon as possible.

Practice your mountain navigation skills in good weather. Learn how to use your compass to take an accurate bearing from the map and to correct for magnetic variation. Practice walking on a compass bearing accurately and then learn to estimate the distance travelled over the ground on a given bearing. This is the basis of “dead reckoning” navigation.

Smartphone App & GPS or Paper Map & Compass?

This is never an either or decision for mountain navigation. The potentially harsh mountain environment and the need for easily readable, detailed route finding information combined with the need for a wide area perspective means that a "hard copy" mountain map is required for safe mountain navigation.  It does not have to be paper, a weatherproof "plastic" map is often better.

Your main map needs to show you a sizable chunk of the route in high detail and under a wide range of lighting and weather conditions - and it needs to be completely reliable.  This just about rules out most current electronic display devices.

We have always used the latest technology at Beacon Maps and our 'chief geek’ (he once liked to be called an engineer) claimed he helped to introduce compact GPS devices into the UK market in the early days when you needed a small truck to carry all the kit around.  GPS is brilliant, it can make good maps even more useful and versatile, but we would never rely solely on any electronic device for mountain navigation, and neither do the experts.

OS Locate is a iPhone & Android App that uses a smartphone's or tablet's built-in GPS location system to display your position (six figure UK grid reference). Its an ideal App for use with Beacon mountain guide-maps and Harvey Maps, as well as OS mapping. Upgrading it to eight figure references - 10m precision - could make it even more powerful.

Later sections in these notes explain how GPS devices can be used as a powerful aid to safe mountain navigation once some very simple navigation basics have been mastered.

Map Appreciation

The first thing to consider is the scale of the map. The most popular map scales used by walkers in the UK are 1:50,000 (2cms to 1km) such as the popular OS Landranger series, and the more detailed 1:25,000 (4cms to 1km) maps such as the OS Explorer/Outdoor Leisure series and of course the Harvey Superwalker maps which are specially designed for hill-walkers.

The 1:40,000 scale Harvey British Mountain Maps are now also becoming increasingly popular. The 1:50,000/40,000 scales are ideal for a wide range of outdoor activities, but in mountainous terrain the extra detail of a 1:25,000 scale map is usually an advantage.

BEACON Mountain Guide-maps use 1:25,000 scale HARVEY Superwalker maps specially designed for mountain walkers and climbers. They generally don't have administrative boundary information that can be confusing to walkers. These maps have grid lines at 1km intervals. Hence a tenth of a grid square represents 100 meters, looking at features with this in mind is a good way of getting a feel for the map’s scale.

Contour Lines A key feature of this type of map is the contour lines joining points of equal height (usually at 10 or 15 metre vertical intervals), the closer these are together the steeper the ground. Where the ground becomes very steep some contour lines may disappear because they would be drawn too close together, or they may give way to the symbols for crags, cliffs, rock outcrops etc.

On HARVEY maps and the Beacon mountain guide-maps the contour lines are normally brown and at 15m intervals, with thicker contours every 75m. Where the ground is predominately rocky (small outcrops, small crags etc) the contours become grey, which gives a very useful clue to the nature of the terrain.

Become familiar with the symbols the map maker uses to show different types of terrain and other features such as streams, walls, paths and rights of way.

There is a wealth of information given in the margins of most maps and this is well worth studying.

Learning to visualise the lie of the land by inspecting the contours and other symbols requires practice but is a very useful skill to acquire in order to plan routes, avoid dangerous ground and estimate the time required to complete a route using Naismith’s estimating techniques (described later). The course of steams and rivers can often help in visualising the lie of the land.

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Compass Use

The main uses of a compass for a mountain walker are as follows:-

a) Setting the map to conform with the ground (orientating the map) and checking the direction of paths and streams etc.

b) Setting a bearing from a map (allowing for magnetic variation) and walking on a known bearing (direction) to get somewhere

c) Identifying distant features by checking their bearing (from your own position)

d) Identifying your own position using back bearings from two or more distant known features on the map, called resection

Compass Design

The heart of a magnetic compass is simply a magnetised “needle” that is free to rotate on a central pivot.

Compass housing ringThe north seeking end of the needle, normally red, points to the earth’s magnetic north pole. In most models the needle is contained in a special oil filled housing which damps the needle movement.

The outer ring of the compass needle housing is marked with the cardinal points of the compass and in degrees (or Mils - used by the military). If the housing ring is rotated to line-up with the needle (north to north), any required direction (bearing) may be read off from the compass ring.

The type of compass used by walkers, such as the popular Silva type, has a transparent protractor base-plate for measuring bearing angles. This has a “direction of travel arrow” which shows the direction to follow when walking on a bearing and is used to point in the desired direction when setting a bearing from the map.

