A key skill in pre-GPS navigation was finding your position using three known points of reference and a compass. It is still taught on Sailing and Mountain Training Courses, so use this puzzle to learn about how we did it before GPS then go and check it with GPS.
The technique works as follows:
1. Identify 3 known landmarks that you can find on a map and are distributed evenly around the points of the compass
2. From the position you wish to fix take a compass bearing to each of the three landmarks - I used a marine sighting compass but a good walking Silva will do.
3. Adjust the bearings for the current local Magnetic Variation to obtain Grid Bearings.
4. On the OS map use a protractor or the Silva compass to draw lines from each of the three landmarks that represent each of the 3 Grid Bearings.
5. Where they cross should be a small triangle called the Cocked Hat.
6. You are somewhere in that triangle and its size indicates the accuracy of your fix.
7. Take a Grid Reference of the centre of this triangle from the map and convert it to Latitude & Longitude to get GPS co-ordinates to go and locate the cache.
The reverse process of finding somewhere from 3 given bearings is slightly more tricky. You would normally use the map/chart on which you plotted the position to navigate to it as described above. However as challenge hungry geocachers you could try finding the cache just using the compass to replicate the fix readings. A good starting point to try this would be the listing co-ordinates position.
I have included a clue that should enable you to find the cache within your Cocked Hat and the Geochecker is set to an accuracy of 8m to get you in the right area.
Here is the compass data:
Bearing to Dumyat Trig Point 28.0 °
Bearing to Wallace Monument 259.5 °
Bearing to Prominent house near Logie Kirk (GR NS 8173 9697) 311.5 °
You can check your answers for this puzzle on Geochecker.com.