Tim: Differential GPS should get you that sort of accuracy in most situations where you are not under dense canopy cover or in a canyon. You do NOT need permission from the govermnent for that precision, as differential GPS is a civilian workaround to get similar accuracy as the military-only GPS bands.
At that level of accuracy, you would want a mapping grade GPS: standard recreational grade GPS just won't cut it, and a survey grade GPS would be overkill and way more expensive. I've only used Trimble GeoXH data collectors, but I believe there are others on the market. They should come with software that will allow you to set up data collection forms (data dictionaries), plan for the best GPS measurement times, retrieve data from the GPS, differentially correct it using the CORS (continuously operating reference station) network, and export the data to a shapefile, table, etc.. With good satellite geometry and no canopy or other obstructions, you can get accuracies in the 10-20cm range without to much difficulty. Recording more GPS readings per record (I usually use 30-60 for a point) will improve your accuracy via averaging. Mapping grade GPS aren't cheap, but I saw at least one used Trimble on Ebay. This tutorial should walk you through DGPS concepts: http://www.trimble.com/gps_tutorial/dgps.aspx Let me know if you need any further clarification. Best, Jay -- Jay Guarneri 403 S Main St Apt 1 Monticello, AR 71655 (631) 813-8218 guarn...@uamont.edu
