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

S Clark smc+gdal at dogphilosophy.net
Fri Sep 10 17:09:50 EDT 2004

On Friday 10 September 2004 01:36 pm, you wrote:
> It sounds like (based on the NASA example you gave) you're looking at a
> very simple "geographic" projection.  Does GPSDrive require a specific
> pixel resolution(i.e. do all images need to use the NASA resolution)?
> Does it read GeoTIFF tags, world files, or any of the standard ways of
> specifying that information?  If not, how do you know what the map
> resolution is?  That's the only extra bit of info that completely
> specifies an image in geographic projection.

Oops, knew I was missing a piece of information - yes the maps can
be at different "scales" (I still don't quite "get" scale when it comes
to digital maps...)  GPSDrive only reads plain image files (along with
the reading the latitude and longitude of the center of the image and
the images' "scale" from a supplementary text file) of any type readable
by the GTK libraries.  I believe that GTK can read TIFF type files, but 
I'm pretty sure none of the extra infromation in GeoTIFF files is used.
GPSDrive further requires that the images be exactly 1280x1024.

In its existing state, GPSDrive gives the "scale" of the Blue Marble
satellite imagery as 1:2614061.  I have successfully generated and used
maps at other "scales", using custom code to create a "zoomed" image of
the Blue Marble data and superimposing line features from the Tiger/LINE 2003
data atop it, and they show up correctly in GPSDrive at every "scale".

Incidentally, CAN you confirm or refute the idea that Expedia's maps
are in a "miller" projection (and therefore if I warp my own map images
to that form, they will work correctly as "map_*" files in GPSDrive...)?
So far I've not been able to find anyone who knows for sure, though I suppose
I can find out myself quickly enough once I've gotten the gdalwarp process
worked out.

Thanks again


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