The protractor base-plate may be rotated with respect to the compass housing for setting bearings.

Remember that there are 360 degrees in a full circle and bearing angles are measured in a  clockwise direction, so a bearing of 90 degrees represents East, 180 degrees South and 270 degrees West etc.

Taking Bearings

When taking a bearing from a map the compass needle can be ignored completely, you will be using the compass housing ring and the protractor base-plate only.

The first stage is to line-up the base-plate with the intended direction of travel on the map. Using the long edge of the base-plate (or any of the parallel lines on it) line-up the base-plate with an imaginary line from the starting point to the destination for that part (leg) of the route. Ensure that the direction of travel arrow is pointing in the desired direction of travel.

Carefully holding the base plate in position, rotate the compass housing ring until its north arrow points to the map grid north. Use any of the parallel lines on the housing to align with a convenient north grid line on the map. Check that the base-plate is still accurately lined-up on the map, you can then remove the compass from the map having successfully set the bearing for that leg.

Short video clip on compass setting  youtube video

Here's another compass video for coffee lovers coffee and compasses

A more recent (and useful) guide to compass use from OS

Magnetic Variation & the Three Norths

The bearing you have set from the map is a grid bearing which is slightly different from the magnetic bearing required for accurate compass navigation. This is because the compass needle points to magnetic north which is not exactly the same as the map's grid north.  The position of the earth’s magnetic north pole is very slowly moving year to year and map makers need to use a fixed grid reference linked to the true geographical north.

The horizontal angular difference between True North and the current Magnetic North is usually called the magnetic variation. Magnetic variation changes with position on the earth's surface and with time. Because of the inevitable limitations of making flat maps the map's Grid North can differ very slightly from the true geographical north, the small difference is often given on the map.

When navigating with map and compass we need to know the difference between Magnetic North and the map's Grid North, this is often called the Grid Magnetic Angle.

Over the last few years the Grid Magnetic Angle in the UK has been getting steadily smaller and smaller as the magnetic pole drifts and in 2016 Magnetic North in the UK is generally only very slightly west of Grid North. West of Penzance Magnetic North is now slightly east of Grid North. Some specific figures are given in the table below.  Look up the actual figure for the relevant mountain area from the BEACON Guide-map or other map.

To correct a grid bearing if Magnetic North is west of Grid North add the small Grid Magnetic Angle to the bearing by rotating the compass housing anti-clockwise.

Example: if after setting the bearing from the map, the compass housing is set at 320 degrees, and the map indicates that magnetic north is 2 degrees west, then rotate the compass housing anti-clockwise by 2 degrees so that it reads 322.

BEACON Tips: most walker’s compasses are marked in 2 degree graduations and setting them more accurately than this is difficult.  Halve the Grid Magnetic Angle to find the equivalent number of compass graduations and round to the nearest whole number. Hence in the example given above the compass housing would be rotated just one small graduation anti-clockwise.

Also, rather than trying to remember special rules for setting the compass, visualise the relative positions of Magnetic North and Grid North on the map, and how many graduation marks to rotate the compass housing, then you will be able to make the small correction quickly and instinctively.

 If you can maintain a 2 degree accuracy in your compass navigation you are doing well.

In Britain at present the angular difference between Grid North and Magnetic North (Grid Magnetic Angle) is so small (see table below) it is possible to ignore it for “rough” course setting, but it is advisable to practice navigating as accurately as possible so that the skills are there when they are required (and you remember what to do when you are away from the UK).

The following was the approximate Grid Magnetic Angle at three UK locations in July 2018:-

Ben Nevis area, Scotland 0 degrees and 28 minutes West of Grid North
Scafell area, Lake District 0 degrees and 52 minutes West of Grid North
Snowdon, North Wales 0 degrees and 19 minutes West of Grid North

These figures are updated every year in July, the current rate of change is about - 9 minutes of arc per year.

Note that there are 60 minutes (mins.) of arc per degree and on Beacon mountain guide-maps we normally round the Grid Magnetic Angle to the nearest half degree, i.e. 30 minutes. Thus in 2018 the Grid Magnetic Angle on Scafell Pike is about 1 degree West, which is half of one of the small divisions on most compass base plates.

This web blog page from the Ordnance Survey gives some interesting information, but the title could potentially be misleading. The magnetic pole has not suddenly moved, it is always slowly on the move. In the extreme south-west of England Magnetic North is now slightly east of Grid North for the first time in more than 220 years.  Refer to the table above for information regarding our main hill walking areas in the northern half of the UK.

