Introduces concept of distortion in map projections. Compares distortion in shape, distance, and area for several common map projections.
Map projections are ways of transferring information from a 3-D sphere (the globe) to a 2-D plane (a piece of paper or a computer screen). Understanding map projections is important to anyone working with GIS because a three dimensional object cannot be represented perfectly in two dimensions. Some information must be distorted. Map projections allow the mapmaker to select which quality (or qualities) should be preserved in a map. The choice of map projection can affect not only the aesthetics (beauty) of the map but also any analyses performed using that map. The ArcView Help system includes a description of all supported map projections.
This exercise explores several map projections and illustrates how map features are distorted in different ways by different map projections. The exercise focuses on projections designed for world maps but could be adapted to include projections for maps of a continent or a single country. The instructions assume familiarity with basic ArcView GIS functions such as adding themes, setting the View Properties, and using the drawing tools. If you need detailed instructions for any of these operations, consult the ArcView online Help.
A. Comparing map projections
1. Create a world base map including Countries, Major Cities, and Ocean
grid
5. Create another view and set the map projection to one of your choice, set the Distance Units to miles
6. Compare the maps
Arrange these View windows so you can see all of them and compare the maps:
7. Find View3 (The World from Space) and focus the map on Honolulu
Confirm that "The World from Space" is the projection Type. Click
the button for "Custom"; enter Honolulu's
longitude (-157.8) as the Central Meridian and its latitude (21.3 N) as the
Reference Latitude.
8. Change the focus of View3 to Sydney, Australia (150.9 E, -33.8 S)
9. Change the focus of View3 to your hometown or favorite place
10. Measure the distance from New York to Los Angeles in each view and
record the
distances below:
View1:
View2:
View3:
View4:
View5:
11. Create another view and select an equidistant map projection centered
on New York
City (-74.1 W, 40.7 N)
12. Using the drawing tool, draw several circles on View1
13. Change the map projection to Mollweide and record the area of several
circles
14. Change the map projection to Behrmann; record the area of several circles
Explore the other categories of map projections (Projections of the United States,
UTM, State Plane) using a base
map of the United States or of a single state. Refer to the ArcView online Help
for an explanation of each
projection's properties and recommended uses.
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