[BC] GPS recommendations

[RichardBJohnson at comcast.net]

GPS and datums????

It is common to presume that GPS position cannot be accurately known without an additional reference signal, perhaps from CORS or NDGPS (http://www.gps.gov/systems/augmentations/)

The government ...

http://www.gps.gov/systems/gps/performance/accuracy/

...says that some receivers provide better than the "theoretical" 7.8 meter accuracy. How is this possible? No, it is not done with smoke and mirrors. If your receiver is not moving like an airplane moves, one can obtain the GPS position over-and-over again and average the result.

This produces a corresponding smaller and smaller CEP (http://en.wikipedia.org/wiki/Circular_error_probable) 

This improves accuracy if the errors are random so that they simulate noise. When you average noisy data, the signal embedded within the noise sums each time you add it to the residual. However, the noise increases as the square-root. Therefore, if you sit still in one place long enough, the GPS receiver will give you an excellent idea of where you are, in fact much better than geological quadrangle data which often seems to have been drawn by someone who never left the office.

Many surveyors trying to survey property find that even the townships are not located where the geological quadrangle data says they are. It seems that only property near railroads has been accurately surveyed and all the rest is a guess off from some benchmark that is misplaced or doesn't exist anymore. Therefore, when checking out GPS surveyor's instruments, you cannot rely on benchmarks as indicators of accuracy. Fortunately, most surveyor's work entails trying to find original land boundaries rather than absolute positions. That's why a plot-plan may read something like:

"From farmer John's barn, proceed south-by-south-west three hundred rods, more or less to the bridge over Benson Pond creek, thence to the painted rock...."

Unfortunately, the FCC wants to know where the transmitter sites are, using the same references for all. Farmer John's barn burned down in 1948 and was replaced by a high-rise condominum near the airport.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Tim Sawyer" <tzsawyer at tzsawyer.com>

Such a pain in the ass isn't it? Only the FCC mass media bureau uses NAD 27
and only for antenna locations in the CONUS, outside the CONUS at one time
you were suppose to use the local datum i.e., Guam, Saipan, Puerto Rico and
Hawaii which was another mess in and of itself - those areas have all
adopted NAD83 as the standard datum now - so for those areas you use NAD83.
So in effect the mass media bureau database has a mixture of both NAD27 and
NAD83 coordinates - "they" just don't want to talk about it or give you the
option to specify the datum on the forms.