GPS Control Points how'd they get there

[SugarJones]

working on a roadway const job, surveying it with a total station but using control points that where placed by a gps crew. can anyone explain the process of what they did to place those control points that are needed for vert and hor control.

 

[IRstuff]

GPS, even with selective precision turned off results in around 10m CEP accuracy. In order to do surveying with GPS, you need to use differential GPS, where a location with known coordinates uses the GPS constellation to determine its location. The GPS location will be in error most of the time, but if the location already known, you can determine the error of the visible satellite constellation. This error correction is broadcast as part of a differential GPS signal that the surveyor's receiver uses to correct its error.

Since both the reference station and the surveying station must be seeing the same constellation for the correction to work, there could be more than one reference point used in a large area survey.

TTFN

 

[carlorom]

Quite right,

The danger lies in the matching of differential surveyed points from different reference points. Also whether there is an overlap of control points taken from different reference points/ different observation procedures / and possibly affected by ground reflections(multipath etc)

All these are basic errors much the same as errors from older methods (ie Trunion axis tilt in theodolite/total stations etc)

GPS is a marvelous tool and phenomenal accuracies can be obtained provided appropriate techniques and cross checks are taken. It would take too long to explain fully how the points are surveyed in detail. The basic advice is use your total station to check between control points supplied by GPS. If they check you can proceed with a bit more confidence / if not: query the point first / has it been disturbed or is it the right point / then query the actual survey data / and if necessary redo or insist on proof of checks.

Heights by GPS are usually less accurate than Y-X position.
Also check what datum the heights have been given to are they based on Mean Sea Level or Ellipsoidal heights directly off a GPS reference such as WGS84.

Another fundamental is to make sure that the initial reference points are in fact accurate - maybe they were done by unchecked GPS or some other method.

Hope this helps a bit.
Good luck