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

Ed McNierney ed at topozone.com
Thu Sep 9 21:29:46 EDT 2004

S -

In order for gdawarp to translate from projection X to projection Y, you
need to be able to completely define both X and Y.  It sounds like X is
not a problem - the "legally free" (see below) maps you describe appear
to be using UTM or some other reputable map projection, even if you
don't know what it is.  But it's not clear what Y - the "projection"
used by GPSDrive - is.

"Equirectangular" actually defines a class of related projections, and
you need a bit more information in order to define it completely - in
particular, an equirectangular projection usually has two standard
parallels, along which the scale is correct.  Do you have this
information for GPSDrive, or can you specify such information to

Are you sure that the projection is equirectangular and not "geographic"
or Plate Carree?  For folks who work with vector data and don't care to
worry about projections, Plate Carree is much more common.

These, and the Miller, are all cylindrical projections and related (more
or less) to each other.  Generally speaking, a projection is defined by
a general descriptive style (i.e. the "Equirectangular projection")
along with whatever specific parameters are required to define the
specific instance of that projection (i.e. the "Equirectangular
projection with standard parallels at 30N and 30S").  The latter is what
you need for gdalwarp; the former leaves out essential details without
which you just can't do the math.

UTM, by the way, can be a little misleading.  It's really a class of 60
very similar projections, each of which is a Transverse Mercator
projection with a different central meridian (i.e. "zone number").

If you can find out a few more details I think we can get you on the
right track.

	- Ed

Re: "legally free" - "legally free" and "technically possible" are not
the same term.  I have spoken to GPSDrive developers in the past and
asked that the code which illegally downloads TopoZone data be removed;
automated map download tools can cause a map provider to spend a lot of
money, fast.  Just because you can figure out how to create an image
with an HTTP request doesn't mean that the provider of that image will
permit you to do so, or that you have some inherent right to do so.
TopoZone offers a subscription service by which you can create and
download custom, seamless GeoTIFF UTM georeferenced topographic maps or
aerial photos - you're completely free to do whatever you want with
those images, but they cost a few dollars.  You are not free to download
topographic map images from TopoZone by any other means for use in your
own applications.  Other data providers have different license and
copyright restrictions; please take the time to read the terms of use on
any map provider web site, and please respect those terms of use.  If
you don't like them, that's fine - choose a different provider.  Thanks!

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: Thursday, September 09, 2004 8:12 PM
To: 'Gdal-Dev (E-mail)
Subject: [Gdal-dev] Help a stupid GIS newbie: gdalwarp for GPSDrive maps

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"

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

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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