Navigation Errors

It is best to assume that you are not perfect - it saves time and arguments. There are several different types of errors that you can make whilst navigating and each can be planned for and dealt with.

Good navigation skills involve planning for these types of errors and trying to eliminate them as much as possible. The best way to eliminate them (apart from lots of practice) is to plan a route that gives you lots of feedback when you are going wrong - even if the weather is bad.

Let's look in turn at the typical errors that can be made.

1. Starting Point

This may seem stupid but if you are wrong about where you start then everything else will be wrong. This sort of mistake can be easy to make in featureless terrain (such as moorland) or in dense woodland.

The way to avoid these issues is to have starting points that are reasonably unique even if that means that the distance between key landmarks is greater.

Remember, that when you started the walk that you were at a well-defined place. You then walked somewhere on a bearing to another landmark - another well-defined place.

What I am always looking for is a land-mark that is reasonably unique within my expected travel time. For example if I am heading for a Trig Point I would check to see that there is not another one that I might accidentally hit if I went off in a wrong direction. If I was using something more common - like a small hillock then I would take careful note of distance to ensure that it might not be a completely different one.

Warning: I have had some 'fun' due to a GPS that lost signal. When It lost signal the display continued to display the last known point that it had - which was some distance away. This meant that in trusting the GPS to tell me where I was I was actually wrong. This is quite fundamental and leads to no end of problems. Fortunately my current GPS will tell me when signal has been lost.

2. Wrong Bearing plotted

This is an easy mistake to make. The most common mistake is that you place the compass on the map and align the compass-housing lines with the wrong grid so that you have effectively used the wrong North. This mistake should be easy to spot since very quickly the terrain and minor landmarks will be quite different to what you expect.

The second most common mistake to make is to forget to include a correction for the difference between magnetic and grid north (which is shown on the map - map basics)

3. Wrong distance/time calculated

The most likely thing that happens is that the compass is not quite placed accurately on the map (or slips) - rubber feet on the compass base-plate tends to fix this issue. The next thing that might happen is simply that you mis-read the scale.

The way that you spot this is that after measuring the actual distance you have walked you either come accross the landmark early or it is not there when you arrive.

If the landmark is too early then what you need to do is to place the compass back onto the map and check that both the bearing and the distance measurements are roughly correct. If they are incorrect then you need to work out where you are now likely to be and adjust accordingly.

If they are correct then you will need to decide if you have miscalculated the distance you have travelled or have been too optimistic. I tend to find that there will be a tendency to over-estimate how far I have travelled - usually due to rough ground) and so I might allow myself an extra 10-25% of distance to find the landmark before I worry about it.

If visibility is bad and I trust my measurement and course calculations then I will consider doing a spiral search.

4. Wrong bearing walked (drift)

It is generally quite easy to walk in a straight line for some distance when following a compass bearing provided that the terrain is relatively easy - such as grassland or a nice path. The problem typically arises when you are following a bearing over rough ground or have to clamber around obstacles or trees block your path etc.

Drift can easily happen over long distances. An easy way to help to avoid drift is to choose some landmark that you can see that is in line with the bearing that you are on and head for that.

Drift can happen when you are tired and may then walk unevenly - one leg is often stronger than the other and so that might cause you to drift in the opposite direction.

Loose ground such as sand or scree may also cause drift since if you try and walk along a scree slope (instead of straight down/up it) you will find that you always are pulled down the slope.

Boulder-fields (that you have to clamber over/around) are almost impossible to cross without drift because you need to make so many small regular corrections to your ideal path.

Peat bog (where there are deep pools) can also be tricky for the same reason.

In all of these things using a distant landmark may help somewhat. Of even more benefit is maintaining a general awareness of how and where you are walking.

5. Error in bearing calculation (drift)

This is the final and more subtle one. When you place the compass onto the map and set it for a bearing there is chance that it will be less than perfect - either you will not place the compass exactly or you forget to adjust correctly for magnetic north.

This picture shows the problem. You started at the start and walked along on a bearing to Actual when you thought you were walking towards Target. The distance between Target and Actual is you error. Now, remember that the best accuracy you are likely to achieve setting a compass from a map is around 0.5 to 1 degree (subject to the scale of the map and the distance travelled etc) and so that will be the minimum error we will calculate. This table shows you some figures:

On the left we have the error in degrees that you started with and I have assumed that realistically the minimum error that you will achieve is 1 degree. Along the top we have the distance that you have travelled in metres.

The table then shows the error (in metres) between where you wanted to be and where you are.

I have colour-coded the table based on the following rules:

• Green - An error of less than 30 metres means that even on a foggy day (in all but the worst conditions) you would probably be able to see your landmark
• Amber - If visibility is OK or good and you are not unlucky then you should be able to see your landmark - but luck is an important factor.
• Red - You are not likely to be able to see your landmark and you might become lost.

What this all means in the real world is that if you are good at setting the compass and following a bearing over various terrains then you can safely navigate some distance between landmarks. If you are not very good at navigation then you will need to use more landmarks.

From this table you can also obtain an idea of how 'safe' you are when in poor weather or on bad ground. So, from past experience I know that my accuracy on average is 2 degrees or better (usually 1 degree) and that as I become tired that accuracy is probably going to drop to maybe 3 degrees. It follows then that if I want to be safe then the sensible thing for me to do is to organise my routes so that landmarks (of any description - even changes in terrain) are available to me ever 500-1000m.

When I cannot do this then I change my route planning so that I will aim to navigate to a linear feature (like a cliff, a tree-line, a stream or whatever) that is much longer than my expected navigation error and would lead me to my desired point.

Let's look at this map (from plotting a course) in a new light and plan for being less than perfect.

If I assume that I am less than perfect on navigation then if I aim for the forest (red line)

you can see that I can still be wrong by around 2-300m (purple lines) either way and still be sure that I will hit the forest. Once I hit the forest (regardless of where I hit it) all I need to do is to follow the forest edge South West.

This makes it a very good route since you can see from the table above that for around 3Km a 4 degree error is 200m and I know that on average I am much better than that.

Remember, that I actually want to pass through the gap between the two forests so let's look at what would happen if I aimed directly for it.

Here you can see that the leeway for error is quite small and only if I am perfect will I hit the gap. However, if I do manage to do that even in fog I would not actually know that I had hit the gap if I could not see the forest either side (due to the fog). Likewise, if I did drift and hit the edge of the forest I could not be certain initially which forest I had hit. I would need to work it out.

From this example you should be able to see that by deliberately choosing to aim for one of the forests my chances of navigating through the gap have been improved.