[Proj] SRS ogr2ogr - cs2cs - postgis

[Paul Kelly]

On Sat, 5 Jun 2004, [ISO-8859-1] Jaime Mejía wrote:

> Hello.
>
> I've simplified my problem, in order to get some help:
> I have a shape file (attached) containing a single point (994097.623824 
> 1003980.4918) in DATUM BOGOTA, called punto_prueba.shp.
>
> When I try to transform it to WGS84 DATUM using ogr2ogr:
> ----------------------
> ogr2ogr -s_srs EPSG:21892 -t_srs EPSG:4326 /home/jaime/resultado_wgs84 
> /home/jaime/punto_prueba.shp
> ----------------------
> The point now have the following coordinates: -74.1341093150,4.6350408295
>
> If I take the shape into postgis, and try to transform it, I get the 
> following:
>
> ----------------------
> SELECT num_radicacion,the_geom,transform(the_geom,1) AS wgs84 FROM 
> expedientes WHERE num_radicacion=199931011;
> num_radicacion |                 the_geom                 | 
> wgs84
> ----------------+------------------------------------------+--------------------------------------------------
>     199931011 | SRID=2;POINT(994097.623824 1003980.4918) | 
> SRID=1;POINT(-74.1306989066892 4.63219422246886)
> ----------------------
>
> Finally, using cs2cs:
> ----------------------
> jaime at fileserver[/maintask/clientes/creg]# cs2cs -v +proj=tmerc 
> +lat_0=4.599047222 +lon_0=-74.080916667 +k=1.000000 +x_0=1000000.000 +y_0=1tl 
> +towgs84=307,304,-318,0,0,0,0 +to +proj=latlong +datum=WGS84
> # ---- From Coordinate System ----
> #Transverse Mercator
> #       Cyl, Sph&Ell
> # +proj=tmerc +lat_0=4.599047222 +lon_0=-74.080916667 +k=1.000000
> # +x_0=1000000.000 +y_0=1000000.000 +ellps=intl
> # +towgs84=307,304,-318,0,0,0,0
> #--- following specified but NOT used
> # +ellps=clrk66
> # ---- To Coordinate System ----
> #Lat/long (Geodetic)
> #
> # +proj=latlong +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
> 994097.623824 1003980.4918
> 74d7'50.516"W   4d37'55.899"N 16.920
> ----------------------
> 74d7'50.516"W   4d37'55.899"N => -74.13069888888 4.63219416666
>
> As you can see, the results are different (+/- 500 m):
> With ogr2ogr: -74.1341093150,4.6350408295
> With postgis: -74.1306989066892 4.63219422246886
> With cs2cs:-74.13069888888 4.63219416666
>
> So, I assume there is an error in the ogr2ogr command, or in ogr2ogr itself.

No reason to assume an error---you gave ogr2ogr and cs2cs different input.

For ogr2ogr you said "EPSG:21892", which doesn't include any datum 
transformation parameters, whereas for cs2cs you gave the parameters 
specifically as +towgs84=307,304,-318
It's not clear to me where the parameters are coming from in the PostGIS 
example. The cs2cs example also looks like it's missing ellipsoid 
information.

If you want to reproduce the erroneous and correct result with cs2cs, try:

echo 994097.623824 1003980.4918 | cs2cs +init=epsg:21892 \
+to +init=epsg:4326

74d8'2.794"W    4d38'5.669"N (wrong?)

echo 994097.623824 1003980.4918 | cs2cs +init=epsg:21892 +towgs84=307,304,-318 \
+to +init=epsg:4326
74d7'50.516"W   4d37'55.899"N (correct?)

So something like -s_srs "+init=epsg:21892 +towgs84=307,304,-318" in the 
ogr2ogr command line should probably work (if these parameters are correct 
for the Bogota_1975 datum).

Paul K