Changes between Version 2 and Version 3 of Help/DEM_scripts


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Timestamp:
Feb 9, 2012 12:44:31 PM (8 years ago)
Author:
adbe
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  • Help/DEM_scripts

    v2 v3  
    1212
    1313=== Creating a DEM from ASTER data using ''asterdem.sh'' script ===
    14 ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is a sensor on the Terra satellite, ASTER is partly being used to obtain elevation maps. The ASTER version 1 have an approximate resolution of 30 metres, version 2 however has an approximate resolution of 15 metres.
    15 [[Image(http://arsf-dan.nerc.ac.uk/trac/attachment/wiki/Help/DEM_scripts/15_metre.jpg)]]
     14==== ASTER tiles ====
     15ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) is a sensor on the Terra satellite, ASTER is partly being used to obtain elevation maps. The ASTER tiles are 1 degree x 1 degree lat/lon. The ASTER version 1 have an approximate resolution of 30 metres, version 2 however has an approximate resolution of 15 metres.[[BR]]
     1615 metre resolution (left), 30 metre resolution (right)[[BR]]
     17[[Image(15_metre.jpg, width=300, title=15 Metre)]]       
     18[[Image(30_metre.jpg, width=300, title=30 Metre)]] [[BR]]
     19[[BR]]
     20ASTER tiles are in wgs84 lat/lon projection.
     21
     22==== Downloading Tiles ====
     23The tiles can be downloaded from either of:
     24  *  http://reverb.echo.nasa.gov/reverb/
     25    *  Create an account or log in
     26    *  Tiles are downloaded by putting on an ftp site, the time period for this depends on the number of tiles, the fewer the faster
     27    *  The best method to select tiles is by a bounding box, drag the box over the desired region
     28    *  Type ASTER into the "Search Terms"
     29    *  Select "ASTER Global Digital Elevation Model V002" in the "Select Datasets"
     30    *  Then click "Search for Granules"
     31    *  Click accept on the Data quality summary
     32    *  Add all items to the cart
     33    *  Click view items in cart
     34    *  Click Order at the bottom of the page
     35    *  An order items list will appear, click set: select the data usage, and agree to the terms, make sure "Use these values for all applicable order items" is selected, then save
     36    *  Click proceed
     37    *  Click "Submit Order" at the bottom of the page
     38    *  A confirmation e-mail should be sent when the tiles are ready for download from an ftp site
     39  *  http://www.gdem.aster.ersdac.or.jp/
     40    *  Create an account or log in
     41    *  A tile count of under 100 can be downloaded immediately, over 100 the download is done in batches
     42    *  Click on Search in the dialog box on the left
     43    *  A page with a world map on it should have been loaded
     44    *  Tiles can be selected by clicking on "Select tiles directly" or "Select tiles by polygon"
     45    *  Select the desired tiles and click next
     46    *  Check that your selection is valid, then click next
     47    *  Select the purpose of the tile usage
     48    *  Then download the tiles.
     49
     50==== Geoid Height Grid ====
     51A geoid spheroid height model is required to elevate the aster tiles to the correct geoid. As the tiles are in lat/lon it is best to elevate it to wgs84, because this is a good standard. To elevate the aster we use a geoid height grid available from: [[BR]]
     52  *  http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/ww15mgh.grd.z
     53When the file has been downloaded decompress it by running the command on the command line:
     54  *  {{{gunzip ww15mgh.grd.z}}}
     55Then to get the file in a format compatible with grass run, this gives a header compatible with grass:
     56  *  {{{sed "s/   -90.000000   90.000000     .000000  360.000000     .250000     .250000/north: 90.000000   \nsouth: -90.000000     \nwest: 0.000000  \neast: 360.000000\nrows: 721\ncols: 1441/g" ww15mgh.grd -i }}}
     57
     58==== Separation Files ====
     59If projecting the aster data to another projection with a differing elevation model a separation file needs to be used to elevate the aster correctly. For example if projecting the DEM to a osgb36 UKBNG projection from wgs84 lat/lon the elevation model is different. To elevate the aster we need a description of the height differences between the geoid and the spheroid, a file describing the differences between the two can be found at:
     60  *  http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/OSTN02_OSGM02files.zip
     61  *  Once the file has been downloaded run the command: {{{mkdir OSTN02_OSGM02files; unzip OSTN02_OSGM02files.zip -d OSTN02_OSGM02files}}}
     62  * 
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