[gdal-dev] Request for reintroduction of an override line into gcs.override.csv

Even Rouault even.rouault at spatialys.com
Tue Jan 6 06:08:16 PST 2015 

Andreas,

Datum shifts are always tricky. I've dug up that issue a bit.

The GDAL EPSG related files are coming from libgeotiff, from where they are 
synchronized with at some time (when a new EPSG database is imported 
typically). The change in libgeotiff that made the override disappeared from 
gcs.override.csv is http://trac.osgeo.org/geotiff/changeset/1819
The comment of the commit seems to suggest that the intent was just to do a 
refactoring on how overrides are handled, and an override was actually defined 
in the new file 1766


####################################################################
#
# CH1903: Per http://bugzilla.remotesensing.org/show_bug.cgi?id=957
#
4149,1766

So it refers to coordinate operation 1766 which is defined in 
coordinate_operation.csv as :

OGRFeature(coordinate_operation):751
  coord_op_code (String) = 1766
  coord_op_name (String) = CH1903 to WGS 84 (2)
  coord_op_type (String) = transformation
  source_crs_code (String) = 4149
  target_crs_code (String) = 4326
  coord_tfm_version (String) = BfL-CH 2
  coord_op_variant (String) = 2
  area_of_use_code (String) = 1286
  coord_op_scope (String) = Accuracy 1.5 metres.
  coord_op_accuracy (String) = 1.5
  coord_op_method_code (String) = 9603
  uom_code_source_coord_diff (String) = 
  uom_code_target_coord_diff (String) = 
  remarks (String) = These parameters are derive from CH1903+ to ETRS89 (code 
1647) and are used at lesser precision from CH1903 to WGS 84 as an 
approximation which is within the accuracy of the distortions in the CH1903 
network. Replaces CH1903 to WGS 84 (1) (code 1753).
  information_source (String) = Bundesamt für Landestopographie. Aufbau der 
Landesvermessung der Schweiz 'LV95' Teil 3: Terrestrische Bezugssysteme und 
Bezugsrahmen. L+T 1999.
  data_source (String) = OGP
  revision_date (String) = 2007/03/22
  change_id (String) = 2001.390 2006.992 2007.043
  show_operation (String) = 1
  deprecated (String) = 0

And whose parameter values are in coordinate_operation_parameter_value.csv :

1766,9603,8605,674.4,,9001
1766,9603,8606,15.1,,9001
1766,9603,8607,405.3,,9001

So this explains why you get the values you see now.

I'm not sure if the intent was really to change the override with the 
refactoring or if it is more or less accidental, but IMHO, it doesn't look so 
wrong since coord_op_code=1766 aboce description is "CH1903 to WGS 84" and 
"These parameters are derive from CH1903+ to ETRS89 (code 1647) and are used 
at lesser precision from CH1903 to WGS 84 as an approximation which is within 
the accuracy of the distortions in the CH1903 network."

Digging more, the previous override you suggest reintroducing is :

OGRFeature(coordinate_operation):494
  coord_op_code (String) = 1509
  coord_op_name (String) = CH1903+ to CHTRF95 (1)
  coord_op_type (String) = transformation
  source_crs_code (String) = 4150
  target_crs_code (String) = 4151
  coord_tfm_version (String) = BfL-CH
  coord_op_variant (String) = 1
  area_of_use_code (String) = 1286
  coord_op_scope (String) = For applications to an accuracy of 0.1 metres.
  coord_op_accuracy (String) = 0.1
  coord_op_method_code (String) = 9603
  uom_code_source_coord_diff (String) = 
  uom_code_target_coord_diff (String) = 
  remarks (String) = This transformation is also given as CH1903+ to ETRS89 
(1) (code 1647). CHTRF95 is a realisation of ETRS89. May be taken as 
approximate transformation CH1903+ to WGS 84 - see code 1676.
  information_source (String) = Bundesamt für Landestopographie. Aufbau der 
Landesvermessung der Schweiz 'LV95' Teil 3: Terrestrische Bezugssysteme und 
Bezugsrahmen. L+T 1999.
  data_source (String) = OGP
  revision_date (String) = 1999/10/20
  change_id (String) = 
  show_operation (String) = 1
  deprecated (String) = 0

With values :

1509,9603,8605,674.374,,9001
1509,9603,8606,15.056,,9001
1509,9603,8607,405.346,,9001

So basically you would want datum shift for CH1903+ to be applied as well to 
CH1903.

