A map projection is a means 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.
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 the principles apply to maps of a continent or a single country as well.
The ArcView Help system includes a description of all supported map projections as well as help topics on how to set the map projection and other important parameters. Please read the ArcView Help topics:
In addition, the following help topic is a useful reference: Map scale and accuracy
In this exercise you will:
At the end of the exercise, you are asked to answer
questions about the properties of specific projections. For this lab, please turn in
your lab report, including answers to the questions posed in this lab (in bold
with a "Q:" for a total of 10 pts).
Also be sure to hand in your lab notebook. Again, please refer to the course documents for complete
guidelines on writing these two documents. 1. Start ArcView and create a New Project. 2. Set your working directory to your
students account and then immediately Save the project to your directory. Call
the project ex3_<yourname>.apr 3. Then add the themes World30.shp, Country.shp and Cities.shp from the directory C:\esri\esridata\World . Your themes should be ordered Cities.shp, Country.shp and
World30.shp in the View Table of Contents from top to bottom so that you can
see the features of each theme.
Turn on the themes. 1. Create a world base map including
Countries, Major Cities, and Ocean grid a. Make the View window active and set the
map projection for the view. Make
sure that the Projection Properties Category is Projections of the World and the Type is Geographic. If this is not the case, set these
accordingly. b. In the View Properties dialog
box, check that the Map Units are set to decimal degrees and the Distance Units to miles. c. Set the
Theme Properties for the Cities.shp theme to show only a few key cities, such as New York City,
Los Angeles, Anchorage, Honolulu, and Sydney. To do this, select Theme/Properties from the menu. Click on the Query Builder icon (which is a hammer with a
question mark) to open the Query builder dialog box. d. In the Field list double click on [Name] to insert it into the text window
below. It will become part of the
logical expression that defines your query. Now click once on the equal to operation button. The operator is
added to the expression. In the Values
list double click
on the name of the city to add it to the query. Continue forming your query by clicking on the OR operator and then reconstructing
the next phrase ([NAME]= name of city) for all of the cities that you want displayed. Pay attention to the syntax. Click OK to close both dialog boxes. e. Notice now that the only cities that
are displayed are those you specified in the query above. 2. Create a second base map and set the
map projection to Robinson a. In the project window with the View document type
active, click on New. This will create an empty view (View2) into which you will copy the
themes from View1. To do this make
View 1 active
and using the shift key make all three themes active. Then from the
Edit menu select
Copy Themes. Now make View2 active and select Edit/Paste. b. Set the map projection for the view. Make sure that the Projection
Properties Category is Projections of the World and select the Type to be Robinson. c. Set the Distance Units to miles. 3. Create another view and
set the map projection to The World from Space a. Copy and paste themes from View1 to View3 in the same
manner outlined above. b. Set the map
projection properties Type to World from Space and the Distance Units to miles. 4.
Create another view and set the map projection to Mercator a. Repeat step
a above copying the themes into View4. b. Set the map
projection properties Type to Mercator and the Distance Units to miles. 5.
Compare the maps Arrange these View windows so you can see all of them and
compare the maps: Q: a. List each projection and describe what appears
distorted in each of the Views? Q: b. Are there obvious limitations to any of the map
projections? What are they? Hint: Read the ArcView help topics for a description
of each projection (Geographic, Robinson, World from Space and Mercator). 6. Find View3 (The World
from Space) and focus the map on Honolulu a. 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. 7. Change the focus of
View3 to Sydney, Australia (150.9 E, -33.8 S) 8. Change the focus of
View3 to Corvallis, OR (-123, 45N).
Q: a. Describe what changes occurred as you changed the
Central Meridian and Reference Latitude. Q: b. Explain why the longitude and latitude values are
negative and positive. Hint:
Again review the information presented as well as references presented
in this exercise to help you. 9.
Measure the distance from New York to Los Angeles in each view and record the
distances. a. Use the Zoom
In tool on the Tool
bar to zoom in closer to the continental US. Then using the Measure tool, also found on the tool bar, measure the
distance between New York and Los Angeles. With
the Measure tool, click on the point location of one city and then double click
on the point location of the other city.