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Walking on a Bearing

Having set the bearing from the map, hold the compass level and close to your body (but see the note below), turn your whole body until the north pointing end of the compass needle (normally red) is accurately aligned with the compass housing north arrow, then you can sight along the direction of travel arrow on the compass base-plate. When walking on a bearing check the compass frequently but try to use some feature on the ground well ahead of you as your aiming point (ideally not a grazing sheep!).

In poor visibility you can work with a companion ahead of you as your marker, but make sure that they are not going to walk over a cliff or cornice. Be very aware of any dangerous ground in the vicinity, don’t take any chances in poor visibility. This situation is where accurate navigation becomes vital.

Important notes re compass use: when using your compass for navigation ensure there are no magnetic (ferrous) metal or magnets close to it, in a pocket for example, that could deflect the needle. Even metal in underwear has been known to cause errors, as well as more obvious items such as cameras, another compass, mobile phones and ice axes.

Recently the Bank of England has introduced new 5p and 10p coins that are basically steel coated with nickel, so now we have another potential source of magnetic compass disturbance. Many of the 1p and 2p coins in circulation have been made with ferrous metal for some time.

Check by moving the compass well away from any possible interference (e.g. at arms length or onto a suitable rock) and seeing if the indicted bearing changes as you move closer. There are a few places in Britain (notably the Cuillin of Skye and Ben More on Mull) where there are magnetic rocks and a compass is unreliable – check guide books if unsure.

Polarity Reversal Recently it has been reported that storing a compass close to magnetised items can result in permanently reversed (or nearly reversed) polarity compass needles, which results in gross errors when using the compass!!  This is a serous problem and quite different from the temporary compass deflection that can be caused by stray magnetic fields around the compass when you are walking on a bearing for example. With so called polarity reversal the problem is permanent and the compass becomes unsafe to use.

We contacted Silva Ltd about this problem and Kevin Thomson their Marketing Manager gave us this short statement that sums up the problem and issues surrounding compass "polarity reversal":-

The cause:

Magnetic fields exist around many items, such as radio speakers, mobile phone speakers, car speakers (in fact any speakers), mobiles & smart phones, iPad’s, most car seat belt buckles, electrical circuits, ferrous metal objects, key’s etc. Self-service shopping scanners!  These all may effect the polarity of the needle.  The polarity of the compass needle may be reversed if the compass is exposed to another magnetic field.

The effect:

The needle either becomes sluggish and slow to settle (it may appear to stick and be out of balance), if it's polarity is partially reversed, or may reverse it's polarity completely.  In the latter case, the 'North' (usually red) seeking end of the needle will point South. There is no truth in the rumour, that certain manufacturers are painting the wrong end of the needle red!


Keep any type of magnetic compass, well clear of magnets and magnetic fields. It is good practice to use a compass case.  A Silva compass is a precision measuring instrument and should be treated accordingly.


Quickly flick the 'South' pole of a strong magnet outwards along the 'North' end of the needle. Repeat vice-versa. Compare with a compass that is known to be correct.

Golden rule:

Keep any type of magnetic compass, well clear of magnets and magnetic fields. Before leaving home, check the polarity of the needle, the general condition of the compass and that the compass functions correctly.

It appears the "reverse polarity" problem can most likely occur when the compass needle is constrained and not free to align itself with a strong local magnetic field, as could happen if it was stored close to electrical or electronic equipment, a mobile phone etc. Clearly the compass needle can rotate freely on its pivot and follow the earths magnetic field in one plane (normally horizontal) and this does not cause any problems, but if the needle can not align itself with a strong local magnetic field the needle can get trapped and de-magnetised or re-magnetised in the wrong direction.

If you have any doubts, check your compass before you need it for navigation in a critical situation on the mountain. Silva will re-magnetise one of their compasses if required, irrespective of its age.

BEACON Tip Be aware of the possible sources of compass error described above, but once these possible problems have been eliminated learn to trust your compass and map reading.  If “things don’t seem right” it’s very likely the compass that’s correct, so look for errors in your assumptions. It is human nature when lost to hold on to a preconceived idea of where we think we are, even in the light of increasing evidence to the contrary!

Measuring Distance

Compass romer scalesThe compass base-plate should have a handy Romer scale (corner measure) for measuring distances on the map and finding accurate grid references.

In this example the outer scale is for a 1:25,000 map and the inner scales are for 1:50,000 and the old one inch to the mile (1:63,360) scales.

Where-wolf made a simple grid reference finder for use with 1:50,000 and 1:25,000 scale maps, they may now be purchased exclusively from us until stocks are exhausted.

