[postgis-devel] Towgs84 parameters in RD (srid 28992)

[Frank Steggink]

Hi,

I'm not sure if this qualifies as a bug, or if it my knowledge which is 
lacking. I'd like to have some clarification, before I eventually file a 
bug report.

I have the impression that the towgs84 parameters of the projection used 
in the Netherlands (Rijksdriehoeksstelsel, RD for short, EPSG:28992) 
contain an error.
This is from the current trunk version of spatial_ref_sys.sql:
INSERT INTO "spatial_ref_sys" 
("srid","auth_name","auth_srid","srtext","proj4text") VALUES 
(28992,'EPSG',28992,'PROJCS["Amersfoort / RD 
New",GEOGCS["Amersfoort",DATUM["Amersfoort",SPHEROID["Bessel 
1841",6377397.155,299.1528128,AUTHORITY["EPSG","7004"]],TOWGS84[565.4171,50.3319,465.5524,-0.398957,0.343988,-1.87740,4.0725],AUTHORITY["EPSG","6289"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4289"]],PROJECTION["Oblique_Stereographic"],PARAMETER["latitude_of_origin",52.15616055555555],PARAMETER["central_meridian",5.38763888888889],PARAMETER["scale_factor",0.9999079],PARAMETER["false_easting",155000],PARAMETER["false_northing",463000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["X",EAST],AXIS["Y",NORTH],AUTHORITY["EPSG","28992"]]','+proj=sterea 
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079 
+x_0=155000 +y_0=463000 +ellps=bessel 
+towgs84=565.4171,50.3319,465.5524,-0.398957,0.343988,-1.87740,4.0725 
+units=m +no_defs ');

The three rotation parameters are: -0.398957,0.343988,-1.87740
I think the sign must be flipped.

Here are the rotation parameters from RDNAPTRANS, the official source 
code for the RD projection:

    Transformation parameters from RD(Bessel) to ETRS89
    Pivot point: center of the ellipsoid
    alfa    1,9342     *10^-6 rad
    beta    -1,6677     *10^-6 rad
    gamma    9,1019     *10^-6 rad
    delta    4,0725     *10^-6
    tx    565,4171     m
    ty    50,3319     m
    tz    465,5524     m


Note that the units of alfa/beta/gamma are in millionths of radians, and 
not in arc seconds. When you convert the values, you'll get the same 
values, except for the sign. Note that the sign of the other parameters 
(delta, tx, ty, tz) are correct. Also note that these parameters are to 
ETRS89 and not WGS84. Proj.4's datum_shift.csv file [1] contains the 
values I'm expecting (see line 561): 
565.4171,50.3319,465.5524,0.398957,-0.343988,1.87740,4.0725.

My question, does PostGIS expect "negative" values for the three 
rotation parameters? I've also compared the values with EPSG:31466 
(DHDN, Gauss Kruger, zone 2). When I compare the TOWGS84 rotation 
parameters with Proj.4, the signs are the same.

Regards,

Frank Steggink


[1] 
https://trac.osgeo.org/geotiff/browser/trunk/libgeotiff/csv/datum_shift.csv

-- 
Steggink Geo-ICT
Frank Steggink
Smaragdplein 61
3523 ED  Utrecht
The Netherlands
+31 6 53 10 13 66
www.steggink.it
frank at steggink.it
KVK: 63767066

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