In the status bar at the lower left corner of the project window, a
Length measurement will appear. Q: What is
your unit of measurement? __________________ Q: View1 _________ View2_________ Q: View3_________ View4_________ 10. Create another view (View
5) and select an equidistant map projection, such as Platte Carree, centered on
New York City (-74.1 W, 40.7 N) Q: a.
Measure the distance from New York to Los Angeles and record the value:
View5_________ Q: b. In
comparison with View5, which is an equidistant projection ,which of the four
Views above gave the "truest" distance? _________ Q: c. When would it be important to use a map projection
that preserves distance? d. Use the Zoom
to Full Extent button
tool to return to the full geographic extent of the themes. 11.
Using the drawing tool, draw several circles on View1 a. With View1 active, use the drawing tool to draw a circle inside one of
the 30x30 degree squares of the World30.shp theme. To do this, select the point drawing tool by clicking on the far right icon
in the tool bar (the icon with the dot in it). This will pull down the drawing tool menu. Select the icon with the circle. The mouse will now appear as crosshairs
in your view. Position the
crosshairs at the center of one of the 30x30 degree squares. Click and hold as you move the cursor
out towards the edge of the square to create a circle within it. The circle graphic will be selected
after you have created it. b. Copy and paste the circle into several more squares by
selecting Edit/Copy Graphics and then Edit/Paste. The
graphic will be pasted on top of the original. You must then select and move the graphic to another location
using the Pointer
tool. Position the circles at the
equator, mid-latitudes and poles and near the edges of your theme (be sure you
are zoomed out to the full extent).
You should have about eight (8) circles positioned throughout the theme. Q: c. Change the map projection to Mercator. What happens
to the circles?________________________ Q: d. Change the map projection to Robinson. What happens
to the circles? _______________________ Q: e. Change the projection to another of your choice.
List the map projection and describe what happens to the circles?
________________ Q: f. Which of the projections distorted the shape of the
circles most severely? ____________________ Q: g. When would it be important to use a map projection
that preserves shape/angles? _____________ 12. Change the map projection
to Mollweide and record the area of several circles a. Using the Pointer tool, select one of the circles near the equator and
record its area. You will need to
click on the edge of the circle, not inside to select that graphic. The area reading will appear in the
status bar at the lower left corner of the project window. Q: Area of circle near
equator______________ Q: b. Select a circle in the mid-latitudes and record its
area______________ Q: c. Select a circle near a pole and record its
area______________ 13. Change the map projection
to Behrmann; record the area of the same circles Q: a. Equator ______________ Mid-latitudes ______________
Pole ______________ Q: b. Are the areas of the circles the same as you
recorded above? __________ Why?___________ 14. Change the map projection
to Mercator; record the area of the same circles Q: a. Equator ______________ Mid-latitudes ______________
Pole ______________ Q: b. Are the areas of the circles the same as you
recorded above? __________ Why?___________ 15.
Change the map projection to another of your choice; record the area of the
same circles. List the projection:_____________________________________ Q: a. Equator ______________ Mid-latitudes ______________
Pole ______________ Q: b. Are the areas of the circles the same as you
recorded above? __________ Why?___________ Q: c. When would it be important to use a map projection
that preserves area? F. Exploring common map projections 16.
Using information available to you from
the ArcView help topics as well as this USGS web site (mac.usgs.gov/mac/isb/pubs/MapProjections/projections.html)
answer the following questions in your own words (DO NOT cut
and paste!). Q: a.
Define the following terms: 1. conformal 2.
equal
area 3.
equi-distant 4.
true
direction (equi-azimuthal) b. Some map
projections are used more frequently than others are and it is important to
know the properties of these different projections. Q: Using the resources referenced above answer the
following questions about the properties of The Globe and these projections:
Transverse Mercator, Azimuthal Equidistant (or Equidistant Azimuthal), Lambert
Conformal Conic, Albers Equal Area Conic and Stereographic. 1.
Describe
their general use and suitability for mapping 2.
Describe
the distortions inherent in each projection 3.
List
the method of projection for each one (conic, cylindrical, etc.) Adapted from: Copyright © 2000 Environmental Systems
Research Institute, Inc., Schools & Libraries Program
http://www.esri.com
Getting Started
A. Comparing map projections
B. Customizing a map projection
C. Map projections and distance
D. Map projections and shape
E. Map projections and area