When it comes to estimating how far you have travelled over the ground, then for short distances, less than a few hundred metres, counting paces is the best technique. Calibrate your own stride by counting double paces (i.e. every time your left foot comes to the ground equals one double pace) over a known distance of 100 metres. Your figure might be 65 double paces on the flat for example, but will vary depending on the terrain being crossed, so you need to know how to modify the figure a little depending on whether the terrain is level, climbing or descending, rocky or smooth etc.

BEACON Tip: Some people transfer small stones from one hand to the other to help them keep count (e.g. one stone equals 100m). Some compasses can have simple counter devices attached to them for the same purpose.

For longer distances, timing is normally the preferred technique of estimating distance travelled. To estimate your speed of travel taking account of the terrain see Naismith’s rule below.

Naismith’s Rule

The modern version of Naismith’s rule for estimating the time required for completing a route or leg is to allow one hour per 4 to 5km on the map (i.e. 12 - 15 minutes per km) plus 1 minute for each 10 metres of height gained. Add extra time for rest stops, food, photography etc.

Hence if your estimated speed is 4km per hour for example, then 1km with a 100 metre height gain will take about 15 + 10 = 25 minutes.

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Route planning – use of line features – aiming-off

Most mountain navigation uses a combination of “dead reckoning” and feature recognition to move from one known waypoint to another where ones position can be re-established with some degree of certainty.

Small errors are bound to build-up on long legs between waypoints, so rather than rely on finding a small isolated waypoint after a long leg, experienced navigators will use intermediate waypoint(s) that are easier to find. The intermediate waypoint(s) could be distinct contour features such as a significant change in slope, a col, or an isolated top (contour ring) for example. This type of feature is not easily obscured by snow cover and not easy to pass by accidentally.

Other very useful features for mountain navigators are extended features such as a wall, a stream, a track or a road, a forest edge, or even a cliff etc (called a line feature), again these are less likely to be missed than point features and can eliminate uncertainty in distance travelled estimates.

Aiming-off is a technique for managing uncertainly in navigation. This technique is useful with line features, and involves deliberately aiming to the left or right of the target point so that the navigator knows in which direction to find the waypoint that will re-establish their exact position on the map.

For example rather than aiming directly for a remote stile over a wall, and when reaching the wall with no stile in sight, be faced with the question “which way do I go now?” aim off to the left by 100 metres say, so that when the wall is reached you can be confident that the stile is to your right. Experience will guide you as to how far to aim off in order to not to miss the target.


Even the best navigators sometimes make mistakes or perhaps become distracted, and sooner or later every hill walker is going to become lost, or perhaps just temporarily “positionally challenged.”

This can be a bit of a shock to the normally proficient mountain navigator, and in these circumstances “don’t panic” is very good advice.

First stop, and then calmly and carefully examine your surroundings to see what clues are available. Note the nature of the terrain around you and any visible features before consulting your map.

Mentally go back to the last position where you were absolutely sure of your position, and consider all the possible areas that you could now be. Again challenge all assumptions and test them against the evidence.

Remember that mist makes it much more difficult to estimate the scale of the features that you can see, and often features appear to be much bigger than they actually are.

Before trying to see where you could be, see if you can eliminate places that you definitely are not, by looking at the slope angle and other features.

For example, if you are on a north-easterly slope then look at the map and eliminate all positions that do not have such a slope. Are there other features such as terrain type that can be used to narrow the possibilities further?

Sometimes this technique will just not work because there are no suitable features available or visible, however you probably know your rough position, to within a km or so. In this case look for a suitable line feature that you can aim for. Set your compass to a “safe” bearing that will cause you to intersect this line feature wherever you actually are within your "area of uncertainty".

Stick to this bearing as close as possible and although progress may be difficult, eventually you should hit the line feature you are aiming for, which could be a forest boundary, a track, a river or a contour feature etc. You can then follow this until you come to a point you can identify.

When setting your “safe bearing” avoid tracks that could go near any dangerous ground, especially in poor visibility. Move your compass around on the map over your area of uncertainty and see where the resulting tracks will take you, adjust your bearing as required to give you the best chance of finding your target safely.

This technique is the ultimate fall-back measure for most lost situations and nearly always works but may involve a long hard walk to get back to were you want to be!

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Global Positioning System (GPS)

GPSGPS, including GPS location smartphone/tablet Apps, can be a great asset when the technoloy is working well and you are able to use the information it provides, which is the grid reference (coordinates) of your current position and quite likely your location on an on-screen map display, but it does not give you the "get you there" functionality of car sat-nav systems.

