Here is a short quiz which should help you understand some old and new navigation techniques. Use the hyperlinks to find out more.
How to find north and south
On a sunny day in winter, standing on the Greenwich Meridian, you can use your watch to find where due south is. How is this done?
The method is based on using your watch as a sundial. But strangely sundials tell the time accurately only four times a year. Why is this?
And why do I have to be careful at Lands End in summer?
Why do compasses not work very well in some buildings and outside tube stations!!!!
A compass points to the Magnetic North Pole – strictly speaking it aligns itself with the Earth’s local magnetic field. But where is the Magnetic North Pole and where is it heading?
Why is it difficult to use a compass for navigation near the Magnetic North Pole?
Grids of different inclinations
When you use the GPS units Latitude and Longitude, lines of longitude are by definition due north. However, OS maps use a Universal Transverse Mercator (UTM) projection to create a flat, square grid on top of the spherical Earth and inevitably this introduces distortions of distance and direction. Only one line on the OS grid is therefore exactly north-south. Where is it, which coast to coast walk does it cover and who wrote the book of the same name?
The OS grid comprises 100 km square sheets and most of VBR's walks are on sheet SK. What is the significance of grid reference SV000000.
The equivalent of the OS in France is IGN (Institut Géographique National) which uses the worldwide UTM/UPS system and you should switch your GPS to this setting if you are to use the IGN maps. So the next question is: which lines on the IGN grid are exactly north-south?
Measuring time
A knowledge of time relative to GMT has always been the basis for worldwide navigation, particularly in determining longitude. Galileo discovered the moons of Jupiter and proposed that they could be used to measure time accurately anywhere in the world. How would this work?
John Harrison designed four ships chronometers which measured time with sufficient accuracy for navigation. His fourth built in 1759 and designated H4, differed markedly from the first three. In what way? The clocks are described in the excellent book Longitude by Dava Sobel.
H4 was accurate to a few seconds a month but GPS measurements depend on distances calculated using the speed of light and this travels 30 metres every one ten millionth of a second. So how accurate is the internal clock inside you GPS? (Tip – leave it off for a few days, switch it on indoors and time it against the pips).
Why is the displayed time inaccurate?
Measuring distance
A distance of fifty four nautical miles is only 8 metres greater than 100 km. Is it chance that this difference is so small?
When my GPS calculates distance, does it include height in the calculations? (Tip – go outside; store a waypoint; edit the waypoint by adding 1000 metres to the height; use GoTo to display the distance to the waypoint.)
And now a simple?? puzzle which you can solve roughly with a pencil, a ruler and a piece of string. But I expect a more elegant answer from at least one of you!
A walker spots a group of other ramblers a kilometre away to the north walking at half his own speed in an easterly direction. Rather foolishly he walks straight towards them at all times, his curved path being longer than the minimum required to intercept them. How far did he walk before he caught up?
The last word
And finally: a rambler walks a kilometre due south, turns left and walks another kilometre due east, turns left again and walks another kilometre due north. He is so surprised to find himself back where he started that he doesn’t notice a nearby bear which eats him. Why didn't he notice the bear?
