I know I'm a math geek, so this may not be something everyone wants to have, but I'm going to put a table of trigonometric values (probably either sines or tangents) in my PSK.<br><br>Why? Because I also plan to put in a steel tape measure, 1 metre long. (I just bought one for a $ in the "Everything for a Dollar" store in the mall. Undid the screw in the back, pried the case apart - presto, a 1 metre long steel tape measure that I think I can fit round the inside of my Altoids tin PSK. With this and a set of trig tables, I should be able to calculate my latitude a lot more accurately than with any protractor I could fit inside. <br><br>To measure your latitude - put a vertical stick in a horizontal piece of ground. At precisely high noon - when the shadow is at its shortest - measure the length of the shadow. Divide it by the height of the stick; this will give you the tangent of your latitude. <br><br>For example: Suppose your stick is measured at exactly 48 cm high. At noon, it casts a shadow 39 cm long. This gives a tangent of 0.8125, which according to my Windows calculater equates to an angle of approximately 39.1degrees. You would be approximately 39 degrees 6 minutes N. (Of course, someone would have to calibrate for the time of year; the above example would only be correct at the equinox, I believe.)