GPS receivers work by receiving signals from a “constellation” of more than 24 satellites orbiting the earth and can operate anywhere on the planet if the signals are unobstructed.

Used properly GPS is a very powerful self location tool and navigation aid for the hill walker and a GPS receiver can make good maps even more useful. If you use GPS always use it in conjunction with a good map and compass to navigate safely on mountains.

It is vital that hill-walkers maintain good conventional mountain navigation skills and not rely on GPS, some of the key issues are detailed below.

GPS is dependent on an external system, the satellite infrastructure, this is unlikely to fail completely or be withdrawn from service without warning, but it is outside the individual's control and users should be aware of the limitations. Modern GPS units are generally reliable, but GPS should not be relied on completely for mountain navigation because of a number of factors:-

a)  The GPS unit needs to be able to receive signals from at least three satellites, four to get a 3-dimensional (3D) accurate fix local terrain and trees etc can easily block the signals. Also the number of satellites "viewable" from any particular location will vary as the positions of the orbiting satellites change - hence some days performance may be worse than expected.

b)  Even the best units are able to display only a small amount of detailed mapping information at a time and the display can be difficult to see clearly under harsh (ie wet/cold or bright sunshine) viewing conditions on a mountain.

c) It has been reported that signals from the satellites can be reflected from rock faces or outcrops etc in a mountain environment, causing signal delays which degrade the position calculations.

d) Any complex electronics unit can fail or be damaged (by being dropped for example) especially in a harsh outdoor environment.

e)   Receiver power consumption is quite high (especially in the case of smartphones) and batteries/power packs can fail just at the wrong time, especially in low temperatures.

f)  And finally, as GPS becomes more ubiquitous, it may, unfortunately, become victim to deliberate jamming or spoofing. The received signal strength is very low which makes it susceptible to interference, deliberate or otherwise. The military sometimes jam GPS signals as part of training exercises news story, but this is unlikely to affect many hill walkers.

Given all the above issues are OK the other requirement is that the receiver has been setup correctly to operate with the British National Grid and map datum (see below).

Also the operator needs to be proficient in using the device, which comes with practice.

Using the menu system set the GPS display format (position co-ordinates or units) to the British National Grid (BNG) and set the Map Datum to Ordnance Survey 1936 which may be abbreviated to OSGB 36 or Ord Srvy GB.  Some GPS units will automatically default to the correct map datum, others will need to have it set by the user.  Failure to set the map datum correctly can result in serious errors in the position co-ordinates.

A GPS will normally display 10 figure references (5 Eastings and 5 Northings). When entering 8 figure references into your GPS, set the last digit of the Eastings and the last digit of the Northings to zero.  Similarly if using only 6 figure references, set the last two digits of both groups to zero.

Batteries Given that the GPS receiver has a good “view” of enough satellites, a “flat” battery is perhaps the most common problem encountered by users of hand-held GPS units. Conserve battery life by turning the receiver off when not required and carry plenty of spare batteries. Use the batteries recommended by the unit manufacturer, usually premium quality alkaline types or Lithium. If battery usage becomes an issue try high capacity rechargeable Nickel-Metal Hydride (Ni-MH) cells, these are initially slightly lower voltage but have a similar capacity to alkaline cells and better low temperature performance. They do not have a long charge retention life, so make sure they are fully charged before each use. However for critical use, and as an emergency back-up, non-rechargeable Lithium AA cells available from most photo shops and now many supermarkets are best, albeit expensive (typically £7 for four AA cells), as they have a long shelf-life, high capacity, low weight and good low temperature performance.

Self Location

Finding your position with the aid of GPS and a map is the first thing to master and requires an understanding of Grid References. The next section gives some details about Grid References used in Great Britain.

Practice self location using your GPS and BEACON Guide or map.  The display format on your GPS should be set to OS British National Grid and the map datum should be set to OS GB (OSGB) 1936.  Usually abbreviated to British Grid and OSGB36.

Select the position co-ordinate screen on your GPS and check the receiver status shows that it has a good 3D (3-Dimensional) position fix (better than 15 metres). Read the full co-ordinate position, first the two 100km grid square letters, then the Eastings (i.e. how far East you are), and then the Northings (i.e. how far North you are). Normally these are both 5 figure numbers, making a 10 figure reference in total which defines your position to 1 metre, but personal GPS units are not as accurate as this. Typically accuracy is 5 to 15 metres, improving to better than 3 - 5 metres with the EGNOS (European Geostationary Navigation Overlay System) currently under development and trial.

If you are entering 8 figure waypoint references into a 10 figure GPS, set the last digit of both the Eastings and the Northings to zero.

