Hi John, Again, thanks for the good question. The advice given so far is excellent. A degree of longitude varies in length with the cosine of the latitude.
One of the problems in doing this kind of conversion is the units that are not natural for this application. Feet and inches are natural for describing length in relation to parts of you body. They are unnatural for describing co-ordinates on the surface of the earth. Navigators use a natural unit, the nautical mile. One nautical mile is equal to one minute of latitude. On a nautical chart, you pick off distances with a pair of dividers and read the distance by moving the dividers to the latitude scale at the side of the chart. 1' = 1 nmile. It is very simple and natural. The French revolutionaries also had a world view in establishing the metric system. The original definition of the meter is based on 10 million meters being one quadrant of the earth's circumference. From the equator to the pole is 10,000 kilometers. For conversion purposes, 90 degrees is 10,000,000 meters. The 1000 meter grid on topo maps can be used for these picking off distances and doing the conversions. Again it is simple and natural. Purists are correct in pointing out that these natural units are no longer accurate (or is that precise). For normal navigation and siting of sundials the natural units are satisfactory and give you a good sense of what is happening. The real solution suggested before on this list is to use MapBlast!. <hppt://www.mapblast.com> In the US you can put in your street address and get back a map showing the latitude and longitude. In other parts of the world you have to move the icon to your precise location but you can still get a reading of your coordinates precise to 4 decimal places. Roger Bailey Walking Shadow Designs n 51 W 115
