== DEM scripts == This page details and explains the use of the DEM creation scripts: '''lidardem.sh'''[[BR]] '''asterdem.sh'''[[BR]] '''nextmapdem.sh'''[[BR]] '''lidar_aster_dem.sh''' The scripts listed are written in bash. These scripts are used to create a DEM from LiDAR and elevation model tiles. The scripts use grass GIS to read in the elevation tiles and perform patching and various other operations on the data to create a DEM. === GRASS GIS === GRASS is a GIS (Geograohic Information System) and details of it can be found at: http://grass.fbk.eu/. There is a detailed api and usage guide, it is available for free download at: http://grass.fbk.eu/download/index.php, as the scripts are written to run on linux, depending on your build of fedora run the following to install grass, the scripts use grass63 by default: * {{{sudo yum install grass}}} * {{{sudo apt-get install grass}}} To get started download the template grass database: [attachment:grass_db_template] === Creating a DEM from ASTER data using ''asterdem.sh'' script === ==== ASTER tiles ==== 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]] 15 metre resolution (left), 30 metre resolution (right)[[BR]] [[Image(15_metre.jpg, width=300, title=15 Metre)]] [[Image(30_metre.jpg, width=300, title=30 Metre)]] [[BR]] [[BR]] ASTER tiles are in wgs84 lat/lon projection. ==== Downloading Tiles ==== The tiles can be downloaded from either of: * http://reverb.echo.nasa.gov/reverb/ * Create an account or log in * Tiles are downloaded by putting on an ftp site, the time period for this depends on the number of tiles, the fewer the faster * The best method to select tiles is by a bounding box, drag the box over the desired region * Type ASTER into the "Search Terms" * Select "ASTER Global Digital Elevation Model V002" in the "Select Datasets" * Then click "Search for Granules" * Click accept on the Data quality summary * Add all items to the cart * Click view items in cart * Click Order at the bottom of the page * 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 * Click proceed * Click "Submit Order" at the bottom of the page * A confirmation e-mail should be sent when the tiles are ready for download from an ftp site * http://www.gdem.aster.ersdac.or.jp/ * Create an account or log in * A tile count of under 100 can be downloaded immediately, over 100 the download is done in batches * Click on Search in the dialog box on the left * A page with a world map on it should have been loaded * Tiles can be selected by clicking on "Select tiles directly" or "Select tiles by polygon" * Select the desired tiles and click next * Check that your selection is valid, then click next * Select the purpose of the tile usage * Then download the tiles. ==== Geoid Height Grid ==== 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]] * http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/ww15mgh.grd.z When the file has been downloaded decompress it by running the command on the command line: * {{{gunzip ww15mgh.grd.z}}} Then to get the file in a format compatible with grass run, this gives a header compatible with grass: * {{{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 }}} ==== Separation Files ==== 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: * http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/OSTN02_OSGM02files.zip * Once the file has been downloaded run the command: {{{mkdir OSTN02_OSGM02files; unzip OSTN02_OSGM02files.zip -d OSTN02_OSGM02files}}} *