ES 551 Lab Exercise

DIGITAL ELEVATION MODELS

The first part of this exercise is based on 10-minute digital elevation models for northern Europe, including the Scandinavian and Baltic regions. These models were derived from the Fleet Numerical Oceanographic Center (FNOC) of the U.S. Navy. Elevation values are in 100-foot intervals (converted to nearest 10 m) and stored in integer-binary format. The horizonal and vertical spatial resolutions are relatively coarse, suitable for broad regional display and analysis, but lacking in fine-spatial details.

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.

Display these images using default display selections. Also view file metadata to examine the documentation files. You can see that autoscaling provides poor displays, because low elevations are shown black, same as the sea. Also notice the intervals of elevation values that appear in the legend.

1. What are the latitude and longitude boundaries of these files?

2. Explain why the legend categories have odd numbers.

Now use RECLASS (under GIS Analysis, Database Query or 5th icon from right) to prepare a classified image of EUR-MAX according to the following elevation intervals. Name your result MAX-CLAS, and provide an appropriate title (incl. your name) and legend with the Metadata module.

Elevation Class Elevation Class
Sea level
0
< 500 m
8
< 30 m
1
< 600 m
9
< 60 m
2
< 800 m
10
< 120 m
3
< 1000 m
11
< 180 m
4
< 1250 m
12
< 240 m
5
< 1500 m
13
< 300 m
6
< 2000 m
14
< 400 m
7
> 2000 m
15

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.

3. Why do you suppose these classification intervals were chosen? What aspect of the landscape does this classification scheme tend to emphasize?

Now use OVERLAY (4th icon from right) subtraction to create an image that shows the difference between maximum and minimum elevations for each cell in the DEM (subtract EUR-MIN from EUR-MAX). Then make a classified version of this image according to the following intervals. Name your result DIF-CLAS, and this time use the IDRIS16 palette for the display.

Elev. Dif. Class Elev. Dif. Class
No dif.
0
< 240 m
8
< 15 m
1
< 300 m
9
< 30 m
2
< 400 m
10
< 60 m
3
< 500 m
11
< 90 m
4
< 600 m
12
< 120 m
5
< 800 m
13
< 150 m
6
< 1000 m
14
< 180 m
7
> 1000 m
15

4. What does this image show about the landscape? Describe the general topography in areas with small values and in areas with large values.

Finally drape DIF-CLAS over EUR-MAX using the ORTHO module (under Display). Select the IDRIS16 palette. Experiment with different combinations of view direction, view angle, and vertical exaggeration. Choose 800x600 output resolution, and save your best result as a file named EURORTH. Include a suitable title and subtitle (with your name and date). Make a digital image file to turn in.

5. Describe the combination of factors you picked for EURORTH, and explain what the image shows about the landscape.



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.

6. List the elevation intervals you picked and explain why you chose those intervals.

The display above is not particularly effective, because is does not show a clear separation between land and sea floor. Now develop a color palette for an improved display of AUSTCLAS. Use blue, cyan and green colors for the seafloor; use yellow, orange and red for land areas. There should be a marked visual distinction between land and sea floor. Name your palette TOPOBATH.

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.

7. Describe the georeferencing system used for the AUST-DLG file. How does this differ from AUSTRAL5?

Now display AUSTCLAS with your TOPOBATH palette, legend and title. Add AUST-DLG with the Composer using the standard symbol file. Notice that the DLG file does not exactly match the DEM. There is a slight offset of the two files.

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.

Min x +0.20 and Max x +0.20 (longitude).
Min y -0.05 and Max y -0.05 (latitude).

After making adjustments in x and y values, you should achieve a close, but not perfect, match between the DEM and DLG files. Notice any discrepancies and think about why the features do not match up properly.

8. How much did you adjust x and y values to make the DEM fit the DLG file?

9. Discuss possible explanations for why the DEM and DLG data are not in perfect alignment on your final image.

10. Based on your experience with this exercise, discuss some problems in GIS in using databases acquired from different sources.

Finally, prepare a map composition, named AUSTRAL, which shows the adjusted AUSTCLAS (with your special palette) overlain by AUST-DLG. Include appropriate title, subtitle (your name and date), legend, and scale bar. Make a digital image file to turn in.

Turn In

Note: this exercise is double credit: 10 points for each part.

Return to computer mapping schedule.
ES 551 © J.S. Aber (2012).