RE: Accuracy of GPS with respect to surveys

["Clement, Christopher" <clementc@aecl.ca>]

I agree that for a one-off survey it is probably not cost effective to
build you own integrated GPS / detection system.  However, another
option is to talk with someone who has already set up and used an
integrated GPS / detection system.  You might find it very cost
effective to have someone else do much of the fieldwork.  We have done
many of these types of surveys, and although we are not a commercial
operation (we work for the Canadian federal government), our office has
also done these types of surveys on a cost-recovery basis for others in
the past.  There are also several other companies out there that do this
type of work on a commercial basis.

I understand that there are also some instruments out there that will
accept GPS signals directly through an RS-232 port, saving both location
data and radiation reading to memory at the same time at the press of a
button.  Although this would be great for a gridded survey, we have
found that you can get much more data if you continuously log location
and radiation data.

------------------------------------------------------------
Christopher H. Clement
Scientific Specialist / Radiation Protection Program Manager
Low-Level Radioactive Waste Management Office
Atomic Energy of Canada Limited
clementc@aecl.ca


> ----------
> From: 	KENNETH POHL[SMTP:POHL.KENNETH@epamail.epa.gov]
> Sent: 	May 1, 1998 12:09 PM
> To: 	Multiple recipients of list
> Subject: 	RE: Accuracy of GPS with respect to surveys
> 
> 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
> ----------------------------------------------------------------------
> 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
>