Spiral Search

Overview - when to use it

On this map there is a path shown. In reality the ground is peat bog and the path is aspirational rather than actual. Even in good weather it is hard to find. In dense fog once you are out of the forest you are unlikely to stay on the path. In that case you will want to plan a route with several easy-to-find landmarks along the way.

I have chosen (for this example) to leave the forest and to head for the trig point (at 389m) and then on to the lake and then I will head North-West via the furthest Cairn.

I will use the map and compass to calculate a bearing from where I leave the forest to the trig point and I will estimate the distance (using the compass scale) at 750m.

Now I will pace count 750m and from 700m onwards I will be keeping a careful watch for the trig point.

At 750m if I cannot see the trig point then I will assume that I am close to it but that it is lost in the fog nearby. I will now use a spiral search to find it!

How to use it

I am at the start point in dense fog.

The first thing that I need to do is to estimate how foggy it is and how far I can actually see. This can be surprisingly tricky to do especially on featureless terrain. I guess that I can see around 10-20m. I will be cautious and assume that I can see 10m.

From my pace counting I know that 10m is about between 5-6 paces. I will use 5 paces.

I will now take my compass out of my pocket and walk North 5 paces. I will turn and look back to see if I can see where I came from. If I can then I will continue.

I will turn right 90 degrees and head East for 10 paces. I will now turn right again and head south for 15 paces.

At the end of every line I turn right, add on 5 paces and walk again.

You should be able to see that by doing this I am walking in an outward spiral and that it should be possible on any leg of the spiral that I can look left and right of the line and see all the ground between two parallel legs of the spiral.

By doing this I can guarantee that I can cover all of the ground around my starting point and not miss anything. Eventually I will find the landmark that I am looking for.

If I lose count or forget which leg of the spiral I am on (N, E, S, W) I can always start a new spiral from wherever I am and start counting at 5 paces.

Efficiency

The spiral search is very efficient. It is the easiest one to do and is the one that will find your landmark the quickest. There are other search patterns that you can use but they tend to be more prone to errors.

Modifications

Let's assume that near the trig point is a cliff. Obviously then, whenever we reach the cliff we will not be able to do the spiral search in a pure form. Instead it will be necessary to take this into account by using the cliff edge as a point where you change direction. This will make the spiral less efficient but it will still work.