[FWTools] conversion/transformation

[Mateusz Loskot]

Christine Spietz wrote:
> Hi there,
> 
> is there a detailed documentation on the mechanism of 
> conversion/transformation with ogr2ogr?
> 
> Only things I was able to find were:
> 
> -a_srs srs_def = assign an output srs
> -t_srs srs_def = Reproject/transform to this SRS on output
> -s_srs srs_def = override source srs
> 
> I need a more detailed description on what especially is going on if
> you assign one of those parameters to ogr2ogr.

-a_srs option is used to "assign" coordinate system to output file e.g.
during copying.

In example, I have Shapefile called 'poland' (.shp|.dbf|.shx) and I know
it stores coordiantes in geographic coordinates sytem WGS 84 but I don't
have a poland.prj file, so I can use ogr2ogr utility to copy poland
shapefile to poland2 shapefile and assign coordinate system:

$ ogr2ogr -a_srs 'EPSG:4326' -f "ESRI Shapefile" poland2.shp poland.shp

$ ls poland2.*
poland2.dbf  poland2.prj  poland2.shp  poland2.shx

$ cat poland2.prj
GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]m

NOTE: I run those commands on Linux, so I use here 'ls' and 'cat' commands


-s_srs option is used to override SRS for input dataset if there is any
assigned.
- t_srs is used to tell ogr2ogr what is the target projection; to which
projection ogr2ogr should transform your input data.

In example, I have poland2 shapefile and in the example above I assigned
WGS84 coordinate system to it.
Now, I want to project this poland2 shapefile from WGS 84 to "Poland
CS92" coordinate system (it's used widely in Poland).
So, I have two choices:

1) I can use poland2.prj file generated in the example above:

ogr2ogr -t_srs 'EPSG:2180' -f "ESRI Shapefile" poland4.shp poland2.shp

2) I can omit the poland2.prj file and tell ogr2ogr to use SRS defined
in -s_srs option as an input SRS :

ogr2ogr -s_srs 'EPSG:4326' -t_srs 'EPSG:2180' -f "ESRI Shapefile"
poland4.shp poland2.shp

In both cases, the output is the same:
shapefile called 'poland4' projected to "Poland CS92" (EPSG:2180)

I hope my explanation is helpful.

Cheers
-- 
Mateusz Łoskot
http://mateusz.loskot.net