Geodetic tools

Geodetic tools

• ILWIS add-on: Inverse Molodensky
• ILWIS add-on: Datum Transformation
• ArcMap add-on: Datum Transformation

The ILWIS add-ons are formally not part of the ILWIS package;

questions on these tools cannot be handled by ILWIS support.

Inverse Molodensky (invmol.exe, 326 KB)

This is a tool that enables the user to find Molodensky Datum shifts (D X, D Y, D Z) between the global WGS84 reference and a local geodetic reference.

The user has to provide the following input:

1. The latitude, longitude and ellipsoidal height of 3 points in the WGS84 system. The latlons must be expressed in arcseconds, and the heights in meters above the ellipsoid.
2. The parameters a, and 1/f of the ellipsoid used in the Local reference system (semi-major axis and inverse flattening).
3. The latitude, longitude and ellipsoidal height of 3 corresponding points in the Local system (in arcseconds and meters resp.).

The Molodensky shifts are then calculated in meters (with cm precision).

Remark: The program uses the average of the three point-coordinates in either system. It works fine also if one has only one or two control points available. In those cases one has simply to repeat the same coordinates in the other (two) boxes, because the actual program expects 3 control points and hence all input fields being used.

Datum Transformation (datrans.exe, 344 KB)

This is a tool that enables the user to apply a full Datum transformation between 2 geodetic reference systems (input, output).

• One of the systems can be the global WGS84 reference for instance, and the other can be any Local Reference System defined on any user-defined ellipsoid.
• The usual reference ellipsoids (Bessel, Clarke, Hayford (International), Airy, Everest etc.) are also provided.
• The transformation method uses 7 parameters (and optionally the 3 geocentric coordinates of a local centroid or pivot point).
• The method is known as Bursa-Wolf, Molodensky-Badekas (Leica) or Position-Vector (Esri) transformation.

The user has to provide the following input:

1. The 3 shift components of the position vector (in meters).
2. The 3 rotations of the coordinate frame about the respective axes (in 10e-9 radians, 'nano-radians').
3. The difference of scale (in 10e-9 m per meter).
4. Optionally the geocentric coordinates (with respect to the axes of the input ellipsoid) of a local centroid (origin of Reference System 1).
5. The ellipsoid parameters a and 1/f of the input system (provided in a combo box for many usual ellipsoids).
6. The ellipsoid parameters a and 1/f of the output system (provided for many usual ellipsoids).
7. An input (terrain-) point P, given in latitude, longitude (decimal degrees) and height (m above the input ellipsoid).

The computed output consists of:

1. Geocentric coordinates of P in the input system (Reference System 1).
2. Geocentric coordinates of P in the output system (Reference System 2).
3. Ellipsoidal latitude, longitude and height of P in the output system (again in decimal degrees and m respectively).

Datum Transformation in ArcMap (ESRI) (VBA source code, 14 KB)

In Esri 's ArcGis environment one can program VBA code to customize Spatial Reference possibilities. In ArcMap there is no tool to find or compute Datum transformation parameters from measured or observed control data.

Below you can find VBA code to implement this as an interactive dialogue that can be started from a button control.

• In the tool, two pointfiles are expected (ESRI shape format) with 3D geographic coordinates (lat, lon, height), each of them defined on a different but known spheroid.
• Common points (pertaining to identical locations) are used to calculate the Datum transformation parameters. The user can select either of the two control point layers; the selected one will serve as the 'source map' whose correction to the spatial reference of the other layer will be calculated. Optionally this transformation is carried out in the current data frame.
• Finally, an option is offerd to store the transformation parameters in a custom geogtran file for later use.