USGS Digital Raster Graphics

[Ralph Root]

MEMO TO:  Interested GRASS GIS users

FROM:  Ralph Root, National Biological Service, Denver, CO

SUBJECT:  Reading USGS DRG's into GRASS

DATE:  March 7, 1995

Last week at the ACSM/ASPRS conference in Charlotte, NC I obtained a free copy
of sample DRG's (Digital Raster Graphics) from the USGS booth containing 
selected 24K topo quads for all 50 state capitals.  They are seeking comments
from the user community on this relatively new product.

This week I looked at the data on the CDROM, and brought some examples into 
GRASS using r.in.tiff.  Although the CDROM is designed primarily for PC users,
and includes a couple of on-line viewers, I was impressed at how quickly and 
easily these data sets can be brought into GRASS in a UNIX environment.  

I set up a generic xy location, with about a 10000 by 8000 default 
region, and brought in a few quads to see what would happen.  R.in.tiff read 
the *.tif files in the CDROM "data" directory with no problem, created the
proper XY cellheader file, and extracted the color table with no difficulty.

I then consulted the "metadata" directory on the CDROM and was impressed to 
find each file prepared to FGDC metadata standards, and fully "fleshed out" 
with full details on how the files were prepared.  The metadata also indicated
that these DRG's have been georeferenced to the UTM grid.  After finding the
coordinates and sample size (different for each quad) I edited the GRASS header
file to reflect UTM.coordinates instead of an XY projection.  After re-
displaying, I checked grid intersections with d.where, and found very 
consistent locational UTM's (within one pixel) in the case of the Boston map 
which I used as a test case.  Topo maps in digital form, and georeferenced 
provide a GREAT backdrop to a GIS data base, with all sorts of hybrid analyses 
and display possibilities.

The documentation and metadata support on this CDROM are outstanding.  It is
an excellent resource, and a promise of good things to come from USGS!

If you are interested in getting a copy of this CDROM contact:

                esic at mcdgs01.cr.usgs.gov

If you have an Internet multi-media browser, take a look at:

                http://mcmcweb.cr.usgs.gov/

for full background on the USGS DRG products.


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Ralph Root
rroot at its.nbs.gov