Locally optimized transformation parameters for Pulkovo-1942 datum
The Soviet topographical maps are a highly detailed source of information for many fields of research, engineering and natural resource management. In combination with accurate satellite imagery and GPS measurement highly valuable baseline information can be derived.
The topographical maps are based on the geodetic datum of Pulkovo-1942 and the ellipsoid of Krassowski-1940. For several reasons an adequate transformation parameter set between the WGS-1984 and the Pulkovo-1942 is not available for Tajikistan. The EPSG database v7.11 provides about 21 different versions of parameter sets. But unfortunately, none of them is accurate enough to combine the Soviet topographical maps in Tajikistan with satellite imagery and GPS measurements based on the WGS-1984 datum. Additionally, local distortions of the base coordinate frame caused by the triangulation methods adds errors to the consistency between the different data sources.
One possible solution to overcome these shortcomings is the calculation of locally optimized transformation parameters between the WGS-1984 and the Pulkovo-1942 datum. For several regions of Tajikistan, such optimized parameters were calculated based on the equation of the Standard Molodensky Transformation.
 | The Standard Molodensky Transformation is based on a shift of the coordinates along the 3 axis with the values dx, dy and dz. As input for the calculations to retrieve the three Molodensky parameters, several identical points were measured in ellipsoidal coordinates of the WGS-1984 and Krassowski-1940 ellipsoids. The three parameters were derived by using least squares adjustments. Depending on the quality of the input data, the 7-Parameters of the Coordinate Frame Rotation or Position Vector Transformation methods could also be provided. |
The following vector field shows the improvement between the standard geocentric translation applied in Tajikistan (left) and the locally optimized Molodensky parameter set (right). The control points were measured in high resolution orthorectified satellite imagery (WGS-1984) and topographical maps 1:50'000 (Pulkovo-1942).
| Existing Geocentric Translation EPSG 1254 | Locally optimized Molodensky parameter |
|---|---|
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The systematic shift of the coordinates caused by the EPSG 1254 transformation was fully removed. The average residuals of the control points were reduced from 53m to 16m. The fitting accuracy of the satellite imagery and GPS measurement was significantly increased.
In another region the vector field of the residuals is showing similar effects. The systematic shift of the coordinates in Western direction was largely removed and the average residual error was reduced from 85m to 21m. The significant difference of the vector directions between the first and the second example may result from local distortion of the base coordinate frame.
| Existing Geocentric Translation EPSG 1254 | Locally optimized Molodensky parameter |
|---|---|
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The method applied can be used in project regions where Soviet topographical maps and satellite imagery with GPS-based field observations shall be combined. It also can be used for the transformation of digitized data layers which are based on the Pulkovo-1942 datum into WGS-1984-based reference systems such as the new coordinate frame of Tajikistan.
 | The calculated transformation parameters can be applied in all standard GIS software packages such as ArcGIS, Quantum GIS or gvSIG. For ArcGIS users the transformation set can be stored as Custom Geographic Transformation file (*.gtf file) which can be distributed among the users of an organization. The *.gtf file can be directly used within the user interface of ArcMap and will be applied to all data sets in the data frame. |
The use of topographical maps at a scale of 1:25'000 or above could improve the quality of the calculation of the parameter sets considerably. GeoIdee.ch offers the calculation of optimized parameters as service, please feel free to contact.
