In a previous chapter, we have used the MapQuery > By User Defined Area ... menu item to select polygons overlapping a user-defined area. You can perform the same operation by using raster zones. In this example, we will find the parcels overlapping a user-defined zone.

This method and the previous one produce identical results. However, the use of zones provides more flexibility - you can create zones with holes, and the zones can be stored away for future use.

In this example, we will use three steps to create a zone with a hole.

1. Create the outer zone.
2. Create the inner zone.
3. Subtract the inner zone from the outer zone to make a ring.

 

 

 

 

Create the outer zone.

1. Pick the Zone > Pixel Resolution... menu item to define a ground resolution of one metre.

2. Pick the Zone > Create > By User Defined Area... menu item.

3. It will be called ZONE_A automatically unless you have previously created a zone with this name.
4. Choose a colour for the zone.

5. Use the crosshair to enter the points for the outer zone.  Remember to close the polygon you can click C on the keyboard.  Enter Y on the keyboard or right click the mouse to accept this polygon.

(288788, 740274), 1:1000

6. A raster zone is created and drawn.

NOTE : The zone will cover the parcels underneath. You would have to turn off the Options > Display > Erase switch and refresh the screen again to show them.

Create the inner zone.

Follow the same procedure as in the previous exercise to create the inner zone. It will be called ZONE_B automatically.

NOTE :  When you are asked to define the outline of the inner zone, the polygon you have used previously will be highlighted. Enter N on the keyboard to reject it.

Subtract the inner zone from the outer zone.

1. Refresh the screen.

2. Pick the Zone > Operation ... menu item.

3. The Zone Operation panel shows that two zones, ZONE_A and ZONE_B, have already been created.  Click on th Zone Operation Panel below to view to entire panel.

4. Click on ZONE_A to select it as the first zone, and then click on ZONE_B to select it as the second zone. Turn on the Subtract operator. This will generate ZONE_C by subtracting ZONE_B from ZONE_A.  Click on the Zone Operation Panel below to view the entire panel.

NOTE If you have turned off the Options > Display > Erase switch previously, turn it back on.

5. Pick the Draw > Zone ... menu item. It shows that we have now three zones. Select ZONE_C and click on OK button .

6. Choose a colour.

7. The ring is drawn on the screen.