[GRASS5] Even More on Projections

[Frank Warmerdam]

Andreas Lange wrote:
> 
> Hi Frank,
> 
> i will try to answer some of your questions, but i must admit that i am
> not sure for some concepts in GRASS myself.
> 
> In GRASS 4.x the projection was defined within the raster header and/or
> the WIND/DEFAULT_WIND files. In GRASS 5 the projection information is
> mainly in the PROJ_INFO file, but for the projections supported in GRASS
> 4.x the WIND/DEFAULT_WIND will suffice. Functions that return values
> from the PROJ_INFO file return default values if this file is missing. 

Andreas,

It hasn't been my experience that G_get_projinfo() returns defaults if
PROJ_INFO is missing.  Are you sure that it does?  I had proposed changing
G_get_projinfo() to generate defaults from the proj/zone settig in the
WIND file if there was no PROJ_INFO file, but I didn't get any feedback
(positive or negative) on this suggestion. 

This would be instead of implmenting the G_get_location_coordsys() call.


> > Currently I prepare a struct Cell_head, attempting to set the projection
> > to match that of the source file if possible.  I then call G_set_window()
> > with that structure, and then G_open_raster_new(), and write the file.
> > This seems to result in a cellhd file being created with the information
> > I passed to G_set_window() whether the projection matches the location or
> > not.  Am I responsible for doing validation to ensure the projection of
> > my new raster matches the location?  Some commands (like r.in.dted) seem
> > to check, while others (like r.in.doq) seem to do the same as r.in.gdal.
> >
> 
> I fear there is no general concept, some modules check if projections
> match, other do not. I think you should always validate matching
> projection.

As part of r.in.gdal I have implemented a projection comparison function
(though it isn't really complete) which currently just lives in with r.in.gdal,
but if folks think it is worthwhile, I would be happy to move it into libgis.a.

static int 
G_compare_projections( struct Key_Value *proj_info1, 
                       struct Key_Value *proj_units1, 
                       struct Key_Value *proj_info2, 
                       struct Key_Value *proj_units2 )

It returns TRUE if they appear to be the same, otherwise FALSE.  

> Tom Pointdexter wrote a script "mklocation.sh" to create the directories
> for a new location. The raster modules IMHO do not create a new
> location, but i remember a script that creates a new location, imports
> the raster data and uses r.proj to transfer this to the current location
> (?).

I have tentatively implemented a location making function in C, that takes 
Cell_head, proj_info and proj_units as inputs to default the projection and
extents of a new projection.  Currently this is just in r.in.gdal, but 
I would like to move it to libgis.a if folks think it worthwhile.

int G__make_location( 
    char *location_name,
    struct Cell_head *wind, 
    struct Key_Value *proj_info, 
    struct Key_Value *proj_units,
    FILE *report_file )

Best regards,

---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerda at home.com
light and sound - activate the windows | http://members.home.com/warmerda
and watch the world go round - Rush    | Geospatial Programmer for Rent

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