ENTS, For those of you still wondering, a GPS unit or a map refers to some starting point or datum. The latitude/longitude shown is based on where you are in relation to that sarting point or datum. Ideally all of these datums would be interchangeable, but they are not. If you use one reference datum on your GPS unit and someone else uses a different reference datum on their unit, then the location readings you get at the same point will be different, The difference could be off by tens or hundreds of feet. If you know what the refernce datum is for the different units, then the reading can be reconcilled. If you have readings for a tree based upon on datum, and you are trying to find it again using a GPS or map set to a different datum, when you arrive at the point where the numbers say the tree should be located - the tree will not be there.
This article describes it fairly well: http://www.alpharubicon.com/prepinfo/mapdatumwmerrin.htm by wmerrin The article says in part:: "For use in the continental US the two primary datums are the older NAD27 CONUS used on many older USGS 7.5 minute topo maps and the newer NAD83/WGS84. NAD is "North American Datum" and 27 or 83 refer to the years of adoption (1927 and 1983). NAD83 is equivalent to WGS84, which stands for "World Geodetic Standard 1984". NAD83 describes the continental US while WGS84 describes the whole world and uses NAD83 data for the US part. Depending on the age of a USGS 7.5 minute topo map, you may see reference marks for both NAD27 and NAD83. GPS units usually allow the choice of NAD27 or WGS84 as well as many others. GPS units usually default to a datum selection of WGS84. However, most USGS topo maps are still keyed to NAD27 unless they are relatively new. Most wilderness area USGS maps that I have seen don't get updated that often (not much changes out there) so they tend to be NAD27. The difference isn't huge, no more than 200 meters, but that can be significant if you are searching for a pointer to a cache." Ed --~--~---------~--~----~------------~-------~--~----~ Eastern Native Tree Society http://www.nativetreesociety.org Send email to [email protected] Visit this group at http://groups.google.com/group/entstrees?hl=en -~----------~----~----~----~------~----~------~--~---
