01-26-2004, 03:25 AM
[font "Comic Sans MS"][size 2]WH2, simply, put, datums are GPS coordinates from "different points of view". Different datums have been used by different organizations over the years. A given set of coordinates in one datum can be a quarter of a mile off from the same coordinates of another datum. The fact that there is more than one datum has something to do with the fact that the earth is a sphere rather than flat. I am VERY far from knowledgeable about datums but I do know that if you are trying to find a specific location (like GeoCashers do), you'd better make sure your GPS unit is set up to match the same datum as the cordinates that are given you. All GPS units allow you to change datums in their setup screens.[/size][/font]
[font "Comic Sans MS"][size 2]The two most common datums used are NAD27 and WGS84. WGS84 is the default for most GPS units. But many maps show Long/Lat in NAD27.[/size][/font]
[font "Comic Sans MS"][size 2]If you use the same GPS to "Mark" a spot as you use to go back to that spot, then datum doesn't matter. But if someone gives you coordinates and you plan to get close to that spot, then you need to pay attention to datums. That is why when ever I give someone coordinates, I also tell them what datum I used to mark those coordinates.[/size][/font]
[font "Comic Sans MS"][size 2]So, how important is it to pay attention to datums? That depends on how close you need to get. If being off by 600 feet matters to you, then pay attention to datum. If being off by 600 feet is close enough, then don't wory about datum.[/size][/font]
[font "Comic Sans MS"][size 2]Here are a few Web sites that describe it much better than I:[/size][/font]
[url "http://gpsinformation.net/main/maps.txt"][font "Comic Sans MS"][size 2]http://gpsinformation.net/main/maps.txt[/size][/font][/url]
[url "http://www.esri.com/news/arcuser/0401/datum.html"][font "Comic Sans MS"][size 2]http://www.esri.com/news/arcuser/0401/datum.html[/size][/font][/url]
[url "http://216.218.220.254/eng/datums.html"][font "Comic Sans MS"][size 2]http://216.218.220.254/eng/datums.html[/size][/font][/url]
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[font "Comic Sans MS"][size 2]The two most common datums used are NAD27 and WGS84. WGS84 is the default for most GPS units. But many maps show Long/Lat in NAD27.[/size][/font]
[font "Comic Sans MS"][size 2]If you use the same GPS to "Mark" a spot as you use to go back to that spot, then datum doesn't matter. But if someone gives you coordinates and you plan to get close to that spot, then you need to pay attention to datums. That is why when ever I give someone coordinates, I also tell them what datum I used to mark those coordinates.[/size][/font]
[font "Comic Sans MS"][size 2]So, how important is it to pay attention to datums? That depends on how close you need to get. If being off by 600 feet matters to you, then pay attention to datum. If being off by 600 feet is close enough, then don't wory about datum.[/size][/font]
[font "Comic Sans MS"][size 2]Here are a few Web sites that describe it much better than I:[/size][/font]
[url "http://gpsinformation.net/main/maps.txt"][font "Comic Sans MS"][size 2]http://gpsinformation.net/main/maps.txt[/size][/font][/url]
[url "http://www.esri.com/news/arcuser/0401/datum.html"][font "Comic Sans MS"][size 2]http://www.esri.com/news/arcuser/0401/datum.html[/size][/font][/url]
[url "http://216.218.220.254/eng/datums.html"][font "Comic Sans MS"][size 2]http://216.218.220.254/eng/datums.html[/size][/font][/url]
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