The raster grids are in latitute-longitude format. Note: these data sets are for land areas only; the seafloor is not depicted. Use FTP to download the following files from the DATASETS/MAP/DEM directory.
|Sea level||< 500 m ||< 30 m ||< 600 m ||< 60 m ||< 800 m ||< 120 m ||< 1000 m ||< 180 m ||< 1250 m ||< 240 m ||< 1500 m ||< 300 m ||< 2000 m ||< 400 m ||> 2000 m |
Note: pay particular attention to upper and lower limits of each interval while doing reclassification. For example, the values for class 0 are from -999 to just less than 1, and the values for class 1 are from 1 to just less than 31, etc. You may wish to save a reclass file (*.rcl) as part of the procedure. Display the image with the NDVI16 palette and without autoscaling.
|Your display should appear similar to this example. The NDVI16 palette provides a particularly bold separation between sea (black) and land (redólowest to greenóhighest). Click on small image to see a larger version.|
|Elev. Dif.||Class||Elev. Dif.||Class|
|No dif.||< 240 m ||< 15 m ||< 300 m ||< 30 m ||< 400 m ||< 60 m ||< 500 m ||< 90 m ||< 600 m ||< 120 m ||< 800 m ||< 150 m ||< 1000 m ||< 180 m ||> 1000 m |
For the second part of this exercise, you will work with both raster (DEM) and vector (DLG) files for Australia. The DEM is derived from the ETOPO5 database: 5-minute gridded elevations in meters for global land and sea-floor areas. The DEM for Australia is named AUSTRAL5. The DLG file is named AUST-DLG; it was derived and corrected from the Micro-World Data Base II.
Begin by displaying AUSTRAL5 using default display selections. Check metadata to examine the file data characteristics. Note the georeferencing system and the range of elevation values. Use the histogram function (tenth icon from right) to see the distribution of elevation values above and below sea level.
Next make a classified verison, called AUSTCLAS, in which elevations are grouped into 16 intervals, as above: from lowest class (0) to highest class (15). You will have to decide how to do this effectively.
Recommendation: use classes 0 to 7 for bathymetry (sea floor) and classes 8 to 15 for topography (land). Notes: your classes should reflect the actual distribution of elevation values--see histogram. Also pay attention to negative values for the sea floor. Display the image with the IDRIS16 palette without autoscaling.
|Your display should appear similar to this example, although it may differ in some details depending on the elevation intervals you used for the classification. Click on small image to see a larger version.|
|Example of a custom palette in which submarine elevations appear in blue, cyan and green colors, and land areas are yellow, orange and red. Note the distinct visual boundary between lowest land (bright yellow) and shallow seafloor (green). Some interior land areas are below sea level, so appear green. Click on small image to see a larger version.|
Complete the title and legend of AUSTCLAS, so that each elevation interval is clearly identified. At this point, you should have an impressive display of the land topography and marine bathymetry for Australia and the surrounding sea floor.
Next turn your attention to the DLG file, which includes coasts, islands, and rivers for Australia and vicinity. Examine the metadata for AUST-DLG. Note the georeferencing system and compare it with AUSTRAL5. Both employ a latitude-longitude grid, but there are significant differences between the two georeferencing systems.
The positional coordinates of the vector (DLG) files cannot be moved and are assumed to be correct, but the DEM position can be shifted by changing its min/max x and y values. Examine the display carefully and determine how much to adjust x and y values to make the DEM "fit" the vector files.
Use the Idrisi text editor (8th icon from right) to open the rdc file and make the necessary adjustments in x/y values. Or you can update the metadata with Idrisi Explorer. Note: after changing and saving the new x and y values, you need to redisplay the image for the changes to take effect.
Note: you may want to experiment on a temporary file; copy AUSTCLAS to AUSTEMP. You must adjust min and max values equally, although x adjustments may be different from y adjustments, for example.
Return to computer mapping schedule.
ES 551 © J.S. Aber (2012).