David Thomson wrote: > 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...
Yes, I wrote that we often use handheld units (read that as less that $375.00 each) for marking and relocating points and plots in the field. These inexpensive units are indeed pathetic when it comes to accuracy, but they are also the best thing since sliced bread when it comes to being a very handy and useful tool. For example we have water level loggers in the middle of cedar swamps along the Mississippi coast that we have to download periodically. After Hurricane Katrina it is very hard to find what used to be a path to the logger. My pathetic little handheld that offers "any position that it acquires" led me directly to the correct point (and back out, I might add). Although that logger is no longer under a canopy, it was when we originally marked the point. We have 54 study plots in the middle of nowhere on Felsenthal National Wildlife Refuge. They are sometimes an hour ATV ride from where we have to leave the truck. That same, inexpensive handheld unit gets us back to those plots, under a full canopy, time after time. The tracks feature of the handhelds is wonderful for helping us stay on the correct route. When we get to where we are going, the unit may say that we are 4 to 7 meters from the plot, but the plot markers are right there, easy to see. For my research, sub-meter accuracy is of no added value. When I was a combat engineer in the US Army we had a saying that we measure a cut with a micrometer, mark it with a crayon and cut it with a chain saw. Sometimes the accuracy of the micrometer is just not worth much. Bottom line is that if you need to know a particular location with extreme certainty, then pay the $1175 per month or $10,000 or $100,000 or whatever. If you are doing a spatial distribution analysis on some critters in relation to specific vegetation, then perhaps you need the accuracy. If you are measuring a parcel of land for a legal description you probably need accuracy. I personally don't believe that very many people in ecology really need sub-meter accuracy. If you are studying trees (like I do) or birds or amphibians you probably need to relocate a plot, not a particular, very small point on the ground. Spend much less on a handheld GPS and much more on other aspects of your study (like $5 for some PVC pipe to mark the plot so that you can see it from a few meters away). Consider what your needs really are and then decide what GPS instrument you need. Don't be fooled into buying much more capability than is really needed. BobK Bobby D. Keeland, Ph.D. Research Forest Ecologist 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/ The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts - Bertrand Russell
