This is useful when local control or benchmarks are not available, as it makes it possible to work "on the ground" rather than on the ellipsoid.The Bernese GNSS Software is a scientific, high-precision, multi-GNSS data processing software developed at the Astronomical Institute of the University of Bern ( AIUB). The benefit using the geoid model for the vertical adjustment is that you can display elevations without having to calibrate on elevation benchmarks. When you select geoid model as the vertical adjustment type, the software takes the geoid‑ellipsoid separations from the geoid file chosen, and uses them to display elevations on the screen. This is illustrated in the following diagram:įor correct results, the ellipsoid height (H) must be based on the WGS‑84 ellipsoid. The elevation (h) of the point above mean sea level (the geoid) is the result. The geoid‑ellipsoid separation value (N) is obtained from the geoid model and is subtracted from the ellipsoid height (H) for a particular point. A geoid model or Geoid Grid file (*.ggf) is a table of geoid‑ellipsoid separations that is used with the GNSS ellipsoid height observations to provide an estimate of elevation. The geoid is a surface of constant gravitational potential that approximates mean sea level. If required you can then perform a site calibration to adjust the geoid model by a constant value. Trimble recommends that you use a geoid model to obtain more accurate orthometric heights from your GNSS measurements than from the ellipsoid.
The Department of Natural Resources Canada (NRCan) makes no guarantees, representations, or warranties respecting the data. If you use the Canadian NTv2 datum grid please note the data is provided on an "as is" basis. When you export a datum grid using Trimble Business Center, the two datum grid files associated with the current project are combined into a single file for use in the Trimble Access software. Two gridded datum files are required for this interpolation – a latitude datum grid file and a longitude datum grid file. The accuracy of a datum grid depends on the accuracy of the gridded data set it uses.Ī datum grid transformation uses interpolative methods to estimate the value of the datum transformation at any point in the area covered by the datum grid files. By interpolation, it provides an estimated value for a datum transformation at any point on that grid. It applies translations and rotations in X, Y, and Z, as well as a scale factor.ĭatum grid – This uses a gridded data set of standard datum shifts. Seven‑parameter – This is the most complex transformation.State Plane 1983 coordinate systems use three‑parameter transformations. The three‑parameter transformation that Trimble Access uses is a Molodensky transformation, so there may also be a change in ellipsoid radius and flattening. The three‑parameter transformation involves three simple translations in X, Y, and Z. Three‑parameter – This assumes that the rotational axis of the local datum is parallel with the rotational axis of WGS‑84. Alternatively, you can choose not to use a transformation at all. Three types of datum transformation are commonly used. To survey in a local coordinate system, the WGS‑84 GNSS positions must first be transformed onto the local ellipsoid using a datum transformation. GNSS is based on the WGS‑84 ellipsoid, which is sized and positioned to best represent the entire earth. To conduct a real‑time survey in terms of local grid coordinates, define the datum transformation and map projection before starting the survey. If a horizontal adjustment is defined, it is applied next, followed by the vertical adjustment. The result is northing and easting coordinates on the local grid. Local geodetic coordinates are transformed into local grid coordinates using the map projection. When WGS‑84 coordinates are transformed onto the local ellipsoid using a datum transformation, local geodetic coordinates result.