Killet Software Ing.-GbR - short: KilletSoft - is a software company, which was established in the year 1991. The society is divided into the ranges "Geodetic Standard Software", "Development Tools for Geoinformatics" and "International Geodata". The focal trades of the companies supplied by KilletSoft are consulting engineers, GIS developers, Internet marketing, public utilities, logistics, telecommunications, security and public services.
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Quality Analysis of the NTv2 Coordinate Transformations with Geosoftware by KilletSoft
Practice oriented Accuracy Verification of the Software TRANSDAT and GeoDLL with
an extremely structured NTv2 Grid File

Kempen, May 22, 2017
KilletSoft - Fred Killet
 Download Press Release as MS WORD file

Logo Kempen, Rhineland (Germany) - The Coordinate Transformation Program TRANSDAT and the Geodetic Development Kit GeoDLL support the NTv2 standard for Coordinate Transformations with high accuracy. For this purpose TRANSDAT uses the Dynamic Link Library GeoDLL as a basis for all geodetic calculations. Already in 2010, Geosoftware by KilletSoft was tested for the accuracy in calculations with the then-current NTv2 grid file BeTA2007. The result of this analysis is documented on the website  http://www.killetsoft.de/t_1005_e.htm. Since then, the requirements on Geosoftware regarding the transformation of geodata of the real estate cadastre have increased. For this purpose almost all federal states provide highly accurate, close-meshed NTv2 files. The most recent examples are the cadastral-exact NTv2 solutions of the federal states Bavaria and Baden-Württemberg. Read more about it in my publication on  http://www.killetsoft.de/t_1704_e.htm.
NTv2-Logo For a current quality analysis and in collaboration with the engineering office KilletSoft the biggest German network operator Westnetz GmbH has again conducted comparative calculations with TRANSDAT / GeoDLL. The NTv2 file used for the quality analysis has been compiled by Westnetz for the conversion of all inventory data with spatial reference. The NTv2 file allows Coordinate Transformation with cadastral accuracy on the basis of official solutions in the area of the old German federal states (without Berlin). The NTv2 file contains 64 sub and sub-sub grids with over 7 million data lines in total. For testing purposes a coordinate file with a grid of 4,328,291 points for transformations from Gauß-Krueger/DHDN90 to UTM/ETRS89 has been generated. FME (Feature Manipulation Engine by the Canadian company Save Software) was used as reference software for comparative calculations and quality assurance measures.
Based on comparative calculations of 4.3 million coordinates with TRANSDAT / GeoDLL and the reference software, the quality could be tested in terms of the following criteria:

  1. Calculation accuracy over the entire surface
  2. Identity of forward and backward calculations
  3. Correctness in border areas of the NTv2 grid file
  4. Transformation performance with big data volume

Calculation accuracy
More than 4.3 million points were transformed with TRANSDAT / GeoDLL and the reference software from Gauß-Krueger/DHDN90 to UTM/ETRS89, and then compared afterwards. There were no deviations bigger than one millimeter. Such minor deviations (0-1mm) are tied to mathematic tolerances and internal rounding of the used software.
Identity of forward and backward calculations
For this test the coordinates were transformed from the source system Gauß-Krueger/DHDN90 into the target system UTM/ETRS89. Afterwards, the result was calculated back to Gauß-Krueger/DHDN90 and compared to the source coordinates. The NTv2 documentation shows that during back transformation it is not possible to find the same grid mesh for every coordinate in border areas of the grids. As expected it leads to a few minor deviations:

CategorieNumber
0-1 mm 4327562
2-3 mm 712
3-5 mm 2
5-7 mm 1
7-9 mm 1
9-10 mm 1
1-2 cm 3
2-3 cm 3
3-5 cm 3
5-6 cm 1
6-7 cm 2
Total: 4328291
Qualität-Logo
TRANSDAT Main Window
Main Window of TRANSDAT Coordinate Transformation

Main Window of TRANSDAT
Click to enlarge

If deviations less than three millimeters are explained by mathematic tolerances and internal rounding of the used software, then 17 calculations with a deviation between three millimeters and 6.4 centimeters remain. This equates to an extremely low rate of only 3.9 ppm or 0.0039 per mill.
Correctness in border areas
The tested NTv2 file contains 64 sub and sub-sub grids. In the western and southern border areas of the sub grids the calculation is quite complicated. This particularly applies when a coordinate lies exactly on a boundary line. Many of the 4.3 million calculated points have been intentionally placed into the border areas. Compared to the reference software there were no remarkable deviations.
In this context the trouble ticket number 209 of the Open Source Library Proj.4 is pointed out. It says that the NTv2 module in Proj.4 does not transform correctly in a minimal area around the edges of the coordinate grids due to a bug when the NTv2 file contains several sub grids. It states it might apply to a lot of programs which contain Proj.4 for geodetic calculations. But GeoDLL and TRANSDAT by KilletSoft use own algorithms for NTv2 calculations and thus they are not affected by the above mentioned error!
GeoDLL and TRANSDAT are the only software on the market which allows the consideration of polygonal validity scopes in NTv2 grid files. Coordinates located outside of a polygon can be excluded from the calculation. For example, the polygon can be the boundary of an investigated area or administrative area. In the publication "Proposal for using Polygonal Validity Scopes in Ntv2 grid files" on  http://www.killetsoft.de/t_1512_e.htm you can read how a NTv2 file is prepared for this purpose.
Transformation performance
The project management of Westnetz has conducted an extensive documentation as part of the quality analysis. Among others, Westnetz stressed the excellent calculating speed of TRANSDAT / GeoDLL. Despite a volume of 4.3 million test data and the usage of the very large NTv2 file of Westnetz, the software worked with high performance and very precisely. Dynamic Link Library GeoDLL used by TRANSDAT for geodetic calculations is very fast due to optimized C Programming.
Result of the quality analysis
The good result in all areas of the quality analysis is not surprising because only strict formulas are used for geodetic calculations. The Coordinate Transformations are conducted based on publications by Maarten Hooijberg (1997 and 2008). The NTv2 geodetic datum shift is carried out based on the publication by D. R. Junkins and S. A. Farley (1995), "NTv2 - National Transformation Version 2 - User’s Guide".
Are you interested?
You can find detailed information about the Coordinate Transformation Program TRANSDAT and the Geodetic Development Kit GeoDLL on the website  http://www.killetsoft.de and you can download free trial versions there. The website also shows a worldwide compilation of NTv2 files that are freely available and can be downloaded directly there. Worldwide all NTv2 grid files and thousands of Coordinate Systems and Datum Shifts are supported by TRANSDAT and GeoDLL.
If you want detailed information about the NTv2 file used by Westnetz for cadastral-exact Coordinate Transformations from Gauß-Krueger/DHDN90 to UTM/ETRS89, please contact the project management of Westnetz GmbH via email  gk2utm@westnetz.de.

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