I agree with Dave Thomson's comments (below) and will add a few of my own.
We have 3 Trimble Pro XR/Asset Surveyor units that we have used under a variety of circumstances. In general, they are very finicky about multipath reflections (the common problem with working under a canopy or near large structures). However, the quality of data you get from the Pro XR is user-selectable to a large degree. You can tell it to accept fewer satellites, lower the horizon acceptable, etc. But by doing that, you defeat the whole purpose of using that kind of unit. For when you decrease the various parameters in order to accept lower quality information, your positional accuracy will degrade, also. The system is water resistant, not waterproof and it can be slow attaining lock even under good circumstances. We have chosen to use the Trimble datalogger and it works for our purposes. We have not used their latest datalogger which I understand has a color display and such. Also, we have used a device running ESRI's ArcPad. I haven't done that and so can't speak to it's utility. Another factor that may play into the decision process that nobody has mentioned (which is probably and indication of its importance), is elevation. The Pro XR will give better than 2 m accuracy in elevation pretty consistently. For establishing points we usually observe a location for 30 seconds to a minute or so. 30 sec is usually sufficient for submeter performance. For relocating points we use the correction signal. that combination has consistently allowed relocation of pin flags in dense marsh vegetation--my gold standard for quadrat resampling. If you're insistent on submeter accuracy then do some planning in advance. Trimble (for example) has software that will predict optimal time for observing the constellation signal, given the parameters that you set. Doing this in advance will prevent a lot of time wasted waiting for another satellite to pop over the horizon. Also, nobody has mentioned proprietary signals for real-time correction. There are some few companies that offer subscription correction which can be invaluable in areas not covered by the USCG signal. Racal (I believe) offers that service. I have heard others do also. I have never used any of these services so cannot speak to their quality or availability or price. A caution about precision and accuracy. Some units will report a fantastic number of digits of precision but the accuracy is not equal to the precision. It's a good idea to take your unit to a benchmark and observe the benchmark to compare the unit's output with the benchmark's actual location. In the USA the National Geodetic Survey (NGS) maintains a database that can be queried for their benchmarks. I think the British Isles have a similar system maintained by Ordnance. Probably other countries offer that service, too. In closing, I caution new users not to overspend. Most people don't really need to have submeter accuracy and all the attendant problems and expenses with attaining it. If all you're doing is establishing or relocating a large plot in a forest or trying to keep yourself oriented in the field, then a cheap unit will serve you much better and you wont feel so bad when it drops off the tailgate or falls out of the boat. ---chris Christopher J Wells, Geographer National Wetlands Research Center, USGS 700 Cajundome Blvd Lafayette, LA 70506 337 266 8651 [EMAIL PROTECTED] Dave Thomson <[EMAIL PROTECTED]> Sent by: "Ecological Society of America: grants, jobs, news" <ECOLOG-L@LISTSERV.UMD.EDU> 03/30/2006 09:35 AM Please respond to Dave Thomson <[EMAIL PROTECTED]> To ECOLOG-L@LISTSERV.UMD.EDU cc Subject Re: GPS units and accuracy data I would agree that the GeoXT is not a high caliber GPS unit. I was dissuaded from using them by the very helpful techs at GeoPlane Services: http://www.geoplane.com/trimble/pathfinderproxrs.html I have no experience with Leica products, but I wouldn't imagine a handheld unit could rival the data quality of a backpack unit. I have experience with the Trimble TSC1 Asset Surveyor/Pathfinder Pro XR combination (dGPS) but was looking for the ability to utilize orthorectified aerial photography in the field during data collection. I thought that the GeoXT might fit the bill; I was told, unfortunately, that the GeoXT's resolution was poor, and in particular the Z-axis was very poor. Instead, he recommended renting one of their Pathfinder Pro XRS backpack dGPS units, which they can pair with an Allegro CX field PC which takes the place of the Trimble data collector. As you will see in the link below the Allegro can hold up to a gigabite of data and display color images very well. http://www.geoplane.com/junipersystems/allegrocx.html Of course not everyone can afford a Pathfinder Pro XR/S and an Allegro CX. To be honest, I am not even sure