[GRASSLIST:4316] Re: GIS visitor requests directions

Nigel McFarlane nrm at kingtide.com.au
Mon Aug 19 05:44:36 EDT 2002

Glynn, thanks for the pointer. After some further study, I
realise there's a gap in my knowledge, thus:

I picked tile 'a' of the GLOBE DEM for a test. That's
50-90N, 180-90E in lat-long, Nth America. I had no
problems loading this.

When I created a second location based on the Universal
Polar Stereographic projection, ups, (one that should merely
makes the re-projection results obvious), I found I don't
understand the concept of 'region' or 'region coordinates'.

In a lat-long projection I can enter the raw lat-longs,
but most other projections require these largish numbers
whose origin I don't know. That left me with the
default of 1, 0, 1, 0 for the ups projection, and an
r.proj operation failed:

   (Central latitude 70N, central meridian 135W)

   r.proj input=ustopo location=PERMANENT mapset=testmap output=outfile

   ERROR: Input map is outside current region

I happened to look at UTM (which I don't need) and now I
understand a bit about Eastings and Northings, but I don't
understand how to calculate them for other projections,
or even how Eastings or Northings could apply to other
projections, especially a polar one. How can you be East
of the north pole?

- where do these numbers come from?
- is a region merely a pixellated subsection of a full projection?
- must the two projections (source and target) somehow 'fit'
   each other? (for example, must one contain the other?)

thanks again,

regards, Nigel.

In response to other kind suggestions, I've now identified
that Adobe Illustrator can read .DXF files, so I shan't
need to rasterise my vector data, just transform it.
I think GRASS/DXF stores per-pixel data, rather than lat-long
data, which should suit Illustrator, but if I'm stepping on
a mine again, please advise.

- N.

Nigel McFarlane                                  Melbourne, Australia
Software, Telecommunications, Internet, Physics  UTC +10 or +11 hours
Analysis, Programming, Writing, Education       Phone +61.3.9415.7080

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