Tuesday, March 27, 2012

SRTM: An Excellent DEM Data Source

Digital Elevation Model or DEM is the digital representation of  earth surface. For instance, if we divide a 100 meter by 100 meter area into 100 by 100 square grids or 101X101 grid points,  we'll end up 10000 squares. Now, if we know the elevation of those 101X101 ( or 10201) grid points with respect to a datum we can easily develop a DEM of that area. The Figure 1 shows a DEM of an 100 m by 100 m area generated by MATLAB.


Figure 1. A MATLAB generated DEM
Importance of DEM is enormous in Water Resources Engineering, especially in hydrological modeling and analysis. In hydrological modeling, the first thing we need to delineate a basin from adjacent area. We can use the GIS algorithm to develop such basins from fine gridded DEM. Besides, we need channel network of that basin for further hydrological analysis (i.e. channel routing, basin outflow determination, etc) which eventually  may be determined from DEM with the aid of GIS software. However, the objective of this article is neither hydrological modeling, nor GIS. Today I'll talk about an excellent DEM data source, namely SRTM (Shuttle Radar Topography Mission) that I have been using in my research and everybody can access and use that data source for free of charge.

SRTM:

SRTM obtained DEM data covering a near-global scale from 56° S to 60° N. Eventually it generate one of the the most complete high-resolution digital topographic database of Earth.  SRTM Project was a joint endeavor of NASA, the National Geospatial-Intelligence Agency, and the German and Italian Space Agencies, and flew in February 2000.  Information on this project can be found HERE. It used dual radar antennas to acquire interferometric radar data and  process to digital topographic data.  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  in both ArcInfo ASCII and GeoTiff format. The technical details on SRTM DEM database is documented on a paper entitled 'The Shuttle Radar Topography Mission' and can be downloaded from following link: http://www2.jpl.nasa.gov/srtm/SRTM_paper.pdf


Downloading SRTM DEM:

The official download interface for SRTM 90 m DEM data is HERE. If you click on the link , a new window will be opened like the Figure 2.


Figure 2. SRTM Download Page Screenshot

The first thing is to select the data server.  I would suggest everybody to keep the default server location. Then we need to select the DEM area. There are three options to select the DEM area: Multiple Selection, Enable Mouse Drag and Input Coordinates.

a)  In multiple Selection, one can just click on the required 5 deg x 5 deg tiles to select the DEM areas randomly. For example, to select the DEM covering Bangladesh, one can select multiple tiles as below:


Figure 3. Multiple Selection

b) In Enable Mouse Drag option, an area can be selected. This area will be Marked on the Map. Only one area can be selected at a time. Results page will display description of each tile belonging to this marked area. For example, to select the DEM covering Bangladesh, one can select multiple tiles as below:



Figure 4, Enable Mouse Drag

c) In Input Coordinate Option, user can input co-ordinates of their choice, in different modes (Decimal Input, Deg/Min/Sec). Minimum and Maximum extremes have to be specified for both longitude and latitude. For example, to select the DEM covering Bangladesh, one can set the maximum and minimum Latitude , Longitude as below:



Figure 5. Input Coordinate Option

Personally, I always prefer to select the DEM area by inputting ordinates.  No mater what selection method has been used, the next step is to select the File format. There are two different formats: GeoTIFF and ArcInfo ASCII. Both formats are supported by a wide number of GIS and remote sensing software. Form my experience, ASCII format are more useful in hydrological analysis.

The next  step is to click " Click Here to Begin Search" written on the yellow shaded rectangle.  For example, as I select my DEM area by inputting coordinates:  Min Lat=20, Max Lat=30 , Min Lon=85, Max Lon=95, after clicking " Click Here to Begin Search" , it will give me four tiles  ( Lat 20-25, Lon 85-90; Lat 20-25, Lon 90-95; Lat 25-30, Lon 85-90; Lat 25-30, Lon 90-95) as shown below:



Figure 6. Download DEM

The final step is to download each and every files either by FTP server or by HTTP server.  The downloaded DEMs are ready to undergo visualization and analysis. Enjoy !!!

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