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

S Clark smc+gdal at dogphilosophy.net
Fri Sep 10 15:12:57 EDT 2004

On Thursday 09 September 2004 07:29 pm, Ed McNierney wrote:
> 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.
Short answer - there are two.  "Geographic" or "Plate Caree'" MAY be
the correct term for what GPSDrive inaccurately refers to as "topo"
maps for historical reasons.  The other is "Whatever expedia maps are, which 
may or may not be a miller projection".

> "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
> GPSDrive?
What I'm referring to (quite possibly incorrectly) as "equirectangular" is a 
result of adding support for a set of NASA "Blue Marble" satellite data to
GPSDrive.  This data has the following characteristics:

It is a rendered image of satellite photo data of the entire globe.  The data 
is separated into two data sets ("Eastern" and "Western" hemispheres).  The
"Western" hemisphere dataset is 21600x21600x24bit data, covering 90.00,-180.00 
to -90.00,0.00.  Each pixel, regardless of where in the 21600x21600 square it 
is, covers 0.0083333[...] by 0.008333[...] degrees.  While that distorts 
distances pretty severely in places, I like it because it's extremely easy to 
work with in program code.  Is that enough information to determine what the
proper name for the (pseudo-)projection should be?

My interest in the other "Miller or whatever Expedia is" projection is just
because I figure at times I may find it's less distorting when it matters to

> 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").

Put that way, it makes a little more sense - I knew about the "every 6 degrees
is a different zone" but it hadn't sunk into my thick skull that each one was
technically a different projection.  Southeastern Idaho, where I'm stuck at 
the moment, should be zone 12 if I'm counting right (and I've thus far got:
-s_srs '+proj=utm +zone=12 +datum=WGS84'
as part of the gdalwarp command line... )

> 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;
Yup, Topozone.com downloads were removed some months ago - just as well, since
being UTM the downloaded maps never quite lined up except right at the center.

I agree completely here - that's actually a primary driving reason behind my
interest in learning to obtain, generate, and possibly redistribute my own
maps in a genuinely legally free manner.  (I use the term "legally free" to
emphasize the difference - too many people think that, for example, Microsoft
Windows is "free" because they were able to borrow the install disks from 
work...)  While my reading of the expedia terms of service didn't seem to 
actually forbid automated downloads, and did seem to permit any reasonable 
"personal, non-commercial use" (which to my mind would include using them
in GPSDrive), they are still not genuinely "free", and yet they are 
currently the only "built-in" automatic download in GPSDrive.  I'd much
rather avoid risking a lawsiege from Microsoft lawyers for copyright
infringement by generating my own maps.

(If I get it all figured out, a HOWTO document will hopefully follow...)

Incidentally, looking at the "Topozone Pro" page, it appears to be an
all-you-can-eat sort of subscription (i.e. pay $49.95 and use it as
much as you want for a year) rather than a per-use service, is that
accurate?  If so, is it possible and reasonable to use THAT service from
an automated, custom-designed program rather than only being able to 
use it "by hand" or via ESRI(tm)-brand ArcProduct for Microsoft(r)
Windows(r)?  (Hey, being able to download the more recent color
aerial photo imagery on demand via a script for use in GPSDrive alone
would just about be worth that to me...)

Thanks for the help!


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