Study your unit’s manual and become familiar with using your GPS for self location before it is needed in a serious situation.

Grid References

The British National Grid divides Great Britain into 100km major grid squares identified by two letters, SH for example. These letters are given on the map of the BEACON Guides, or in the margins of a conventional map.

The grid lines marked on the maps are at 1km intervals. These grid lines have two-digit numbers written against them giving the distance in km (from 00 to 99) within the 100km squares. The vertical grid lines, running north to south, show how far east you are and are called Eastings. The horizontal lines, running east to west, show how far north you are and are called Northings. In a grid reference the Eastings are always given first, then the Northings:-

GR Format:  Two Letters (representing a 100km square)  Eastings,  Northings

Hence a grid reference with two letters and four figures, such as SH 24 19, identifies a unique 1km by 1km grid square. Greater accuracy is achieved by using more digits to measure down to 100 metres, 10 metres, 1 metre etc. There is always the same number of digits used in the Eastings and the Northings.

Estimating your position to within 10 metres or so (i.e. four digits on both the Eastings and the Northings to make a eight-figure reference) requires careful measurements on the map inside the appropriate 1km grid square on a 1:25,000 scale map. Eight-figures (10 metres) is the precision used on BEACON Guides. Six-figure references are not really accurate enough for mountain navigation and do not exploit the full potential of GPS.

A six-figure position fix, accurate to 100 metres, is easy to estimate on a 1:25,000 scale map, and may be done by eye with practice, but a Romer scale (or corner measure) on your compass will enable you to find your position on the map with the maximum accuracy, use the top-right corner of the Romer to pin-point the position while reading the last figures of the co-ordinates from the side scales.

If you ignore unwanted figures in the position co-ordinates from your GPS, remember to ignore the same number of digits at the end of both the Eastings and the Northings.

The following table shows the size of grid square defined by a grid reference preceded by two grid letters.

No of Figures

(after letters)

Area Defined

four figures

1 km grid square

six figures

100 metre square

eight figures

10 metre square

ten figures

1 metre square

The reference of a whole grid square is given by the co-ordinates of its south-west (bottom left) corner.

Note: Some GPS receivers (and grid references) do not quote the 100km grid square letters, but use an extra digit in front of the Eastings and Northings instead.  For example 2xxx, 3xxx is a “SH reference”. If required, these extra prefix digits may be found in the margins of an OS or HARVEY Map, but if you use these digits do not quote the letters as well.  Do not confuse this alternative format with the practise of sometimes simply omitting the letters when it is thought that the letters are obvious and unnecessary.  It is best to quote the full reference including the letters to avoid any possible source of confusion.

Position Checks Check your GPS positional accuracy at known positions such as your starting point and OS Triangulation Pillars (trig points). Ideally enter key waypoints such as path junctions, turning points and the start of emergency escape routes, into your GPS before you set out.

BEACON Guide-Maps give eight-figure grid references for key waypoints (field surveyed by GPS) that are rounded to the nearest 10 metres, rather than truncated. These may be entered into your GPS either by hand or by using a computer with a suitable interface cable and software.

Selecting and using a GPS Receiver for Mountain Navigation

GPS receiver technology is advancing rapidly and new models for the outdoor enthusiast are being introduced regularly. Look at specifications and visit a specialist retailer who will be able to demonstrate the features of different models before you decide to purchase.

Hand held GPS units designed for mountain walkers and climbers have a unique set of requirements. Detailed mapping software is now available for many models, but unlike a unit designed mainly for road or lowland use, a mountain walker's GPS needs to be capable of being used with “paper” topographical maps. The following check-list covers most of the technical things to consider when choosing a personal GPS for mountain navigation in Britain:-

     A GPS for outdoor activities should be waterproof to at least IPX7 standard, if it is not it will need additional protection on the hill.

     Look for a model with at least 12 receiver channels (almost standard now)

     Check that it supports British grid position co-ordinates and map datum

     Check that the operating temperature range is adequate for your planned use

     Check battery life – remember that battery performance is dramatically reduced at low temperatures (i.e. below about 4C)

     Select the position co-ordinate screen and check that the position data is readable in strong sunlight, in low light and when dark.  Ditto other important screen data.  Under adverse viewing conditions it can be very difficult to see the screen displays, especially map details.

     Check that it clearly shows the quality (validity) of the current position report

     A model that is WAAS / EGNOS enabled will be capable of higher accuracy when and where GPS accuracy augmentation systems are available.  Note that using WAAS will normally require more power and hence shorten battery life.

