Re: GPS restrictet on continents or can all be used Global ?
18 Rabbit,<br /><br />Map Datums are interesting. They come from the fact that the earth is not a perfect sphere and so pinpointing spots on a map using Latitude and Longitude won't give you absolute accuracy in relationship to a known point. Its a little hard to explain actually but I'll give it a try, and then a absolutly useless bit of trivia about one Datum.<br /><br />If you could drive a nail at the exact spot where the equator crossed the zero longitude line (just off the coast of Africa) you could measure the distance from that exact point to your front door it would probably be somewhere in the neighborhood of 8,000 miles. The thing is that if you calculated exactly where 8,000 miles came out to be it would not be at your front door. In fact it would probably be off by a couple of hundred yards. The reason is, once again, that the earth isn't perfectly round. When a mapmaker is plying his trade its necessary for the indicated reference lines, usually longitude and latitude, to match known spots. Unfortunately as you move farther and farther from the starting point the accuracy of the known location falls further and further off. So what map makers do is segment the earth into smaller patches (I call them smaller but in fact they are usually quite large) and then adjust the map to match the indicated reference lines. Each one of these smaller segments is named and that name is the 'map datum'. It is just an adjusted section of map. If you take a look at the (I can't think of the right name for it right now, maybe the 'legend') little inset on the chart that tells when it was printed and what scales are used and all that sort of stuff it will also tell you what map datum was in use when the map was created. In order for your GPS to agree with indicated points on the map you need to be using the same datum that is indicated on the chart itself. If both the chart and the setting for Map Datum in the GPS are in agreement then if you are at a location shown on the chart the GPS should agree with the positon as indicated on the chart. Were you to then change Map Datums in the GPS to one of the others what would happen, if you did not move, is that a different longitude and latitude would then be indicated on the GPS.<br /><br />So basically what happens is that every now and then some law making body will designate the datums to be used, usually on a country-wide basis. In every case there is some known point, that may or may not actually be on the chart you are using, that is accepted as the starting point for everything in that particular map datum. They are updated every now and then, but not at all often. We are now using World Geodetic System 1984 as the norm in the USA, but before that it was something-1927. This is the bit of trivia, in the old system, the 1927 whatever it was called, the starting point was the front porch of the house on a ranch in central Nebraska. I have been told that it was the home of the parents of whoever the guy was who was in charge of such things at that time. It was used as the norm in the USA for 57 years.<br /><br />Anyway the point is that in order to get good coordination between indicated positions from a GPS and known locations on a chart it is imperative that the GPS be set to match the map datum used in creating the chart.<br /><br />Thom
