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

[Ed McNierney]

Sean -

As for the Expedia projection, it is NOT a Miller projection - at least
not at small scales.  My bet is that it is a simple orthographic
projection, with the central meridian and the latitude of origin both
set to the coordinates of the center point on the current map view.  I
cannot confirm that this projection doesn't change when you zoom in, but
I don't see any reason why it would need to do so.

Pick an Expedia map and zoom out as far as you can.  Note that the outer
limit of the projected Earth appears to be a circle - a big hint right
there.  Note that the meridian lines (not shown, but many US states have
convenient east-west boundaries) curve up towards the top of the map and
down towards the bottom.  Now use the scroll buttons to spin the globe
around.  The projection behaves reasonably and always appears least
distorted at the center, meaning that the projection parameters change
as the map center changes.  Also note that the projections behave nicely
at the poles as you scroll around.

The orthographic is simple to use and implement, and it makes maps most
people consider "nice" at small scales.  It is what the Earth would look
like if viewed from an infinite distance (with, of course, infinite
magnification to go with it).  The scale bars displayed on Expedia maps
are a little bogus - they probably show the scale at the center of the
map, but orthographic maps don't preserve distances; the distances at
the edges of the circle are badly distorted.  But when zoomed in this is
relatively irrelevant.

So if you're going to work with Expedia maps - and if my guess is
correct - then you will need to know the latitude and longitude of the
center point of each map.  You might want to see if GPSDrive works
correctly with small-scale Expedia maps, where assumptions that might
work for large scales will begin to break down.

	- 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
maps

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

Sean
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