Towgs84 parameters in RD (srid 28992)

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Towgs84 parameters in RD (srid 28992)

Frank Steggink-2

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
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079
+x_0=155000 +y_0=463000 +ellps=bessel
+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):

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.


Frank Steggink


Steggink Geo-ICT
Frank Steggink
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