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demfits - Read DEM (Digital Elevation Maps) from USGS
dem [parameter=value]
demfits (previously called dem) converts ASCII DEM (Digital
Elevation Maps) from the USGS into a FITS file. Currrently it converts
the 1:250,000-scale 1 degree (with 3 arcseconds resolution ) which have
a size of 1201 by 1201 pixels, as well as the 7.5 arcmin maps (with 1 arcsec
resolution) The output file is a FITS data-unit (no header currently written).
You need to use a script (see EXAMPLE below) to add a header and flip
the X and Y axes. Note that North will be Right, and East Up if you display
this block of data in the usual convention of the first data in the lower
left corner
The following parameters are recognized in any order
if the keyword is also given:
- in=
- Input DEM file
- out=
- Output FITS
data unit file
- naxis=nx,ny
- Preset the size of the output map
- crval=x0,y0
- Preset the lower left corner of the image. The output dataset will have
pixels not within the region set to 0.
- newfmt=t|f
- New format has to be used
if data items 17-28 are present in the A record. Default: f
- c=t|f
- Force reading
of a trailing C record (not implemented yet). Default: f.
- tab=
- If given,
this will be the name of an ascii table file that contains a listing of
the extracted X,Y,Z values. Default: not used.
Units are directly taken
from the USGS maps. E.g. the 1-degree maps are in standard arcseconds, maps
are 1201 by 1201 pixels. The 7.5 arcmin maps are in Universal Transverse
Mercator (UTM) projection, units in meters (x=Easting, y=Northing; after
the proper transformation). Height is in meters in both cases.
Lots
of header variables not properly checked, and hardcoded/assumed in the
source code. Once a proper FITS file is be written, this will be cleaned
up.
1 degree maps should be processed with dem1 (the first version), 7.5
arcmin maps with dem, but not after the proper coordinate system (naxis=,
crval=) has been found out, otherwise you will probably wind up with a
map with lots of zeros :-)
Here is a very simple shell script that
adds a FITS header to a block of data of which you know the size:
#! /bin/csh -f
# Usage: SCRIPTNAME NX NY BASENAME
# Input: BASENAME
# Output: BASENAME.fits
set nx=$1
set ny=$2
set base=$3
set tmp=tmp$$
echo "SIMPLE = T /" > $tmp.txt
echo "BITPIX = 16 /" >> $tmp.txt
echo "NAXIS = 2 /" >> $tmp.txt
echo "NAXIS1 = $nx" >> $tmp.txt
echo "NAXIS2 = $ny" >> $tmp.txt
echo "END" >> $tmp.txt
fitshead in=$tmp.txt out=$tmp.fitshead
cat $tmp.fitshead $base > $base.fits
ftp://edcftp.cr.usgs.gov/pub/
NEMO/usr/pjt/usgs - more formal releases
/bootes3/teuben/usgs - development
Peter Teuben
10-dec-96 V0.0 handle 1-degree maps (kludged) pjt
11-dec-96 V0.1 handle 7.5 arcmin maps (more kludged) pjt
23-mar-99 V0.3 added some doc and better 7.5arcmin support Pjt
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