Exercise 2-DEM downloading

Transcription

1 Exercise 2-DEM downloading Downloading and importing DEM in to ILWIS 1.1. PURPOSE The purpose with this exercise is to exemplify how to download a DEM to your area of interest, import it to your GIS software and make it ready for further processing. You will use a global DEM from a public domain called The CGIAR Consortium for Spatial Information (CGIAR CSI) BACKGROUND The The CGIAR Consortium for Spatial Information (CGIAR CSI) is an initiative by geospatial scientists working to apply and advance Geospatial Science for International Sustainable Agriculture Development, Natural Resource Management, Biodiversity Conservation, and Poverty Alleviation in Developing Countries. The CGIAR-CSI works to facilitate collaboration and capacity building for Data Sharing, Data Dissemination, and Geospatial Analysis amongst the fifteen CGIAR Centers and their many regional activities; with geospatial laboratories, scientists and researchers throughout the developing countries; and within the broader Global Research and Development Communities. (Source: You will practice on how to locate the data you required for your work. The path to the global database called the The CGIAR Consortium for Spatial Information (CGIAR CSI) is: Geo-referencing information: Projection: Geographic Note: The coordinate pairs are measured by latitude/longitude Units: Decimal Degrees Datum: WGS84 The SRTM digital elevation data is produced by NASA. The SRTM digital elevation data provided on CGIAR CSI site has been processed to fill data voids, and hence no need for gap filling operations as in the original data download site. The SRTM 90m DEM's have a resolution of 90m at the equator, and are provided in mosaiced 5 deg x 5 deg tiles for easy download and use. The data is available in ArcInfo ASCII and GeoTiff format to facilitate their ease of use in a variety of image processing and GIS applications. Data can be downloaded using a browser or accessed directly from the ftp site. Compiled by Essayas K. for SCWRE, BDU-iOT 1

2 1.2.1 Downloading DEM data from CGIAR CSI DEM Data from the CGIAR CSI can be downloaded from the web interface (fig. above). The web-interface is made of a grid having 24 rows and 72 columns forming 1728 tiles. Each tile is 5 deg x 5 deg amounting 555 km x 555 km at the equator. The grids are sensitive to a mouse click. You can select a single tile or set of contagious tiles or separated tiles. On top of the grid you will see three selection panels. On the first panel you will select the location for the data. As we are downloading from CGIAR CSI leave the default selection as it is. You have three options to select data. You may click on a tile to select or enable the mouse drag option and hence you will draw a rectangle by dragging you mouth or enter your coordinate boundaries if you have the information. Use the file format GeoTIFF as this is the format that we will use with ILWIS. Compiled by Essayas K. for SCWRE, BDU-iOT 2

3 Note that if you select a tile mistakenly you can erase your whole selection by clicking the Clear Area button and start all over again. For this exercise select the tile containing Lake Tana and click the Click here to begin search button. Your search will result in the following display. You have two options to download the data, the HTML and the FTP option (Refer details). Note the extents of the data on the left pane of the download window. The HTML option shall be used in this exercise as the FTP option is inaccessible from BDU Importing data to ILWIS Follow the following steps to import the files On ILWIS main window File>Import Compiled by Essayas K. for SCWRE, BDU-iOT 3

4 The Import dialog box will open. Select the srtm_43_10.tif file. In the Output Filename text box click on the ellipsis and select the folder where you are currently working. Click OK. The importing process should start immediately. You will see the progress manager in the top left side of ILWIS window. Compiled by Essayas K. for SCWRE, BDU-iOT 4

5 Wait patiently for the process to finish as this is a big file. When this is finished your data will appear in the ILWIS main window. You will see three objects here; the coordinate system icon, the georeference icon and the raster map icon. Double click on the raster map icon to display the DEM and observe the coordinates of the location of your mouse pointer. Do the numbers give meaning? At this stage the data representation gives less sense and hence you may want to enhance its visibility. Close the raster map and re-open it by double clicking. On the Display Options Raster Map dialog box change the stretch from 100 to 5000 (Why?) and click OK. Close the map and display the Coordinate system icon by double clicking on it. You may see here that the coordinate system is as specified on page one above. Click OK and return back to the main window. Double click on the georeference icon and see its contents. You may see here that a georeference uses a coordinate system and contains the pixel size information. Note that the pixel size for this raster file is 3 arc seconds. What size does this amount to in meters? Click OK and return back to the main window. As this is the DEM for the whole Ethiopia its size is considerable and takes time to open, zoom, pan etc. You may create pyramids so as to enable your GIS tool to display the image quickly. Let s now select our area of interest and cut it out for use in the next process Sub-mapping data to ILWIS The Sub-map of raster map operation copies a rectangular part of a raster map into a new raster map. The user has to specify row and column numbers or XY-coordinates of the input map to indicate the part of the input map that should be copied into the new raster map. This operation allows you to cut out a part of your raster map. You can choose to specify the part of the input map that should be copied as: Compiled by Essayas K. for SCWRE, BDU-iOT 5

6 start line and column (upper left) and the number of lines and columns, start line and column (upper left) and end line and column (lower right), start XY-coordinate and end XY-coordinate (opposite start XY-coordinate). Now open the raster map again and display it with the same stretch that you used to enhance it before (100 to 5000). Zoom to the Lake Tana area. You may observe here that the visibility of the map is not up to your expectation. On the map window double click on the raster map icon to enter a different stretch values. Stretch your raster map to the range of 1500 to At this stage you should see a very rough outline of the lake. Use your mouse pointer to locate the pixel value on Lake Tana (What does this value represent?). Zoom to the Lake Tana catchment adequately and start the sub mapping task. File > Create > Submap Choose the Corners option and enter the following values. What does this represent? First line: 2688 First Column: 7893 Last line: 5271 Last Column: 9946 Use your mouse pointer to locate the pixel value on Lake Tana (Are the values still the same?). E n d o f e x e r c i s e Compiled by Essayas K. for SCWRE, BDU-iOT 6