how much they would run you currently. I did see used Pathfinder Pro XR backpack units sell on eBay in the $5-10,000 range! The big questions to ask yourself are: 1) do you need this level of precision in your data, and 2) do you need to own one of these setups? If the answers to the above are yes, and no, respectively, then consider renting the units from a business such as GeoPlane. The last quote I had from them for that setup was $1175 a month. The techs at GeoPlane can help you decide what you require without the pressure of a sales department. Hope that helps, David Thomson PS - I noted that one person discussed the ability to hold a lock on satellites underneath a tree canopy with a GPS unit (handheld?). This is not common. In fact I was surprised that someone had been 'offered' data by their GPS unit in such a situation. Perhaps this is because (I believe) most handheld units do not have good data quality assurance functions and will offer any position they 'acquire'. User beware... High quality units will not even offer the user any data in such situations. The Pathfinder Pro XR/TSC1 combination that I have used in the past was only able to acquire a 3D lock in the near absence of canopy; certainly the gap in the canopy had to be quite large (i.e. expose sufficient sky to adequately 'see' enough satellites). In some situations where I needed to acquire whatever data I could, I would alter the settings to acquire the less accurate 2D data. Later, while using the data, I was surprised at it's obvious innacuracies. The techs at GeoPlane did note that they have range poles can cabling that they offer with their backpack units, which allow the user to raise the antenna (what was it?) 10 feet (meters?) - sorry it's been awhile. They said that this can help one acquire a signal in and around tree canopies, but (of course) it is no guarantee. -----Original Message----- From: Ecological Society of America: grants, jobs, news [mailto:[EMAIL PROTECTED] On Behalf Of Malcolm McCallum Sent: Tuesday, March 28, 2006 7:22 AM To: ECOLOG-L@LISTSERV.UMD.EDU Subject: Re: GPS units and accuracy data I have not used a Leica GS20, how does it compare to the GeoXT? also,=20 =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: Ecological Society of America: grants, jobs, news on behalf of = Marlow G. Pellatt Sent: Mon 3/27/2006 11:26 PM To: ECOLOG-L@LISTSERV.UMD.EDU Subject: Re: GPS units and accuracy data There are several GPS units with as good or better precision than the = GeoXT. You will need to decide what Basic mapping grade handheld units include = the Leica GS20, Trimble Pathfinder series, and several other survey and = mapping level GPS units. The Trimble GeoXT is a good handheld unit with decent accuracy but is not the most accurate or precise GPS on the market. Geodetic and Survey grade GPS units are highly accurate but expensive. = I have used several Garmins, Trimbles, CMT, and Leicas. I have been happy with with Leica GS20 and Trimble GeoXT for collecting spatial data for = most ecological applications. Take a look at how the software will interface with your GIS and weather = you need to look at maps and air photos etc of you site in the field. Here are some websites that may help you. http://www.digitalgrove.net/GPS.htm http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap4/chap4.htm Cheers, Marlow On Mon, 27 Mar 2006 14:41:46 -0600 [EMAIL PROTECTED] wrote: > Forumites > > I wanted to find out more about GPS units, a topic many people have = asked > about on this listserv in the past. What I am trying to figure out is = the > accuracy between an item like the Trimble Geo-Explorer XT compared to > something more economically available like a Garmin 12 or Magellan > product? =20 > Are people happy using simple GPS units like the Garmin 12 to collect = data > especially if you want to conduct geospatial analyses and if so how do = you > deal with spatial accuracy and post processing data if it can be done. = =20 > Also, what is the spatial accuracy compared to sub-meter GPS units. I > would > assume that it couldn't but it might also be difficult to do. I know = the > Trimble GPS unit and Pathfinder Office makes this easier and you have > better > accuracy and precision, you even get a root mean square error with you > data > so that could be discussed. I appreciate any input and help from any > one and > look forward to hear what has to be said. You can reply to me and = then 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 > Marlow G. Pellatt, Ph.D. Coastal Ecologist Parks Canada, Western Canada Service Centre 300 - 300 West Georgia Street Vancouver, British Columbia V6B 6B4 Canada, and School of Resource and Environmental Management Simon Fraser University Burnaby, British Columbia V5A 1S6 Canada Phone: (604) 666-2556 Email: [EMAIL PROTECTED]
