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Note: Shortly after this tip was written, SRTM Level 3 data ("SRTM 90m
[research Data]") for the
western hemisphere became available through the Seamless Data
Distribution System (e.g.
http://edcsns17.cr.usgs.gov/EarthExplorer/ ) in ArcGrid format.
Comparable data for the eastern hemisphere are not yet available.
Using the Seamless site, none of what's written below is now necessary, but
1x1 degree tiles are still available at
ftp://edcftp.cr.usgs.gov/pub/data/srtm in the .hgt format
described below.
DTED Level 1 SRTM data is now available for purchase on CDs.
See http://edc.usgs.gov/products/elevation.html
for further information.
Note added Nov., 2007: SRTM
version 2 was
released in 2006. Data can be obtained by anonymous ftp
to:
ftp://e0srp01u.ecs.nasa.gov and moving to the directory srtm
where both version 1 and version 2 directories may be found.
Read the appropriate documentation, also found in the
directories. SRTM30 (30 Arc Second) data and header files
can also be found here.
For an up-to-date status of SRTM products see the
CGIAR CSI website.
Note added Nov., 2013: The best available global
elevation data, the 30 meter seamless
ASTER GDM
v2, was released in Oct. 2011 and is available for
download from several sites.
Preliminary 3-arc second (c. 90 meter resolution) Shuttle Radar
Topography Mission data, available through EROS data center,
provide for many areas of the world the best available digital
elevation data. These are Level 3 products, still relatively
crude, with data voids and elevation discrepancies for nominally
flat (e.g. water) areas. Even in their crude form, they
are better than GTOPO30 data for some areas. Data tiles are
substantially smaller than those
available within GTOPO30, so are useful for this reason as well.
Once obtained, importing SRTM-3 data into ArcInfo requires the
following steps:
1) Creating a header file
2) Importing the data in a fashion similar to that for GTOPO30
3) Reclassifying "no data" cells
4) Assigning a spatial reference.
I. CREATING A HEADER FILE
Unlike GTOPO or SRTM30 data, SRTM-3 data contain no ancillary
information, only x, y and z values in a single .hgt-format file.
A header file, having the same name as the .hgt file but with a .hdr
extension, is required for importing into ArcGIS. To create one, modify the ULXMAP and ULYMAP
values in the code below to correspond to the lat. and long. of
the upper left corner of the .hgt data tile, i.e. nearly
the values contained in the file name (which are the lower left
coordinates of the data tile). The header example given
below is, for example, appropriate for the 3
arc-second SRTM file N37W105.hgt.
Finally, if perchance you are lucky enough to 30m rather than
90m data, NROWS and NCOLS should be replaced with 3601 and
XDIM and YDIM should replaced with 0.0002777778.
----------------------------------------------------------------
BYTEORDER M
LAYOUT BIL
NROWS 1201
NCOLS 1201
NBANDS 1
NBITS 16
BANDROWBYTES 2402
TOTALROWBYTES 2402
BANDGAPBYTES 0
NODATA -32768
ULXMAP -105.0
ULYMAP 38.0
XDIM 0.000833333333333
YDIM 0.000833333333333
---------------------------------------------------
After copying and pasting the example above
into Notepad or Wordpad and modifying the ULXMAP and ULYMAP
values, save this new header file with the same
name as the data file, but with an .hdr extension. This
should be a simple text file, but without a .txt extension.
To prevent automatic addition of a .txt extension, be sure to set
the file type to "All Files" when saving from Wordpad or Notepad.
2) IMPORTING INTO ARCINFO - CONVERTING THE IMAGE
TO A GRID
Once a header file has been created, this process is identical to
that for importing GTOPO30 data.
Follow those directions through to the step that talks about
reclassifying no data cells.
3) RECLASSIFYING NO DATA CELLS
Unlike GTOPO30 data, "no data" raster cells have a dummy value of
-32768. Use the steps given in the importing GTOPO30
directions to reclassify these dummy values to "no data".
4) ASSIGNING A SPATIAL REFERENCE
SRTM-3 data use the WGS84 datum and have x and y coordinates in
decimal degrees (FYI, the vertical datum is EGM96). An .aux
file containing this spatial referencing information needs to be
created so that your newly created grid file will properly overly
other data sets. Do so in ArcCatalog with a right-click on
the new grid file, select Properties, and then assign a WGS84 GCS.
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