[OSRS-PROJ] Re: Datum-shift

[Frank Warmerdam]

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,

-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent


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