MapViewer 6: Using a Mask to Limit the Display of a Map

Sometimes you may want to display only a portion of a larger map. This is commonly and easily done by deleting the objects you don't want or by setting reduced map limits using the Map | Limits command.

However, there is another technique that is less well known than can be used and that is sometimes a better alternative. You can create a mask that obscures part of a map, while letting other portions show.

Think of a mask like a mat in a picture frame. The outer part of the mat covers up what's underneath. The hole in the middle of the mat allows that portion of the underlying picture to be seen.

A customer who called our tech support group recently wanted to make a hatch map of the counties of Alabama, but only see the hatch fills in a certain circular region. He still wanted to see the whole state, but only have a portion of the hatch fills visible.

How to make this map in MapViewer 6?

This map is made by creating three layers. The bottom layer contains the hatch map. The middle layer has the mask, which obscures parts of the hatch map layer. The top layer has the county outlines for the whole state. Since they are on top of the mask, they all appear.

The County Outlines layer is drawn on top of the Mask layer,
which partially obscures the Hatch Map layer.

Creating the hatch map and the county outlines base map are standard MapViewer operations. Nothing special was done on these layers. The only part that is more complicated is creating the mask on the middle layer. To create the mask shown in this example, do the following steps:

1. Make the Mask layer the active layer.
2. Use the Draw | Rectangle tool to create a rectangle surrounding the entire state.
    
   
   

   Draw a rectangle surrounding the entire state.

3. Use the Draw | Circle tool to draw a circle around the portion that you still want to see.
    
   
   

   Draw a circle around the part to see through the mask.

4. Select both the rectangle and the circle. Since they are the only objects on this layer, you can use Edit | Select All (Ctrl-A).
5. Convert the symmetric shape objects (the rectangle and the circle) into Area objects with the Boundary | Change Boundary Type | Symmetric Shape to Area command.
6. The two Area objects should still be selected after the previous command. Combine them into a single Area object with the Boundary | Combine Islands/Lakes command. The single Area object created is a rectangular shape with a circular hole.
7. Give the area object a solid white fill color by setting the Foreground color in the Fill Properties section of the Property Inspector.

This white mask with a circular hole creates the effect the customer wanted. Only a circular portion of the underlying hatch map is seen, while all of the county outlines are still visible.