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Posted by Peter N. Schweitzer on April 22, 2005, 8:26 pm
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I'm trying to write a .hdr for a BIL image consisting of 16-bit
signed integers. I haven't found any directives that would indicate
to GIS software that the cell values are signed, and I think that
when I import them (into Arc as a grid), the software thinks they're
unsigned. This is etopo5 data (I'm interested in the bathymetry),
so a histogram of the cell values is strongly bimodal, with the
depths represented as high positive values and the heights as relatively
low positive values.
Have I asked for something that GIS software just cannot do--that
is, think of a grid as containing signed integers--or is there some
magic word I can speak that will cause Arc to interpret the numbers
the way I'd like it to? Or do I have to write a separate program
that converts the image so that it has unsigned values and in order
to interpret the values properly I will need to subtract some number
from them?
Thanks for any advice you can offer.
Peter
--
Peter N. Schweitzer (MS 954, U.S. Geological Survey, Reston, VA 20192)
(703) 648-6533 FAX: (703) 648-6252 email: pschweitzer@usgs.gov
<http://geology.usgs.gov/peter/>
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Posted by Dean Gesch on April 25, 2005, 7:58 am
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Peter -
This is similar to how we distribute GTOPO30 data, as 16-bit BIL files.
GTOPO30 contains negative values, so we run into the same problem
getting Arc to recognize the data as signed integers.
See the following section in the GTOPO30 README file for directions on
how to get Arc to recognize the negative 16-bit values (it involves use
of a CON statement in Arc Grid):
http://lpdaac.usgs.gov/gtopo30/README.asp#h15
I hope this helps.
Dean Gesch
U.S. Geological Survey
National Center for Earth Resources
Observation and Science (EROS)
Sioux Falls, SD 57198
Phone: 605-594-6055
Fax: 605-594-6529
Email: gesch@usgs.gov
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