Re: Problem solved with the lakes

> >VMAP0 is 1:1 million scale (ie quite crude), and uses the NAD83 map
> >datum on a GRS80 ellipsoid.
> >
> >SRTM-1 is 30m resolution, and uses the WGS84 map datum in the
> >horizontal and the EGS96 datum in the vertical. (difference between
> >WGS84 geoid and EGS96 is mostly minor; just ignore)
> >
> >
> >I think for U.S. coverage (ie where SRTM-1 exists) it might be better
> >for you to use TIGER data (US Census) instead of VMAP0. Use the
> >ogr2ogr program from GDAL/OGR to reproject into WGS84 and convert to
> >the data format you need. (VMAP0 is supported by the OGDI driver)
> >see  http://www.gdal.org/ogr/
> >
> >probably need PROJ.4 as well for any reprojections and datum
> >transforms:
> > http://www.remotesensing.org/proj
> >
> >(GDAL & PROJ4 are maintained together)
> >
> >I know of an article to be published soon on SRTM+VMAP0 written by a
> >colleague, can post a link to it here once it is in press if anyone
> >is interested.
>
> 
> That's very interesting.  Is the Tiger data available online
> someplace?

homepage:
http://www.census.gov/geo/www/tiger/index.html

data:
http://www2.census.gov/geo/tiger/tgrcd108/
http://www.census.gov/geo/www/tiger/tgrcd108/tgr108cd.html

(NAD83 datum for the 48-cont.)


> Is there anyone out there interested in going through the
> process of  remaping vmap0 datums to see if it helps any?

Use cs2cs from PROJ.4 on some test points. http://proj.maptools.org
If you have a hard time figuring out the options let me know and I'll
spit some out for you. (GRASS GIS: g.proj -jf; PROJ.4 EPSG code for
vmap0: 4269; SRTM is close enough to WGS84 to just use that)
http://proj.maptools.org/

General feeling is that for VMAP0 don't worry about it as the error is
probably much smaller the resolution of the data; for TIGER it'll
probably be more important to get it right.

Datum/ellipsoids are all fairly modern so I'd guess they will be
somewhat close, i.e. closer than the vmap0 resolution, but I don't have
much experience with NAD83 so couldn't really say. Best to do things the
"right" way wherever possible I suppose, as data may get better
tomorrow.
?


> Anyone interested in  playing with tiger data and figuring out the
> exact tool chain needed to use it in FlightGear?

Terragear can read from shapefiles, correct?
use ogr2ogr from GDAL/OGR to make TIGER files into shapefiles, then
proceed as usual (same goes for vmap0->shp).   http://gdal.org



Hamish

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