[PROJ] Helmert transform

[Even Rouault]

On mercredi 20 mars 2019 15:31:14 CET Dean Howell wrote:
> Hello all,
> 
> We have a project that is using the helmert transformation and we are
> trying to get an understanding of what level of residuals is acceptable and
> how we interpret the results.
> 
> If anyone has experience in using the helmert transformation, I would be
> interested in your feedback.

Dean,

if you use Helmert transformations from the EPSG dataset, they are accompanied 
with their accuracy.

For example, if you use latest PROJ 6,

$ projinfo -s EPSG:4214 -t EPSG:4326 --spatial-test intersects
Candidate operations found: 7
-------------------------------------
Operation n°1:

EPSG:15921, Beijing 1954 to WGS 84 (4), 1.0 m, China - Tarim - 77.5°E to 88°E 
and 37°N to 42°N

PROJ string:
+proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert 
+xy_in=deg +xy_out=rad +step +proj=push +v_3 +step +proj=cart +ellps=krass 
+step +proj=helmert +x=15.8 +y=-154.4 +z=-82.3 +step +inv +proj=cart 
+ellps=WGS84 +step +proj=pop +v_3 +step +proj=unitconvert +xy_in=rad 
+xy_out=deg +step +proj=axisswap +order=2,1

WKT2_2018 string:
COORDINATEOPERATION["Beijing 1954 to WGS 84 (4)",
    SOURCECRS[
        GEOGCRS["Beijing 1954",
            DATUM["Beijing 1954",
                ELLIPSOID["Krassowsky 1940",6378245,298.3,
                    LENGTHUNIT["metre",1]]],
            PRIMEM["Greenwich",0,
                ANGLEUNIT["degree",0.0174532925199433]],
            CS[ellipsoidal,2],
                AXIS["geodetic latitude (Lat)",north,
                    ORDER[1],
                    ANGLEUNIT["degree",0.0174532925199433]],
                AXIS["geodetic longitude (Lon)",east,
                    ORDER[2],
                    ANGLEUNIT["degree",0.0174532925199433]]]],
    TARGETCRS[
        GEOGCRS["WGS 84",
            DATUM["World Geodetic System 1984",
                ELLIPSOID["WGS 84",6378137,298.257223563,
                    LENGTHUNIT["metre",1]]],
            PRIMEM["Greenwich",0,
                ANGLEUNIT["degree",0.0174532925199433]],
            CS[ellipsoidal,2],
                AXIS["geodetic latitude (Lat)",north,
                    ORDER[1],
                    ANGLEUNIT["degree",0.0174532925199433]],
                AXIS["geodetic longitude (Lon)",east,
                    ORDER[2],
                    ANGLEUNIT["degree",0.0174532925199433]]]],
    METHOD["Geocentric translations (geog2D domain)",
        ID["EPSG",9603]],
    PARAMETER["X-axis translation",15.8,
        LENGTHUNIT["metre",1],
        ID["EPSG",8605]],
    PARAMETER["Y-axis translation",-154.4,
        LENGTHUNIT["metre",1],
        ID["EPSG",8606]],
    PARAMETER["Z-axis translation",-82.3,
        LENGTHUNIT["metre",1],
        ID["EPSG",8607]],
    OPERATIONACCURACY[1.0],
    USAGE[
        SCOPE["unknown"],
        AREA["China - Tarim - 77.5°E to 88°E and 37°N to 42°N"],
        BBOX[37,77.45,41.99,88]],
    ID["EPSG",15921]]

[...]

Even

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com