Geography Lab

Geography 256

Instructions and answer sheet (Graded, 10 points)

Exercise 6: Distortion in Maps: distance and direction

Purpose: to explore distance and direction distortion in map projections. To introduce GeoMedia GIS.

Problem: In the previous exercise we explored two of the ways in which projections distort map objects. The distortional effects of a map projection are often complex, especially with respect to distance and direction. A particular projection may be able to maintain distance relations along one orientation but not along others, or maintain direction relations along a standard line but not elsewhere.

In this exercise we continue to examine how projections change our perception of the mapped world. The goal of this exercise is visualize and measure changes in map distances and directions as you change projections.

Materials and Methods: Copy Exer6 folder out of the X:// drive and paste it into your Z:// drive. Navigate to your Z:// drive and open the project Exer6 and examine its content. Exercise 6 consists of a U.S. base map and two sets of reference lines assembled in a GeoMedia GIS project. The US data have 3 layers: states, rivers, and major cities. Notice again how this GIS software distinguishes each layer as point, line, or polygon data by symbolization in the legend. There are other two-line feature layers: LatLon_grid maps meridians and parallels, and Rhumb_Lines represents airline routes between major U.S. cities.

In this lab, you will change the projection of the map and observe what happens to the length and orientation of meridians, parallels and Rhumb_Lines features. As you change the projection, the scale of the map will also change. In order to make valid comparisons of length and orientation between projections you’ll need to make sure that the re-projected maps are displayed at the same scale. To re-scale the maps, Open “ Map Display ” in the Home menu tab. “Display scale” should read 1:30,000,000 and “Nominal map scale” should read 1:50,000. If either of these is incorrect, change the entry in the box to the correct value and hit the Apply button; then Close. Use the “Pan” (Note: the “Pan” tool is the little hand, found in the Home menu) tool to center the U.S. within the map window.

NOTE: When you open GeoMedia, an Error Message will sometimes occur. In this error message, click BROWSE. Go to your Z:// drive and open Exer6 folder. In this folder, open WAREHOUSES and select Exer6. Click OK. The map should display in GeoMedia.

The table on the next page identifies the various map projections you will explore. To access the map projection tool, open Coordinate System (next to Map Display) from the Home menu tab. Notice that this first map is “unprojected” – the General tab is marked Geographic (radio button) and the Projection Space tab is inactive (pull-down menu). Verify that the display scale is correct.

1. Choose at least two meridians, two parallels, and four rhumb lines. For each projection you will measure the length and angle of these eight lines. The simplest way to do that is to use the coordinate display in GeoMedia. The display can be found outside of the menu tabs—in the lower part of the display. By default, the coordinate display is set to Lat, Lon. Use the pull down list and change the coordinate display to the “Projected distance, Azimuth.” Now click on one end of the line (Projected distance & azimuth should read “0.0, 0:00:00.0”; move the mouse to the other end of the line and read the projected distance and azimuth. Note: this may already be changed to the correct setting.) Record these measurements in the attached table.

2. To re-project the map, change the entry on the General (in Coordinate System) tab to Projection then choose an entry on the Projection Space tab – scroll down the list to find the projection you want. Choose one of the map projections from the table. Verify the display scale (=1:30,000,000) and center the map. Re-measure the same meridians, parallels and Rhumb_Lines you measured the first time—using the same starting points. Record your measurements.

3. For each projection, use Snapshot in the GeoMedia File menu tab (left of the Home menu tab) and paste into your answer sheet following the table. Make sure you label the name of the projection beneath the image. Crop the image appropriately and enlarge it so that you can read the graticule lat-long labels.

4. Now do the same for each of the other projections. For the Mollweide and Local Space Rectangular projections, you’ll need to adjust the Projection parameters so that the projection is centered on -90º longitude. In the Projection Space tab, click on Projection Parameters. Change the Longitude of Origin to -90. Hit Ok and OK. The map should change. Be sure that you change the display scale to 1:30,000,000 before measuring the lines on each map.

Lab Report : There are three parts to this lab report: the table, the projection images, and the report. In several paragraphs, discuss what you see happening to the lengths, orientations, and shapes of the graticule & rhumb lines as the map projection changes; make sure that you look up the definition of “rhumb line.” Submit your summary table and projection images with the report.

Points:

Table: 6 points, Should have 64 entries.

Maps: 2 points, Should have 8 projections

Report: 2 points, 3 paragraphs (3 to 4 sentences each)

*When measuring the angle of everything (meridian, parallels, & rhumb lines) pay attention to detail.

If you look at the rhumb lines below and see Cleveland ( San Francisco make sure to get that angle and not San Francisco ( Cleveland…It does make a difference.

Also with the meridians and parallel define your line such as 65N,70W ( 45N,70W.

Exercise 6

Summary table of map projections & distortion measurements*

Line Segment

Projection

unprojected

Mercator

Miller Cylindrical

Albers Equal Area

Lambert Conformal Conic (1SP)

Mollweide (Longitude of origin = -90º)

Universal Transverse Mercator

Local Space Rectangular (Longitude of origin = -90º)

Projection surface:

plane

cylinder

cylinder

cone

cone

cylinder

cylinder

plane

length

angle

length

angle

length

angle

length

angle

length

angle

length

angle

length

angle

length

angle

meridian

#1

#2

parallel

#1

#2

Rhumb line: choose 4

Boston-Atlanta

Boston-Dallas

Boston-Seattle

Cleveland-San Francisco

Miami-Seattle

LA-Chicago

LA-Washington, DC

Phoenix-Minneapolis

* Choose at least 2 meridian lines, 2 parallel lines, and 4 rhumb lines. Measure length and angles using Projected distance, Azimuth coordinates.

Insert Projections, followed by the lab report, here.

270 degrees

180 degrees

90 degrees

0 degrees