Create ntv2 (National Transformation Version 2) grid shift file using Helmert transform from identical point pairs.
The difference between the coordinates of the grid point in both reference systems is entered as a shift value in the NTv2 file. The calculated shift values include the compensation of regional and local tensions and they consider the different dimensions and bearings of the ellipsoids used in both reference systems. Accurate NTv2 file is generated using small-scale three-dimensional 7-parameter Helmert transformations around each grid point and subsequent Natural Neighbour Interpolation for residual gap distribution.

Format:
https://github.com/Esri/ntv2-file-routines/blob/master/README.md#gsa-file-syntax

Option to calculate valid 7 parameter +towgs84 value that could be used with proj.4/WKT

we have now opened Pandora's box. Should we support other projections, too?

https://en.wikipedia.org/wiki/List_of_map_projections

❓ Release?

🔗 See https://github.com/Geo-Linux-Calculations/helmparms3d/releases

Maybe add a link:

• https://github.com/Geo-Linux-Calculations/gnumeric-ols/blob/master/spreadsheets/conformaltrans/helmert3d.gnumeric

• xyz_src_infilename, xyz_dest_infilename - required
• [parms_outfilename] - optional

• xyz_src_infilename, param_infilename - required
• [xyz_transformed_outfilename] - optional

• {blh|xyz}, src_infilename, ellipsoid_infilename - required
• [outfilename] - optional

• xyz_dest_infilename, xyz_transformed_infilename - required
• [xyz_diff_outfilename] - optional

Hi @dr-ni .

At the moment there is only one ellipsoid:

GRS80 6378137.0 6356752.3141

A list of commonly used ellipsoids known to me:

BESSEL 6377397.155 6356078.962822
GRS80 6378137.0 6356752.3141
GSK2011 6378136.5 6356751.758
KRASOVSKIY 6378245.0 6356863.018773
PZ90 6378136.0 6356752.3142
WGS84 6378137.0 6356752.314245

Add those that you see fit (and wherever you see fit) to this repo.

PS: Should we change the form of a b to the more common a 1/f, where b = a * (1 - f)? See:

• https://en.wikipedia.org/wiki/Bessel_ellipsoid
• https://en.wikipedia.org/wiki/SK-42_reference_system
• https://en.wikipedia.org/wiki/PZ-90
• https://en.wikipedia.org/wiki/World_Geodetic_System#WGS84

Hi @dr-ni .

See public params:

• https://github.com/OSGeo/PROJ/blob/master/data/ITRF2000
• https://github.com/OSGeo/PROJ/blob/master/data/ITRF2008
• https://github.com/OSGeo/PROJ/blob/master/data/ITRF2014
• https://github.com/OSGeo/PROJ/blob/master/src/ellps.cpp
• https://github.com/OSGeo/PROJ/blob/master/src/datums.cpp
• https://github.com/locationtech/proj4j/blob/master/src/main/resources/proj4/ellipsoid.csv
• https://github.com/locationtech/proj4j/blob/master/src/main/resources/proj4/gcs.csv
• https://github.com/OSGeo/gdal/blob/master/data/gt_ellips.csv
• https://github.com/OSGeo/gdal/blob/master/data/gt_datum.csv
• https://github.com/OSGeo/gdal/blob/master/data/pci_ellips.txt
• https://github.com/ofmooseandmen/jord/blob/master/resources/transformations.txt

Report residuals in output

Residuals:
1.9319 Maximum distance
0.9056 Standard deviation
0.5823 Root Mean Square residual (RMS)