[OSRS-PROJ] datum shift Gauss Krueger

Wed Jun 12 22:24:30 PDT 2002

Hello All

I got the same problem as Harald Wehr when tranforming from wgs84 to Gauss-Krueger (zone 4) . My transformation will also be about 55 meters wrong.

I use cs2cs as follow
cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc +ellps=bessel
+lat_0=0 +lon_0=12 +x_0=4500000 +towgs84=583,68,399

What I know from some other program I should use 

Gauss Krueger 4, Bessel, Potsdam_IST_V2_0 
583.00     - Translation dx in meter to WGS84
68.0       - Translation dy in meter to WGS84
399.50     - Translation dz in meter to WGS84
0          - Rotation ro (omega) in Radiant to WGS84
0          - Rotation rf (phi) in radiant to WGS84
13.6e-06   - Rotation rk (Kappa) in Radiant to WGS84
11.3e-06   - Mapscale factor in ppm (parts per million) to WGS84

And tranformation should calculate

ID  lon (WGS84)  lat (WGS84)    X (GK4)      Y (GK4)
1   11.60608768  48.25167084    4470860.075  5346044.934
2   10.96510029  50.25949478    4426313.445  5569780.810
3   7.46         51.57          4185407.951  5724830.919

When I use cs2cs I get the X-value to be about 55 meters wrong! The Y-value will just differ 0 - 3 meter.
I think that I should in some way add the rotation kappa in the calculation! Does anyone know how how I should do this?

If I change the dy value from 68 to 18 I got some better values. Now the X and Y values will differ about 8 meters but I don't think I should do this.

Frans

		Dear Frank,

		The Europeans like to do their rotations in an opposite direction from the
		rest of the world when doing 7-parameter Datum Shifts.  Translation and
		Scale Factors remain the same.  e.g., you will get weird and unexplainable
		results if you rotate in the wrong direction.

		A Helmert transform done with geocentric coordinates in three dimensions is
		called a Helmert Direct from Geodetic to Geocentric, and is called a
		Helmert Inverse from Geocentric to Geodetic.

		Molodensky-Badekas does a 3-d Helmert direct to geocentric coordinates,
		subtracts the geocentric coordinates of the classical Datum origin, then
		does the 7-parameter, then finally does the Helmert inverse back to the new
		Datum's geodetic coordinates.

		Bursa-Wolf does a 3-d Helmert direct to geocentric coordinates, does NOT
		subtract the geocentric coordinates of the classical Datum origin, then
		does the 3-parameter, or 4-parameter, or 7-parameter, then finally does the
		Helmert inverse back to the new Datum's geodetic coordinates.

		How do you tell which direction to rotate?  It is impossible unless you
		have a test point.  Since Harald does have a test point ...

		So now you know.

		Cliff

		Prof. Clifford J. Mugnier (cjmce at LSU.edu)
		Chief of Geodesy
		CENTER FOR GEOINFORMATICS
		Department of Civil Engineering
		LOUISIANA STATE UNIVERSITY
		Baton Rouge, LA  70803
		Voice and Facsimile:  (225) 578-8536
		Pager:              1-(888) 365-5180
		================================
		http://www.ASPRS.org/resources.html
		http://www.ce.LSU.edu/~mugnier/
		================================

		Harald Wehr wrote:
		> Hi Frank,
		>
		> first i apologize, that i contact you directly.
		>
		> In the PROJ.4-mailing-list no one answered to my problem. Attached you
		> find my mail from february 26th that I send to the list.
		...

		> I want to convert some data from latlong/wgs84 to tmerc/bessel with
		> datum shift from wgs84 to potsdam-datum here in germany.  How do i have
		> to use the function? Are helmert-7-parameter as well as
		> molodenski-3-parameter implemented? I tried the following:

		Harald,

		I didn't answer because I am always getting confused about the terminology
		for some of the different transforms.  There is a three and seven
		parameter transform implemented; however, I am not certain that the
		interpretation of the parameters (sign, handedness, etc) is exactly what
		is defined for helmert and molodenski.  I think it is though.

		>
		> --------
		> cs2cs -v +proj=latlong +to +proj=tmerc +ellps=bessel +lat_0=0 +lon_0=12
		> +x_0=4500000 +towgs84=-586,-87,-409 <<EOF
		> 10.69079E 51.86134N
		> EOF
		> --------
		>
		> As result i get the following:
		>
		> ---------
		> # ---- From Coordinate System ----
		> #Lat/long (Geodetic)
		> #
		> # +proj=latlong +ellps=WGS84
		> # ---- To Coordinate System ----
		> #Transverse Mercator
		> # Cyl, Sph&Ell
		> # +proj=tmerc +ellps=bessel +lat_0=0 +lon_0=12 +x_0=4500000
		> # +towgs84=-586,-87,-409
		> #--- following specified but NOT used
		> # +ellps=WGS84
		> 4409820.23 5748070.07 698.769
		>
		> ---------
		>
		> The result is not good enough. I expect something like that:
		> 4409920 5748280.
		>
		> Whats wrong here? I got the "towgs"-parameters from a another
		> helmert-calculation i found in the internet which work in another
		> program i have here and show the results i expected.

		First, no actual datum shift is being applied in this case because PROJ
		doesn't equate +ellps=WGS84 with the WGS84 datum automatically.  I realized
		this when I did the conversion without the +towgs84 parameter and got the
		same results.  Please use +datum=WGS84 to force treatment of the lat/long
		coordinates as being the WGS84 datum.

		Howwever, this doesn't seem to get us any closer to your expected values.

		warmerda at gdal[355]% cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc
		+ellps=bessel
		+lat_0=0 +lon_0=12 +x_0=4500000 +towgs84=-586,-87,-409 < test.dat
		4409793.19      5747857.54 1386.040

		I tried reversing the sign of the offset, as this is a frequent
		incompatibility,
		and get:

		warmerda at gdal[356]% cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc
		+ellps=bessel
		+lat_0=0 +lon_0=12 +x_0=4500000 +towgs84=586,87,409 < test.dat
		4409847.28      5748282.64 11.504

		This is close to what you expect in the northing (2m), but no so close in
		the
		easting - still off by 70m or so.  I am no sure what else to say.

		Best regards,

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