Site Calibrations Print E-mail

Measurement to local control must be performed to ensure that control and coordinates are relative to your job site. A site calibration to the control of your choice yields flexibility and integration into the local grid coordinates. 

Traditionally survey activities were conducted entirely by survey crews where quality control - quality assurance was the sole duty of that survey crew. These procedures must not be over looked. The Can-Net correction system is a only a small shift in survey practice, that being, no base station set-up. Users should still check the quality of control and corrections every day.

The Can-Net corrections should be checked at the begin and end of your survey activities, in accordance to best management practices. Since the correction system is so productive, the time savings should be used to validate the correction system.  Most users will perform a thorough site calibration for a sizable region (town, city, quadrant) only once and copy that site calibration and control in to every subsequent job. New site cals can be created for any type of coordinate system (local site, municipal, provincial, or HPN). Daily use then dictates a simple check into control at the begin and end of survey. 

 

Getting to Ground (or Grid) with GPS: Site Calibrations - Fundamentals and Execution
To work with a space-based positioning system (GPS/GNSS) in a ground-based coordinate system (local grid coordinates), a precise mathematical transformation has to be created. Some of these transformation parameters are determined by a map projection, datum and geoid definitions. To work accurately in a grid coordinate system, an additional transformation needs to be applied. This accurate transformation is performed by a site calibration where ground control points are tied into GPS measurements.

This scenario is analogous with a traditional traverse. One control point gets you in the local coordinate system, ie gets you on site. Two points, with a coordinated backsite, allows the calculation of an inverse which produces an orientation to grid north and a local scale factor. The traverse can now progress through the job site, with side shots and topo shots. The traverse will conclude on some coordinated control. With two control points at the end of the traverse, a closure can be calculated and all points in between can be adjusted. Without the closure control there is no quality assessment of the survey. Also, the survey is usually tied into local elevations. The elevations are in the form of local benchmarks. This vertical control is brought onto the job site with a level loop or vertical traverse. Good vertical control ties around the job site bring in quality elevations to the survey. This is how a traditional survey is calibrated to the local grid control.

The same goes for the GPS site control. The survey needs to be tied to the local control, otherwise the quality of the survey is not maintained. With modern field survey software this procedure is simple and rigorous at the same time. GPS measurements are made to local control points and a few clicks later a site calibration (a rigorous least squares adjustment) brings the survey to the local grid. Quality control parameters are easily interpreted.

Site Calibration Summary
The GPS site calibration process establishes the relationship between WGS-84 data collected by GPS receivers and local control positions (expressed as a local map grid with elevations above sea level). This relationship is defined by a series of mathematical transformations. The GPS site calibration functionality allows you to pair up the GPS and local control points to be used in the calibration. (The GPS coordinates must be derived from GPS points and observations, and the grid points must be derived from grid points and terrestrial observations). Trimble field data collection software (SC, SP, TGO, TBO) then computes and applies the mathematical transformations using least squares.

The mathematical transformations that are applied in order to convert WGS-84 positions to grid coordinates are:

  • A datum transformation to convert the WGS-84 latitude, longitude, and ellipsoidal height coordinates to latitude, longitude, and ellipsoidal height coordinates relative to the ellipsoid of the local map grid.
  • A map projection to convert the local ellipsoid latitude and longitude coordinates into local map grid Northing and Easting coordinates (the height value is not altered in this process).
  • A geoid model to WGS-84 height to get approximate elevation above sea level.
  • A horizontal adjustment of the transformed grid coordinates to best fit local control data. This adjustment allows for any local variations in the projection system that cannot be catered for in the overall datum transformation.
  • A height adjustment to convert the heights above the local ellipsoid or elevations derived from the geoid to local control elevations above sea level.
  • The above horizontal and vertical adjustment are stored as part of the coordinate system definition for the project. All GPS points in the database are updated using the calibration parameters, resulting in more accurate local grid coordinate values.

sitecal-scheme

To save the new coordinate system definition (which includes the calibration parameters) as a site for use in future projects in the same area, click Save As Site from the Calibration dialog. If you save a calibration as a site with the intention of using the site in another project, make sure that the project area is fully enclosed by the points used in the calibration. If you use the key in Trimble Survey Controller to start a Real Time Kinematic (RTK) base, and transfer the Survey Controller (.dc) file to Trimble Geomatics Office, the base position (and therefore all rover points from the base) are of unknown quality (for all components, horizontal, height, and elevation.) If you are confident that the GPS site calibration has improved the quality of the points, use the Properties window to change the quality of the base point.

NOTE: By default, if you perform a GPS site calibration using an RTK base station point with WGS-84 coordinates, the grid coordinates for the point will be disabled. However, you can use the grid position for a GPS site calibration. If you choose to do this, you need to enable the grid coordinate (make sure it has a higher quality than the WGS-84 coordinate), explode points by data type, and then recalibrate. If you do not do this and GPS vectors do flow out from the grid position: any GPS point derived from the base cannot be used in the calibration an error, equivalent to the calibration residual error, may be introduced.

NOTE: If you have performed a GPS site calibration and select the Use ground coordinates check box in the Local Site Setting dialog, the calibration that you have defined in the project becomes invalid. An error message warns you that continuing with the operation will remove the calibration.

Geoid Models

If the coordinate system for the project defines the use of a Geoid model for point elevation determination then elevations are determined directly off the WGS-84 heights by interpolation on the Geoid model grid. However, it is still possible to apply a height adjustment on top of the elevations produced by a Geoid model to allow for small local variations that a large scale Geoid model cannot take into account. Elevations determined from a vertical calibration are given a survey quality.

Geoid Ellipsoid Earth Relationship