| 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 ==== |
| | 15 | 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 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]] |
| | 16 | 15 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]] |
| | 20 | ASTER tiles are in wgs84 lat/lon projection. |
| | 21 | |
| | 22 | ==== Downloading Tiles ==== |
| | 23 | The 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 ==== |
| | 51 | A 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 |
| | 53 | When the file has been downloaded decompress it by running the command on the command line: |
| | 54 | * {{{gunzip ww15mgh.grd.z}}} |
| | 55 | Then 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 ==== |
| | 59 | If 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 | * |
| | 63 | |
| | 64 | |
| | 65 | |