[D21] GPSability of Geodetic Survey Marks

[KPolt@aol.com]

Dear D21 Members:

The following information from D/Lt Stephen Schmidt, AP has been posted to 
the D21  Website.  The PDF file posted on the Coop Charting page includes 
pictures that help illustrate the points that are explained below.  

You can access the file at 
http://www.usps.org/localusps/d21/CoopCharting/GPSableornot.pdf

Cdr Kay Polt, AP - LRGV
D21 Webmaster
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Determining GPSability of a Mark
  
When we determine a mark to be GPS’able, we are saying that it is useable for 
setting up a specialized antenna over it in order to collect data from the 
GPS satellites.  In order to collect the weakly transmitted data from the 
satellites, we have to have about 75% of the sky open for line of sight between the 
antenna and the satellites.

The antenna is a flat disc that is centered about five or six feet directly 
above the mark. The antenna is set on a tripod.  The mark itself can be below 
the ground or hidden from the sky it doesn't matter - the antenna is where the 
signal is intercepted and the position is calculated.  Of course we have to 
measure how high the antenna is above the mark to get a good elevation at the 
mark. 

If the sky is open, you pick up lots of satellites and the results are good.  
If there are a lot of obstructions, the results are poor and you have to 
spend hours on the mark.  If there is less than about 75% of the sky open to the 
antenna, it will probably not be possible to get enough data to use this mark 
as a reference for the survey of the project marks.  A consumer grade, handheld 
 GPS unit is no indicator of the GPSability of the mark since the typical 
$40,000 unit uses different criteria to obtain the extremely accurate results – 
about 1/100 of a foot.
 
Notice the picture at the Hartman bridge – the bridge is far away and blocks 
only about 25% of the sky - this is a marginally GPS’able point.  All the 
others have really good  view of the sky.  The one in the hole in the courthouse 
lawn was about 2 ft. below the surface but with the antenna up about 7 ft it 
would be no problem.  It should be reported as good for GPS.
 
Obviously, if the mark is within a few feet of a building or under a tree or 
set vertically on a wall it is not GPS’able.  A telephone pole within about a 
foot of the mark can block as much as 40% of the sky.  A chain link fence 
soaks up the signal and might as well be a brick wall.    

There is some guesswork in our reporting.  Just assume the antenna will be 
about 5 or 6 feet up and then imagine a ring around the antenna about 15 degrees 
above the horizon (you always set the GPS receiver to ignore anything below 
15 degrees because it is dirty data) and then if less than about 25% of the sky 
is blocked, it is good enough.  If you are not sure, mark the box on the NGS 
recovery site “not sure”.
 
The GPS’ability of a mark is important to the private or government surveyor 
in his mission planning – a typical static GPS survey involves lots of 
equipment and many employees collecting data over these marks in a well orchestrated 
and timed operation that can cost thousands of dollars. 
 
Power Squadron members should take very seriously the reporting of survey 
marks and whenever in doubt about the GPS’ability of a mark, always report it as 
an unknown. 
 
 D/Lt. Stephen R. Schmidt, AP, Dist 21 COOP Charting Chairman,  May 2006