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 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 use Metadata (4th icon from left) 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 2nd 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 Elevation Class
Sea level
1
< 300 m
6
< 1000 m
11
< 60 m
2
< 400 m
7
< 1250 m
12
< 120 m
3
< 500 m
8
< 1500 m
13
< 180 m
4
< 600 m
9
< 2000 m
14
< 240 m
5
< 800 m
10
> 2000 m
15

Note: pay particular attention to upper and lower limits of each interval while doing reclassification. You may wish to save a reclass file (*.rcl) as part of the procedure. Display the image with the Idris16 palette and without autoscaling.

Your display should appear similar to this example.
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 (under GIS Analysis, Mathematical Operators or icon to far 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 provide an appropriate title and legend. Display the image as before, with the Idris16 palette and without autoscaling.

Elev. Dif. Class Elev. Dif. Class Elev. Dif. Class
No dif.
1
< 150 m
6
< 500 m
11
< 30 m
2
< 180 m
7
< 600 m
12
< 60 m
3
< 240 m
8
< 800 m
13
< 90 m
4
< 300 m
9
< 1000 m
14
< 120 m
5
< 400 m
10
> 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, as before. 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 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. Also use Metadata and Histogram Display to examine the file data characteristics. Note the georeferencing system and range of elevation values.

Next make a classified verison, called AUSTCLAS, in which elevations are grouped into 15 intervals, as above: from lowest class (1) to highest class (15). Recommendation: use classes 1 to 6 for bathymetry (sea floor) and classes 7 to 15 for topography (land). You may use the RECLASS or ASSIGN modules to create AUSTCLAS. Display the image with the Idris16 palette and without autoscaling.

6. List the elevation intervals you picked.

Class Elevation Class Elevation Class Elevation
1
6
sea level
11
2
7
12
3
8
13
4
9
14
5
10
15

Now develop a color palette for displaying AUSTCLAS. Use blue and cyan colors for the seafloor; use green, yellow, orange and red for land areas. Name your palette TOPOBATH. 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 surroundings.

Next turn your attention to the DLG file, which includes coasts, islands, and rivers for Australia and vicinity. Use Metadata to examine the documentation file 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. Make the necessary adjustments by using the Metadata module to change x/y values.

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 overlain by AUST-DLG. Include appropriate title, subtitle (your name and date), legend, scale bar, and north arrow. Make a digital image file to turn in.

Turn In


Return to computer mapping schedule.
ES 551 © J.S. Aber (2004).