G 424/524 GIS for the Natural Sciences
D. Percy
e-mail: percyd@pdx.edu

Spring Term 2006

Tissot's Indicatrix and the effect of projection on areal measurements

In this assignment you will examine the effect of different projections on a simple spatial object, the circle. You will also increase your understanding of using the ArcCatalog and Editor to create data. You will then digitize an object onscreen and observe the effect on its measurement of projecting it to various coordinate systems.

  1. Open Arc Catalog
  2. In the directory that you have chosen to do this exercise create a new shapefile of type polygon. Define the coordinate system to be WGS84. Add a field called area and one called perimeter, make them Type Double.
  3. In ArcMap add this newly created layer. Also, for context, add some counties so you can tell where you are.
  4. Turn on the Editor, and choose Start Editing. Be sure that the Target is set to your shapefile.
  5. Create a circle using the New Feature tool, use Advanced Editting to get to the Circle Tool.
  6. Stop editing.
  7. Use Toolbox->Data Management->Projects and Transformations->Features->Project to project your data to a local planar coordinate system, for example:
    1. NAD 1983 StatePlane Oregon North FIPS 3601 Feet Intl
    2. NAD 1983 UTM Zone 10N
  8. You will need to set a geographic transformation. Why?
  9. Observe the effect on the shape. Comments?
  10. Now add a raster layer and digitize a football field. This is chosen because you should be able to visualize the size of this feature. Feel free to substitute another feature of similar size.
  11. Use Toolbox->Data Management->Projects and Transformations->Features->Project to project your data to a variety of coordinate systems, including:
    1. NAD 1983 Oregon Statewide Lambert Feet Intl.prj
    2. NAD 1983 StatePlane Oregon North FIPS 3601 Feet Intl
    3. NAD 1983 UTM Zone 10N
    4. Alaska Albers Equal Area Conic
  12. North America Lambert Conformal Conic.
  14. In each case append a code to the filename that will help you keep track of what coordinate system it is in. Also make sure that you keep track of what HORIZONTAL UNITS each coordinate system uses, feet or meters.
  15. Compare the area and perimeter values that you get between NAD 1983 StatePlane Oregon North FIPS 3601 Feet Intl and NAD 1983 Oregon Statewide Lambert Feet Intl
  16. Similarly compare all of the values from the metric coordinate systems.
  17. Make a table comparing your results. You can find a formula for converting m^2 to feet^2 and then compare all values...

Look at the results, think about them, and discuss your confidence in these measurements.

Modifued Jan 8, 2006