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Author Topic: Mountain Navigation  (Read 6629 times)

666_pack

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Mountain Navigation
« on: July 27, 2009 »

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 compete 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?”

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


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) and 1:25,000 (4cms to 1km). The 1:50,000 scale is 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.

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 the contour lines are coloured grey on predominately rocky ground. 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.

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)

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

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

Compass Design

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

The 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.

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.




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 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. Actually, map grid north differs very slightly from the true geographical north, the small difference is normally given on the map.

In the UK at present, magnetic north is only a small number of degrees west of grid north, look up the actual figure for your location from your  Guide or map. . To correct for westward magnetic variation add the variation 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 the variation is 4 degrees west, then rotate the compass housing anti-clockwise by 4 degrees so that it reads 324.

 most walker’s compasses are marked in 2 degree graduations and setting them more accurately than this is difficult.  Halve the variation 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 two small graduations 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.

In Britain at present magnetic variation is quite small )and 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 – even so, if you can maintain a 2 degree accuracy in your navigation you are doing very well.

 


Note that there are 60 minutes (mins.) of arc per degree and on Beacon guide-maps we normally round the magnetic variation to the nearest half degree, i.e. 30 minutes.


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 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.

Note: when using your compass in this way ensure there is no magnetic metal 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. 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) where there are magnetic rocks and a compass is unreliable – check guide books if unsure.

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

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


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.


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.



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.




Lost!
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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Celt_Ginger

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Re: Mountain Navigation
« Reply #1 on: July 27, 2009 »

Good post. Some sound advice there.
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RedLeader

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Re: Mountain Navigation
« Reply #2 on: July 28, 2009 »

This is an excellent post, I'll have to peruse it at length. Cheers.
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666_pack

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Wolf_Larson

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Re: Mountain Navigation
« Reply #4 on: August 19, 2009 »

Can you get the computer to read it out, man its long  ;D i read it later hehe
« Last Edit: August 24, 2009 by wolflarson »
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666_pack

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Re: Navigation
« Reply #5 on: September 14, 2009 »

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Mourneman

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Re: Mountain Navigation
« Reply #6 on: October 21, 2011 »

Very well written,lots of information
 

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