ES 551 Lab Exercise

MAP PROJECTION

This lab exercise is designed to explore some common map projections. The exercise is based on a digital elevation model (DEM) from the eastern Baltic region, centered on Estonia. The DEM was acquired from the U.S. National Geophysical Data Center's Global Land One-km Base Elevation Project.

Digital elevation model of the Baltic region, northern Europe.
Adapted from the U.S. National Geophysical Data Center.

GLOBE Project.

Use FTP to download the file EST_DEM from the DATASETS/MAP/PROJ directory. Display the Estonian DEM image using default display selections. Compare the Estonian DEM display to the regional map above. Examine the metadata for EST_DEM.

1. What is the georeferencing system for this dataset? Note reference system, reference units, and resolution. Explain what each cell (pixel) represents in terms of grid dimensions.

2. What kind of map projection is employed for your simple display of the Estonian DEM? Name the map projection and describe its characteristics.

Next make a classified map based on the following elevation categories. Name the classified file EST_CLASS. Display your classified map with the NDVI16 palette (for dramatic colors), and complete the legend with appropriate captions.

Table of elevation classes
Class Elevation Class Elevation
0
< zero
8
101-125
1
1-10
9
126-150
2
11-20
10
151-175
3
21-30
11
176-200
4
31-40
12
201-225
5
41-50
13
226-250
6
51-75
14
251-275
7
76-100
15
> 275

Sample map with elevations shown in the NDVI16 palette.
Click on small image to see a full-sized version.

Compare your image with the regional map below. Note the distortion in scene geometry shown in your map display—features seem to be stretched east-west.

3. Explain why the scene geometry seems distorted in your map display.

Map of Estonia and surrounding countries.
Map obtained from CIESIN.

Open the PROJECT module (under Reformat). This module makes transformations in grid projections. It is a type of resampling. Click on the button for "Reference file for output result" and then click on [+] \Idrisi_Selva\Georef\. This will bring up a list of all projections that are available. Scroll down to the UTM selections. You will find a selection for each UTM zone—both southern (S) and northern (N) hemispheres.

Your goal is to create a UTM grid projection for the Estonian DEM display. Estonia lies between about 22.5° and 28° E longitude. Recall that UTM zones are 6° wide; numbering of zones begins at 180° W longitude and counts eastward.

4. What is the appropriate UTM zone to use for Estonia?

To run the Project module, indicate EST_CLASS as the input file, and name EST_UTM34 as the output file. Input reference system should be "latlong." Select the "UTM-34N" grid projection under reference file for output results. Accept other default selections. Next click on "Output reference information." You will see a note about columns and rows, which you may ignore. Click OK, then click OK on the Project window. When the image comes up, change to the NDVI16 palette for better visual display.

Sample Estonia map converted
into UTM zone 34 projection.

5. What happened to the map geometry? Explain changes in size (area), shape, and orientation of the map display compared to EST_CLASS and to the map above.

6. Examine the metadata for EST_UTM34. What are the new georeferencing system and reference units?

7. What are the new values for X and Y resolution? What does this mean in terms of pixel (cell) size and shape in the image? Hint: enlarge image so that you can view individual pixels.

Estonia spans two UTM zones. Zone 34 is 18° to 24° E longitude, and zone 35 is 24° to 30° E longitude. In your EST_UTM34 display, the western edge of the map is oriented with north nearly straight up, but the eastern side is noticeably tilted. The straight north (up) longitude is called the central meridian of the map projection. For zone 34, the central meridian is 21° E longitude—well to the west of Estonia.

Now make a new projection based on the reference file for "UTM-35N." The central meridian for zone 35 is 27° E longitude, which runs through the eastern edge of Estonia. Call the new image EST_UTM35, and display it as before.

8. What differences do you see between EST_UTM34 and EST_UTM35 map projections? Which of these do you prefer for display of the Estonian DEM and why?

9. Examine metadata for EST_UTM35 and compare with EST_UTM34. What differences do you find?

10. How does X and Y resolution of EST_UTM35 compare with EST_UTM34?

Choose the UTM image that you like the best and prepare a complete map composition named EST_UTM. This will involve "trimming" the black borders from the map--use the WINDOW function under Reformat. Also add a scale bar, legend, and appropriate title and subtitle (your name and date). The legend should give the elevation values for each class. Save a digital image file (jpg or gif) to turn in.

Turn In

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