[Proj] Transformations with height data to bessel bring unrealistic results

[Clifford J Mugnier]

Also, you could use EGM96 to approximate the geoid height and back that out of the elevation to get a SWAG of the ellipsoid height.

Sent from Cliff Mugnier's iPhone

> On Sep 21, 2017, at 5:14 PM, Melita Kennedy <mkennedy2 at earthlink.net> wrote:
> 
> 
> Hello Mark, 
> 
> The z values are being treated as ellipsoidal heights. Nowhere have you given any 
> information to PROJ.4 that they are any particular vertical coordinate reference 
> system. The transformation methods supported by the +wgs84 parameter work strictly 
> with ellipsoidal heights, not geoidal or other gravity-related heights.
> 
> If you know you have gravity-related heights, you can either not pass them and ignore 
> the output ellipsoidal heights or pass them, but replace them with the input gravity-
> related heights. One reason to go ahead and pass in the gravity-related heights is 
> that they may approximate the ellipsoidal heights in the area better than using zeroes.
> 
> Melita
> 
> -----Original Message-----
>> 
>> Date: Thu, 21 Sep 2017 13:00:22 +0200
>> From: Mark Johnson <mj10777 at googlemail.com>
>> Subject: [Proj] Transformations with height data to bessel bring
>>    unrealistic    results
>> To: proj at lists.maptools.org, a.furieri at lqt.it
>> 
>> When transforming positions with DHHN92 (EPSG:5783) height data, to Bessel
>> based projections are returned.
>> For my area (Berlin, Germany) the lowest height is just over 24 meters.
>> Transformations using 34.79 all return  -6.13930772151798 when using
>> ST_Transform, which performs a pj_transform in the background.
>> Using -6.13930772151798 to transform back will return the original value.
>> 
>> Between WSG84 and epsg:25833 there are no problems.
>> 
>> I would be interested in learning the reason why this is so.
>> 
>> Here some cs2cs samples with the results:
>> 
>> ETRS89 / UTM zone 33N [epsg:25833]
>> # +proj=utm +zone=33 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
>> 
>> # DHDN / Soldner Berlin
>> # +proj=cass +lat_0=52.41864827777778 +lon_0=13.62720366666667 +x_0=40000
>> +y_0=10000 +ellps=bessel +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7
>> +units=m +no_defs
>> echo "389918.0707613 5819698.6197819 34.7900000"  | cs2cs +init=epsg:25833
>> +to +init=epsg:3068
>> 23183.38 21047.17 -6.14
>> echo 23183.38 21047.17 -6.139 | cs2cs +init=epsg:3068 +to +init=epsg:25833
>> 389918.07 5819698.62 34.79
>> 
>> # DHDN
>> # +proj=longlat +ellps=bessel
>> +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +no_defs
>> echo "389918.0707613 5819698.6197819 34.7900000"  | cs2cs +init=epsg:25833
>> +to +init=epsg:4314
>> 13d22'45.996"E 52d31'3.638"N -6.139
>> echo 13.37770460000026 52.51627039998215 -6.139 | cs2cs +init=epsg:4314 +to
>> +init=epsg:25833
>> 389796.59 5819544.81 34.80
>> 
>> # DHDN / 3-degree Gauss-Kruger zone 4
>> # +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=bessel
>> +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7 +units=m +no_defs
>> echo "389918.0707613 5819698.6197819 34.7900000"  | cs2cs +init=epsg:25833
>> +to +init=epsg:31468
>> 4593626.95 5821241.65 -6.14
>> echo "4593626.95 5821241.65 -6.14"  | cs2cs +init=epsg:31468 +to
>> +init=epsg:25833
>> 389918.07 5819698.62 34.79
>> 
>> Mark Johnson, Berlin Germany
>> _______________________________________________
>> Proj mailing list
>> Proj at lists.maptools.org
>> http://lists.maptools.org/mailman/listinfo/proj
>> 
>> End of Proj Digest, Vol 156, Issue 4
>> ************************************
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