      A model with an electronic compass built-in is more versatile because it can point to a waypoint even when stationary. Some models feature electronic compasses accurate to about 2 degrees, most are a little less accurate than this. With a simple GPS unit you need to be moving before the receiver can give directional information. Sometimes this can be inconvenient in a group situation.  An electronic compass in a GPS may be programmed to use Grid North, True North or Magnetic North references, but you should still carry a conventional walkers compass.

     GPS is not very accurate on elevation.  Some GPS units incorporate a barometric altimeter which can give accurate height information if calibrated properly, it can also give information about what the weather may be doing.

     Is it easy and intuitive to use – can you manually enter and edit waypoints and navigate to them easily?  Can you still do this in the dark?

     Do you need to be able to use it wearing gloves?

     Do you want to link it to your computer, Laptop, Pocket PC / mapping software package - is a suitable interface available?

     Is the unit's software upgradeable - to be able to take advantage of future developments?

Example GPS Models for Mountain Navigation  - The choice of GPS is a matter of personal preference, but the check list given above can be used to help select a suitable model.  Don't forget that keeping things simple is generally best in mountain navigation and normally you will be using the GPS as an aid to navigation with "paper" mountain maps. Never rely completely on electronic maps in hand-held devices.  Hence being able to enter and use waypoint position co-ordinates (grid references) easily is important.

Unfortunately many recreational GPS user interfaces and their User Guides are not really designed for mountain walkers and assume that the majority of users are not using detailed topographical maps with their GPS, which can make finding the required information difficult.

Garmin publish a fairly detailed beginners guide to using GPS with paper maps. This is written with US users in mind rather than UK users, but we have tried to cover all the relevant information for British users here.  The Garmin guide / manual can be downloaded here as a pdf.

Very often units will default to Latitude and Longitude co-ordinates and before they can can used with British topographical maps the position units (or co-ordinates) have to be changed to British National Grid (BNG) and the map datum set to Ordnance Survey 1936 often abbreviated to something like Ord Srvy GB or OSGB 36.

Also it may be necessary to program the unit to show the current position co-ordinates on one of the main display pages.

To enter a new waypoint or "Point of Interest" (POI) into some GPS models you may have to first mark a new waypoint and then edit the name and co-ordinates to those required.  Other models will allow you to directly enter a new waypoint into the units memory.

If you have a computer available most models will enable you to store and load (upload and download) waypoints and POI to/from your computer using a simple interface cable and proprietary software or freeware.

One of the best map-display personal GPS units we have seen so far is the Satmap Active 10 Sports GPS and we use this model ourselves when surveying routes, often alongside a professional non-mapping unit.  The Satmap is a relatively expensive unit, but it can be loaded with detailed Harvey mountain maps or OS mapping by plugging in the appropriate SD card.  It has a generous sized screen compared to many other models and other features that make it suitable for mountain use, including an electronic compass.  However we would never go on to the hill relying on only a GPS unit or smartphone App for navigation.

If cost is a factor, and bearing in mind the "keep it simple" rule, we would normally recommend a good quality basic non-mapping unit designed for mountain use.

Two older models still worth looking at are the well specified SILVA Multi-Navigator (known as the Brunton MNS in the US) - an "expedition" model well suited to serious mountain use with large display and good features (but no maps), it has an accurate compass and barometric altimeter, but no WASS, and the classic Garmin GPS 60 - a good general purpose non-mapping GPS with WASS and a large easy to read programmable display, but no electronic compass. The SILVA Multi-Navigator has been discontinued for some time now, but is still occasionally offered for sale on eBay etc and it features direct entry of waypoints and a low power GPS-off, compass-only mode which conserves battery life. If you purchase a second-hand Multi-navigator check it has current software installed, otherwise it may not work very well. The current software version is 1.9 and SILVA will upgrade the software free of charge if you send the unit back to them - but phone them first to arrange. The Garmin GPS 60 series may also still be available, but has been replaced recently by the GPSMap 62 series which we have not tried yet, the 62s model has an electronic compass built-in.

The key thing to remember is that a GPS should be used as a supporting tool for use with a good map and compass, not as a replacement for them.


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Use of GPS waypoints and route menus

As well as finding your current position, your GPS can be used to mark and store the co-ordinates of waypoints on your route and on some models mark a “home” position for later use. Normally waypoints can be entered in one of three ways:-

     by marking your current position

     by entering or editing a grid reference manually via the unit’s keys/buttons

    by scrolling to a new position on an on-screen map display and marking the position

     and with many models, using computer software via a special interface.

BEACON Guide-maps list key waypoints that can be entered into your GPS in advance. These are normally 8 figure (10 metre precision) grid references.

