[GRASSLIST:2179] Re: Projection questions

Frank Warmerdam warmerdam at pobox.com
Fri Jul 27 13:41:33 EDT 2001

Rich Shepard wrote:
>   Thanks very much. Just to confirm my understanding. If the map is
> projected in State Plane Coordinates/NAD27, then the file would read:
> unit: foot
> units: feet
> meter: 0.3048
>   Correct?


Yes, it should.  Whether it would depends on a variety of things, such as is
GDAL's SDTS support really smart enough to recognise horizontal feet units,
and does r.in.gdal write it out properly.
>   When you write that r.in.gdal "isn't establishing this correctly", do you
> mean that it's just not writing the PROJ_UNITS file correctly?

Right, but it might be because of GDAL itself.  I am not sure, and would have
to look into it in some detail which I don't have time for this afternoon. 
>   I'm reorganizing my entire data structure to make GRASS5 happy (I'm told
> the existing structure should work, but my problems with r.in.gdal+r.proj
> prove otherwise). As most of my current projects are in the Pacific
> Northwest of the U.S., I'm setting up a top-level location called PERMANENT

A "location" called PERMANENT?  That would seem to be very confusing given the
convention of calling the primary mapset within a locatin PERMANENT.  Why not
a location called "pnw" (Pacific North West) with a PERMANENT mapset? 

> that defines the boundaries of this left third of the country. The
> projection is Albers Equal Area (aea on the /usr/local/grass5/etc/prjections
> file list). How do I specify that in a numeric equivalent?

99 - other 

99 is basically the catchall for generic projections support.  Only UTM, 
Lat/Long and Unknown (and perhaps state plane?) are handled otherwise.  This
has to do with how the software evolved.
>   I know that '99' is used for both 'user specified' and 'spc', but I cannot
> find a list of codes for other projections.
>   And, can we assume that all GRASS modules that work with/report on
> projections know what the projection is? If this is the case, do we need to
> specify it in the PROJ_INFO file?

This seems like a big questions.  I don't know how pervasive use of PROJ_INFO
is in the various GRASS commands.  Some get all the information they need from
the WIND files. 

Best regards,

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

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