Investigating disparate CRS's

Overview

This activity is designed to help students gain experience with on-the-fly transformation. Specifically, it is designed to show how desktop GISystems “mask” some processes. This is often convenient for the user, but can produce unexpected results without proper knowledge of what’s going on behind the scenes.

This activity is designed to be guided by the instructor without students having access to it. Since much of this activity involves contemplation, prediction, and evaluation, it’s better if they don’t blast ahead by filling in all of the blanks and ignoring instructions.

Activity

In this activity, we will explore a feature common in modern desktop GISystems: on-the-fly projection (or on-the-fly transformation). We will work with two shapefiles: one with a projected coordinate system, and one with a geographic coordinate system.

  1. Download and unzip the on_the_fly.zip file. Inspect the coordinate reference system (CRS) of co_geo by either (a) observing the .prj file or (b) by adding the file to a GIS map document and investigating the CRS through dialog boxes. What is the CRS?

  2. View the extent of the x,y data, and add the dataset to the map document if you didn’t do this in the previous step. What are the minimum and maximum x,y values?

Now, we will inspect some information about co_proj, the second shapefile, independent of co_geo. It is important that you do not simply add the shapefile to the map document at this phase. First, close out of the GIS that you are currently using, and restart it.

  1. Now view co_proj‘s’ CRS and minimum x,y values. What are they? How are they different from those of co_geo?

  2. Now, add co_proj to the map. Clearly these are similar datasets but with a different CRS’s and different minimum and maximum x,y values. What do you think will happen when both are added to the map at the same time? Will the features be aligned? Why or why not?

  3. Now, add co_geo to the map and ensure that the co_proj layer is on top of co_geo. Are the features aligned? What happened?