If you are entering 8 figure waypoint references into a GPS that displays 10 figures in total, i.e. 5 figure Eastings and Northings, then set the last digit of both the Eastings and Northings to zero.

Normally in the mountain environment you can not expect to walk in a straight line from waypoint to waypoint, but must follow the ground or path where there is one. Even so, using pre-entered waypoints / POIs with the “distance to go” and bearing functions of your GPS is a very good way of keeping track of your progress and avoiding mistakes in navigation.

When you reach a waypoint, you can select the next waypoint on your intended route as the new “goto” point.  Its usually quicker and easier to use your map and compass most of the time and use the GPS as a cross check on your position when required. Its a good idea to check that the distance and bearing to the next waypoint given by your GPS is what you expect from inspection of the map and use of your compass. This is a vital check that the data has been entered correctly and the GPS is working properly.

Don’t forget to carry and use your map and compass even though you have a GPS unit.

Experienced navigators cross-check all the information they have available to them.

If you make a habit of doing this even when the visibility is good you will soon develop the confidence to navigate accurately day or night whatever the weather.

Route Menus etc

Many GPS units have facilities for storing routes in the form of a set of waypoints which have been entered manually or downloaded from external route planning / mapping software. These sophisticated tools have features designed to automate point to point route navigation and record tracks for later use / analysis. These features can be fun to use, especially on fairly straightforward terrain, but should be used with extreme caution on mountains as they are more difficult to use safely on complex mountain terrain.

Using these route finding facilities usually requires the GPS to be left-on for the complete trip, with the increased risk of battery drain. The key recommendation is to master the basics of conventional map and compass navigation, self location and GPS aided navigation before starting to use the more sophisticated route tools. You will then be able to achieve the maximum benefit from using your GPS in a wide range of circumstances.


Sarloc is worth a special mention even though it is not a conventional navigation aid.

Sarloc is a web based software tool that can be used by mountain rescue to locate someone needing help who has a suitable GPS enabled Smartphone, but is for whatever reason, unable to report their position. They could be lost and without a map or a compass.

Once contact is established with the casualty Mountain Rescue sends a special web link to the Smartphone by text which the user then opens in their phone's web browser. The website prompts the Smartphone for its best geolocation info. (this will use the phones GPS chip-set if installed) and it will report the position co-ordinates back to the rescue team where the casualty's position can be displayed on a digital map using a system called MRMap.

The Smartphone also sends an estimate of the accuracy of the position co-ordinates.

Its not a Smartphone App as such because it does not need anything new to be installed on the users Smartphone, it uses the phones built-in geolocation capability and existing web infrastructure to report its position to Mountain Rescue by a convenient and reliable mechanism.

The Sarloc system was developed by Russ Hore who served as a member of the Ogwen Valley Mountain Rescue Team for many years, so he has plenty of experience of the real world problems that MR faces.

Some of the key issues around this systems deployment are that the Smartphone user may not know how to get their phone to display its position coordinates without help. They may be stressed and/or injured, have a weak signal and a failing battery. Hence anything which facilitates the location process is a big help. However not all Smartphones will have the capability to use the system and it does rely on adequate signal reception conditions.

At present the UK Sarloc system can only be used by Mountain Rescue/Police to help locate a missing person, but a publically accessible system could easily be developed which would enable anyone with a GPS enabled Smartphone to go to a website and get an estimate of their position, on an interactive map for example, without having any special apps. on their Smartphone.

Ordnance Survey have introduced the useful OSLocate App for GPS enabled Smartphones and this is ideal for use with Harvey Maps and Beacon Maps as well as OS mapping. This does require the App to be installed in advance of course.

Finally... next steps

All the techniques described can be used in combination or on their own and form a basic tool kit of navigation techniques that can be used by outdoor enthusiasts. There is always more to learn, and techniques to practice. An excellent way to start or brush-up skills is to go on one of the many mountain navigation courses available where mountain navigation is taught on its own or as part of a more general outdoor activities package.

Have a look at the links page for details of some good navigation training course providers.

Please don't hesitate to contact us if you would like advice on suitable equipment or training courses to suit your needs.

Once the basic skills have been mastered, navigation (with or without GPS) can be a lot of fun and give a great deal of satisfaction whatever your activity.

GPS waypoints / POIs, complete with dates and times can be collected and stored on your computer to form a record of past trips and achievements.  Maybe your personal Munro log, or possibly a record of your Three Peak Challenge event times, or perhaps even your favourite picnic spots!

Also, geocaching games, like GPS aided treasure hunts, have become popular recently, see for example or

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Good luck, & enjoy our great outdoors

BEACON Mountain Maps


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