[Liblas-devel] liblas writing proj4 string (using python api)

[Howard Butler]

On Jun 3, 2009, at 12:31 PM, Wes Toews wrote:

> hi, I'm attempting to write the following python code to change the  
> projection information stored in a las 1.1 file in the manner of a  
> demo python script on the liblas site.
> setup is python2.5, liblas 1.0, using the PyPI python bindings.  I'm  
> also not a C programmer.

Hmm... I have not updated the examples since releasing the libLAS 1.2  
release.  libLAS 1.2 changed things quite significantly in the area of  
spatial reference systems.

The basic problem is you cannot change the values of header items in- 
place when doing so would change the size of the header.  You must  
create a new file if this has to happen.  Because the coordinate  
system is stored in variable length records, changing them does indeed  
change the size of the header and therefore requires that you rewrite  
the file...  (note that the code below is for using the 1.2 version of  
libLAS)

> from liblas import file, srs, header
>
> f = file.File('srs.las')
>
> # this makes a copy of the header, it is not a reference
> h = f.header
>
> proj4 = '+proj=lcc +lat_1=47.33333333333334 +lat_2=45.83333333333334  
> +lat_0=45.33333333333334 +lon_0=-120.5 +x_0=500000.0001016001 +y_0=0  
> +ellps=GRS80 +datum=NAD83 +to_meter=0.3048006096012192 +no_def'
>
> s = srs.SRS()
> s.proj4= proj4
>
> h.srs = s
>
> new_file = file.File('newfile.las',mode='w', header=h)
> for p in f:
>     new_file.write(p)
>
> f.close()
> new_file.close()


Feel free to register for Trac and update the example(s) with code  
that you get working with :)

>
> this returns LASException: LASError in "LASWriter_Create": PROJ.4  
> string is invalid or unsupported
>

This error is related to the issue, and it likely means that proj4  
cannot find its data files to look up values.  Which libLAS Python  
binaries are you using?  The ones from PyPI, or the ones from  
OSGeo4W?  If you are using the ones from PyPI, I don't think they  
include the proj4 data files.  A quick way to get those would be to  
install OSGeo4W and then set an environment variable PROJ_LIB to point  
at c:\osgeo4w\share\proj

Hope this helps,

Howard