[Gdal-dev] Help a stupid GIS newbie: gdalwarp for GPSDrive maps

S Clark smc+gdal at dogphilosophy.net
Thu Sep 9 20:11:53 EDT 2004

I hope a "stupid newbie" question doesn't annoy too many people.  

I'm trying to educate myself on the necessary GIS skills and 
knowledge necessary to accomplish just one thing (at the moment):
converting maps and map data from various public ("legally free") 
sources into maps that can be used with the GPSDrive[1] program.  Most
of the sources are not formal GIS data, though (no "world files" included,

The "short" version of my question:
   A jpeg or png image from a publically available source (terraserver,
topozone, etc.)
   The coordinates (in decimal form) of the center pixel of the map
   The "scale" of the map
   The projection of the source image (nearly always UTM at the moment)

Can someone give me, idiot that I am, an example gdalwarp command line for
converting said image to equirectangular projection?

The "full" version:

GPSDrive supports two different projections:  Equirectangular and "Whatever
maps downloaded from Expedia are in" (I have been annoyingly unsuccessful at
finding out what projection that is - anyone know?  So far all I've gotten is
one person mentioning that they think it may be a miller projection[2] -
judging by the way that UTM maps end up being "off" when given to GPSDrive,
this seems plausible).

I have a variety of "legally free" (i.e. no-cost plus either public domain
or otherwise legally redistributable) sources of maps and map information,
such as the USGS topographical maps and aerial photos, plus NASA satellite
imagery, US Census Bureau "Tiger/LINE" map data, and so on.  I'd like to use
this information to generate custom maps for GPSDrive.

Generating equirectangular maps from raw data is trivial,and working just
fine for me (e.g. zooming in on the NASA "Blue Marble" imagery that GPSDrive
can work with natively, rendering road features atop it from the Tiger/LINE
data, and then adding it to GPSDrive's map files.  Trying to use USGS maps
and aerial photos doesn't work, though, because they're projected as UTM.
(the center of the map is correct, but naturally the further one travels from
center, the further "off" the map is.)

I am unfortunately starting from more-or-less scratch trying to learn what I
need to know to understand what I'm doing here, so I'm not quite sure how easy
it will be to calculate any parameters that I don't already have that I need
to feed to gdalwarp to accomplish what I'm trying to do.  I went searching
for a FAQ of some type that might discuss converting (for example) USGS
"Orthoquads" from online sources with GDAL, but haven't had much luck.
This may very well be because I'm an idiot, but still...

Apologies in advance for sucking up space on the mailing list with what, to 
me, sounds like a really ignorant question, and thanks in advance for any 
assistance anyone can offer.

[1] - http://gpsdrive.kraftvoll.at/
[2] - http://s2.selwerd.nl/~dirk-jan/gpsdrive/archive/msg03568.html
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