[OSRS-PROJ] Re: Datum-shift

[Clifford J Mugnier]

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,

--
---------------------------------------+--------------------------------------
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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