Dear Users,
I'm trying to understand how to create the hdr file for the .bil srtm
dem I downloaded....
I have found too much different information... so I try with you....:
if I have a N11E044.hgt file I have to rename the file to N11E044.bil
and then to create a N44E011.hdr file...
can you help me to insert the right value in the ULXMAP and ULYMAP
sections of the hdr file below.... ???
BYTEORDER M
LAYOUT BIL
NROWS 1201
NCOLS 1201
NBANDS 1
NBITS 16
BANDROWBYTES 2402
TOTALROWBYTES 2402
BANDGAPBYTES 0
NODATA -32768
ULXMAP ???
ULYMAP ???
XDIM 0.000833333333333
YDIM 0.000833333333333
Thank you very much....
Ivan
I had to wrestle with this and here's what I figured out. First, the cells in the SRTM are center-based, so you need to take into account 1/2 the cell for the UL corner. And there is a 1 cell overlap between tiles. So, if you have a cell as your example:
1. add 1.00041666666667 to latitude for ULYMAP
12.00041666666667
2. subtract 0.00041666666667 from longitude for ULXMAP
43.9995833333
this will put the corner in the correct position 1/2 cell outside the upper-left.
On May 17, 2006, at 4:50 AM, ivan marchesini wrote:
Dear Users,
I'm trying to understand how to create the hdr file for the .bil srtm
dem I downloaded....
I have found too much different information... so I try with you....:
if I have a N11E044.hgt file I have to rename the file to N11E044.bil
and then to create a N44E011.hdr file...
can you help me to insert the right value in the ULXMAP and ULYMAP
sections of the hdr file below.... ???
BYTEORDER M
LAYOUT BIL
NROWS 1201
NCOLS 1201
NBANDS 1
NBITS 16
BANDROWBYTES 2402
TOTALROWBYTES 2402
BANDGAPBYTES 0
NODATA -32768
ULXMAP ???
ULYMAP ???
XDIM 0.000833333333333
YDIM 0.000833333333333
Thank you very much....
Ivan
-----
William Kyngesburye <kyngchaos@charter.net>
http://webpages.charter.net/kyngchaos/
All generalizations are dangerous, even this one.
On Wed, May 17, 2006 at 11:50:51AM +0200, ivan marchesini wrote:
Dear Users,
I'm trying to understand how to create the hdr file for the .bil srtm
dem I downloaded....
I have found too much different information...
In fact, there are many wrong documents in the Web.
Even the USGS docs were silently (!) fixed in December 2003.
so I try with you....:
if I have a N11E044.hgt file I have to rename the file to N11E044.bil
and then to create a N44E011.hdr file...
- SRTM filename coordinates are said to be the *center* of the LL pixel.
N51E10 -> lower left cell center
- BIL uses *center* of the UL (!) pixel:
http://downloads.esri.com/support/whitepapers/other_/eximgav.pdf
- GDAL uses *corners* of pixels for its coordinates (?)
[...]
Please have a look at
http://mpa.itc.it/rs/srtm/index.html
where you can grab ready-to-use scripts:
srtm_generate_hdr.sh: general shell script to make GeoTIFF from original
SRTM90 files. Requires GDAL tools to be installed.
r.in.srtm: GRASS script to import original SRTM files. Requires above script
to be installed in PATH. Fixes the INTEGER encoding, applies color table.
To use in Lat/Long LOCATIONS (WGS84, to be precise EGM96).
Even the NASA is using it 
http://xserve.flids.com/pipermail/geotiff/2004-August/000052.html
Hope this helps
Markus
Oh blast! Just figured out that GDAL takes care of the cell center -> cell edge conversion of the header. So my method shifted everything NW 1/2 cell. So it's even simpler: the ULXMAP is the tile longitude, and add 1 to the latitude for the ULYMAP.
Good thing I noticed before importing the whole US next week. And that's probably why the VMAP0 coasts didn't match very well before
On Oct 7, 2004, at 12:29 PM, William K wrote:
I had to wrestle with this and here's what I figured out. First, the cells in the SRTM are center-based, so you need to take into account 1/2 the cell for the UL corner. And there is a 1 cell overlap between tiles. So, if you have a cell as your example:
1. add 1.00041666666667 to latitude for ULYMAP
12.00041666666667
2. subtract 0.00041666666667 from longitude for ULXMAP
43.9995833333
this will put the corner in the correct position 1/2 cell outside the upper-left.
On May 17, 2006, at 4:50 AM, ivan marchesini wrote:
Dear Users,
I'm trying to understand how to create the hdr file for the .bil srtm
dem I downloaded....
I have found too much different information... so I try with you....:
if I have a N11E044.hgt file I have to rename the file to N11E044.bil
and then to create a N44E011.hdr file...
can you help me to insert the right value in the ULXMAP and ULYMAP
sections of the hdr file below.... ???
BYTEORDER M
LAYOUT BIL
NROWS 1201
NCOLS 1201
NBANDS 1
NBITS 16
BANDROWBYTES 2402
TOTALROWBYTES 2402
BANDGAPBYTES 0
NODATA -32768
ULXMAP ???
ULYMAP ???
XDIM 0.000833333333333
YDIM 0.000833333333333
Thank you very much....
Ivan
-----
William Kyngesburye <kyngchaos@charter.net>
http://webpages.charter.net/kyngchaos/
All generalizations are dangerous, even this one.
-----
William Kyngesburye <kyngchaos@charter.net>
http://webpages.charter.net/kyngchaos/
"Time is an illusion - lunchtime doubly so."
- Ford Prefect