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

Ed McNierney ed at topozone.com
Fri Sep 10 21:43:55 EDT 2004

Sean -

Is that "supplementary text file" an ESRI-format world file?  If so, it
would be a text file with six lines in it, each of which contains a
number - and the second and third lines are almost certainly 0.0.

If that's the case, then the first and fourth lines will contain the X
pixel size and the Y pixel size, in units of degrees.  The fifth and
sixth lines are the X and Y coordinates of the upper-left corner of the
image (also in degrees).  Does that look right?

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 978 251-4242   Fax: +1 978 251-1396  

-----Original Message-----
From: gdal-dev-bounces at xserve.flids.com
[mailto:gdal-dev-bounces at xserve.flids.com] On Behalf Of S Clark
Sent: Friday, September 10, 2004 5:10 PM
To: gdal-dev at xserve.flids.com
Subject: Re: [Gdal-dev] Help a stupid GIS newbie: gdalwarp for GPSDrive

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
> 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

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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