GDAL actually has the right values for EPSG:4150 / CH1903+ :

$ gdalsrsinfo EPSG:4150

PROJ.4 : '+proj=longlat +ellps=bessel +towgs84=674.374,15.056,405.346,0,0,0,0 
+no_defs '

OGC WKT :
GEOGCS["CH1903+",
    DATUM["CH1903+",
        SPHEROID["Bessel 1841",6377397.155,299.1528128,
            AUTHORITY["EPSG","7004"]],
        TOWGS84[674.374,15.056,405.346,0,0,0,0],
        AUTHORITY["EPSG","6150"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.0174532925199433,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4150"]]

I'm not qualified enough to know if it is always appropriate to apply CH1903+ 
datum shift for CH1903 and unfortunately the bugzilla ticket is no longer 
available (I think the bugzilla database crashed and couldn't be recovered).
But looking at 1.4 "transformation parameters CHTRS95/ETRS89
⇔ CH1903+" of the link you provide 
http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/sys/refsys.parsysrelated1.2487.downloadList.82881.DownloadFile.tmp/swissprojectionen.pdf 
, "Without any restrictions they can also be used for the system ETRS89 and 
for many applications also for CH1903. But in the case of CH1903, one must be 
aware that because of the local distortions of this network the transformed 
coordinates can be false by up to 1.6 meters compared to the official 
coordinates in CH1903", which would seem to suggest that the CH1903+ 
parameters don't necessarily apply to CH1903.

If you can still think the previous override should be applied in the 
CH1903==>WGS84 case, it would be great to find clear references about that.

Isn't your issue that datasets should be referenced as EPSG:4150 instead of 
EPSG:4149 ?

Even

Le mardi 06 janvier 2015 14:02:33, Schmid Andreas a écrit :
> Help? Anyone?
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: gdal-dev-bounces at lists.osgeo.org
> [mailto:gdal-dev-bounces at lists.osgeo.org] Im Auftrag von Schmid Andreas
> Gesendet: Mittwoch, 17. Dezember 2014 17:29
> An: gdal-dev at lists.osgeo.org
> Betreff: [gdal-dev] Request for reintroduction of an override line into
> gcs.override.csv
> 
> Hi
> 
> I hope this is the right mailing list to ask the following:
> 
> I've realized that in GDAL 1.9.2 the "towgs84" parameters of the Swiss
> "CH1903" GCS (EPSG:4149) appear incorrectly. According to [1], they should
> be 674.374,15.056,405.346,,,,
> instead of
> 674.4,15.1,405.3,,,,
> It seems that the origin of these rounded values is the EPSG database. In
> order to override them, I had to add the following line to the
> gcs.override.csv file:
> 4149,CH1903,6149,CH1903,6149,9122,7004,8901,1,0,6422,1766,1,9603,674.374,1
> 5.056,405.346,,,,
> 
> The same (almost same) line used to be included in the gcs.override.csv
> file until GDAL 1.7.3: [2] Would it be possible to re-include this
> override line into the gcs.override.csv?
> 
> Thank you,
> Andy
> 
> 
> [1]: Formulas and constants for the calculation of the Swiss conformal
> cylindrical projection and for the transformation between coordinate
> systems, chapter 1.4:
> http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/sys
> /refsys.parsysrelated1.2487.downloadList.82881.DownloadFile.tmp/swissprojec
> tionen.pdf [2]:
> https://github.com/OSGeo/gdal/blob/733829bf6f88530a92c25457a0f4d0ef0eb5f03
> a/gdal/data/gcs.override.csv#L29-L31
> 
> 
> Andreas Schmid
> GIS-Informatiker
> 
> 
> Amt für Geoinformation
> SO!GIS-Koordination
> Rötistrasse 4
> 4501 Solothurn
> 
> Telefon +41 32 627 75 93
> Telefax +41 32 627 75 98
> andreas.schmid at bd.so.ch
> http://www.so.ch
> 
> 
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