A good, simple, review of ASTER data and the conversion program
used below is at
http://www.terrainmap.com/rm22.html#ASTER. With slight
modifications and additions, here's an
abbreviated, ten step summary of the same, specific to ArcGIS
software.
1) See this
link for data downloading directions.
2) Once data are downloaded, see directions in the above
pages and the description below for importing:
- Download the program geotiff4.exe to your storage space.
Double click on geotiff4.exe to open it.
- Print and read the directions for using the program in the
Help menu.
3) Aster elevation data are in HDF format. The geotiff4.exe
program converts HDF into USGS DEM format, which can then be
converted to Arc Grid format using a program within ArcToolbox. Do the first step by follow the directions in the Help file you
have printed. You will encounter the following difficulties:
a) the program needs the UTM coordinates of the southwest
corner of the image for georeferencing, yet the only
coordinates available are in decimal degrees in an obscure
location within an accompanying metadata file. You must find
the data, then convert it to UTM using an online program
designed to do so. Do the following:
- To find the coordinates needed for georeferencing, find
the file with the same name as the .hdf file but with a .met
extension and open it with Windows WordPad (found under the
Windows Accessories menu).
- Scroll through the document; toward the top you will see
references to GRING, and then numbers that are lat/longs in
decimal degrees. Four pairs are present: you want The
southwest coordinates of the extent of the image, usually
(?) the last pair of coordinates. Draw yourself a box and
label the corners with the coordinates if necessary.
- enter this pair of coordinates in the online Lat/Lon
applet at http://www.nacgeo.com/j2dpoint/.
Click "Calculate"
and record the zone number and the easting and northing in
meters. Enter
these values into the geotiff 4 program.
b) Once the UTM coordinates of the SW corner are entered,
be sure the "Generate ASTER image" box is checked and select
"Run" from the menu to run the program. You were earlier
prompted to enter the UTM Zone. The Geotiff program does not
recognize and can not distinguish northern from southern
hemisphere zones. This will be dealt with later with the Arc
software.
c) After the program is finished, a separate window opens
that contains the image. You can scroll to see a grayscale
version, and by clicking four corner locations the program
will report coordinates that can be used to subset the data.
See the help file for details. Four values are needed
regardless; these are enter in the lower right boxes of the
program window (the row and column limits).
d) Once these values are in place, choose the File menu,
and export ASCII DEM to generate a USGS DEM formated dataset.
A full~60x60 km Aster tile will generate a file of about 40
Mb.
4) To use these data in ArcMap, they must be converted to a
format (TIFF or GRID) that can be recognized. Use ArcToolbox.
Navigate to Conversion Tools>Import to Raster>DEM to GRID
and double click. Tip: make a new folder to store the new grid
before running the DEM to GRID conversion.
5) Once the grid file is created, it can be viewed in
ArcCatalogue or ArcMap. It will have to be symbolized to the
approp. ramp or classification in order to see all z cell
values. Z values are in meters above sea level, x and y are in
meters.
6) For southern hemisphere data, the spatial reference
file does not distinguish south latitudes. Edit the spatial
reference from within ArcCatalog by double clicking on the grid
file, selecting the Spatial Reference tab, then "edit" to open
the "Define Projection" Wizard. Step through the wizard, giving
UTM as the projection, and "offsets" of 0 for the easting and
10,000,000 for the northing. The datum is WGS84.
7) The new grid file may need to be clipped. See Software
Tip 12. If clipping by the
method described doesn't work, restarting ArcMap and trying
again will generally solve the problem.
8) Finally, "No data" values are stored as negative numbers
in all (?) data sets. These will need to be reclassified to "No
Data" if any analysis or contouring will be done. Do this with
the Spatial Analyst reclassify function.
9) After reclassification, the finished grid file may only be
a "temporary" file - right click on the file to "Make Permanent"
and save to a location somewhere on your permanent storage.
10) To match DEM color ramps across adjacent tiles, see the
Software Tip "Copying raster symbology".
