What Keeland states is exactly true! The selection of one of these = units is 100% based on what your needs! NPS required us to us to have = 99% precision in order for others to come in and duplicate the study = with a large degree of assurance. Most studies aren't worried about = replication 50years from now, and the exact position isn't so vital. = With small amphibians and reptiles and inverts, microhabitats within a = few meters can significantly alter data (some salamanders spend their = whole life under the same rock!). Bison on the other hand are more = dependent (at least its my impresssion) on wider ranges of habitat so = the precision and accuracy are less vital. =20 =20 Bottom line, your going to have to consider your study and decide for = yourself! :) =20 Malcolm L. McCallum Assistant Professor Department of Biological Sciences Texas A&M University Texarkana 2600 Robison Rd. Texarkana, TX 75501 O: 1-903-233-3134 H: 1-903-791-3843 Homepage: https://www.eagle.tamut.edu/faculty/mmccallum/index.html =20
________________________________ From: Bobby D Keeland [mailto:[EMAIL PROTECTED] Sent: Tue 3/28/2006 9:15 AM To: Malcolm McCallum Subject: Re: GPS units and accuracy data While Malcolm is right about the accuracy I must say that we use and really like the inexpensive GPS units (like the Garmin etrex). Yes, = these units are not as accurate as other, much more expensive units on the market, but they are more than accurate enough for many purposes. While = I may "have no idea what the real data looked like" I really don't care = what the real data looks like. The etrex and similar inexpensive units will = get me back to within a few meters of the study site. If I am within a few meters of my study site and cannot find it I need to find another job. = In my experience, if you talk to a GPS guru they will always tell you that you really need the most accurate unit available. For my purposes I do = not need that accuracy and I doubt that most other people need it either. Bottom line is buy what you need, nothing more or less. Check out http://www.fs.fed.us/database/gps/receivers/acc04_01.htm for a comparison of accuries of various GPS units, but also check out the = costs and weigh that against your actual needs. BobK Bobby D. Keeland, Ph.D. USGS, National Wetlands Research Center 700 Cajundome Blvd. Lafayette, LA 70506 Phone: 337-266-8663 FAX: 8592 Email: [EMAIL PROTECTED] Web: http://www.nwrc.usgs.gov/ Malcolm McCallum <[EMAIL PROTECTED]> Sent by: "Ecological Society of America: grants, jobs, news" <ECOLOG-L@LISTSERV.UMD.EDU> 03/27/2006 04:01 PM Please respond to Malcolm McCallum <[EMAIL PROTECTED]> To ECOLOG-L@LISTSERV.UMD.EDU cc Subject Re: GPS units and accuracy data The GeoXT is the most accurate and precise GPS on the market. As you = =3D are probably aware, when you record a data point using a GPS unit you = =3D can have varying degrees of accuracy and precision. Most of the =3D over-the-counter GPS are fairly simple to use, but also give fairly =3D unreliable data. They average all of the datapoints together. In order = =3D to have a true 95% precision you must record about 150 data points when = =3D you log your location, for a 99% precision you must use 180 data points. = =3D You cannot do this with Garman or Magellan. This data distribution =3D then allows you to manually correct your data and even remove =3D influential outliers for good reasons. Most Garmins and Magellans =3D automatically remove the outliers and do averaging so you have no idea = =3D what the real data looked like. It gets even worse if you are dealing = =3D with a 3D environment such as in the mountains. In other words, if need = =3D to get back within 3 ft of your location, you better use a Tremble =3D Geoexplorer XT or comparable GPS. If you only need a rough idea of =3D where you are, any will do. But a garmin can get you anywhere from 3ft = =3D away (on a perfect day with no canopy) to a football field away. You = =3D should talk to a rep about the differences and they should be able to = =3D explain it to you. Or talk to a GIS guru. When I was working on NPS =3D work, they required Tremble. =3D20 =3D20 Malcolm L. McCallum Assistant Professor Department of Biological Sciences Texas A&M University Texarkana 2600 Robison Rd. Texarkana, TX 75501 O: 1-903-233-3134 H: 1-903-791-3843 Homepage: https://www.eagle.tamut.edu/faculty/mmccallum/index.html =3D20 ________________________________ From: Ecological Society of America: grants, jobs, news on behalf of =3D Arvind Bhuta Sent: Mon 3/27/2006 2:41 PM To: ECOLOG-L@LISTSERV.UMD.EDU Subject: GPS units and accuracy data Forumites I wanted to find out more about GPS units, a topic many people have =3D asked about on this listserv in the past. What I am trying to figure out is = =3D the accuracy between an item like the Trimble Geo-Explorer XT compared to something more economically available like a Garmin 12 or Magellan =3D product?=3D20 Are people happy using simple GPS units like the Garmin 12 to collect = =3D data especially if you want to conduct geospatial analyses and if so how do = =3D you deal with spatial accuracy and post processing data if it can be = done.=3D20 Also, what is the spatial accuracy compared to sub-meter GPS units. I = =3D would assume that it couldn't but it might also be difficult to do. I know = =3D the Trimble GPS unit and Pathfinder Office makes this easier and you have = =3D better accuracy and precision, you even get a root mean square error with you = =3D data so that could be discussed. I appreciate any input and help from any one = =3D and look forward to hear what has to be said. You can reply to me and then = =3D I can summarize what was said to the listserv if need be. Thanks, Arvind Master's Student Department of Geography Virginia Tech http://www.geography.vt.edu/people/Grad%20Students/bhuta.htm
