RE: Accuracy of GPS with respect to surveys

[KENNETH POHL <POHL.KENNETH@epamail.epa.gov>]

There are a large number of options for using GPS in surveys.  Each has its own
accuracy, hassle, and cost.

Assuming 

a) you don't already own a $20k survey grade receiver, don't intend to buy one,
and don't plan to sit at each location for the >~2 minutes required, anyway, and


b) you fall within range of a Coast Guard / Army Corp of Engineers beacon (see
the Coast Guard web site (http://www.navcen.uscg.mil/dgps/coverage/default.htm)

then you can use a no-frills GPS receiver/beacon receiver combination.  The cost
would be around $1500 for the combination.  The beacon receivers generally
aren't handheld, but can easily be put in a backpack.  

If (a) is true, but (b) isn't, you might be OK if you're near a major
metropolitan area.  Look for FM subcarrier DGPS services.  These piggyback the
correction signal on commercial FM broadcasts.  You would still need to buy a
GPS receiver, along with a FM subcarrier receiver (there's one for sale which is
built into a pager) and a subscription to the subcarrier service.  Expect to pay
about $1200, with about $250 for each additional year of subscription.

There are vendors you can find on the WWW which sell all these components. 
There are other equipment systems that will also work, often for > $10k. The
accuracy of either system described here will really be around 5-10 meters RMS
(WARNING: Take ALL accuracy statements with a grain of salt.  Especially those
from vendors or from people offering free advice.)  Both systems will give you
"real-time" results at a slow walk.  The gotcha is this: What are you gonna do
with the data?  It's showing on the GPS at one reading per second, but how will
you correlate the position with the meter reading?  You can get GPS's which have
RS-232 ports, so you can read out the position electronically, and you can buy
or make a meter system that does the same.  Throw in a palmtop with custom
software and a mapping program.....

Basically, my advice is to just grid the area as originally suggested.  You'll
end up spending > $1000 and hours (weeks?) of extra time to do it the fancy way,
and unless you're planning to do these surveys often, it's not going to be worth
it. 

These opinions are mine alone, of course.  The mention of any equipment is not
an endorsement thereof.


Kenneth R. Pohl
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US EPA/Natnl. Air & Rad. Env. Lab.        pohl.kenneth@epamail.epa.gov
540 S. Morris Ave                                 vox:  (334) 270-3475
Montgomery AL 36116                               fax:  (334) 270-3454