The diagram below shows the result of the previous chapter. The zone is the high-risk area common to the 200-metre buffers around the three gas stations, and the highlighted parcels are those that could be affected by a leakage from any of the gas stations.

In this chapter, we will compute the area of the affected parcels, and this is done in two ways. In the first analysis, the area is computed from the parcels in vector format, which is the area sum of the parcels highlighted here. This is because the zone-over-vector polygon overlay analysis does not cut up the polygons into parts inside and outside of the zone.

In the second analysis, a more rigorous method is used to exclude those parts of the parcels not in the high-risk zone. This is done through a zone-over-zone analysis.

 

 

 

 

 

The First Analysis

Compute the area of the affected parcels in vector form.

1. Select the MapQuery > Polygon Area ... menu item to compute the total area for the current selection.

2. The display shows that the total area is 7.806 hectares. It is computed from the shaded area in the following diagram.

The Second Analysis

If we want to be more exact and wish to exclude those part of the parcels not within the high-risk zone from the area calculation, we would have to use a zone-over-zone overlay. This can be done by first converting these parcels into a zone and creating an intersection between this and the high risk zone.

Compute the area of the parcels within the high risk zone.

1. Select the Zone > Create > By Polygons ... menu item to convert the current selection into raster zones. Call this zone PARCEL.

2. Use the Draw > Zone ... menu item to draw the zone.

3. Select the Zone > Information ... menu item to compute the area of the PARCEL zone.

Point at the zone from which you need to obtain information.

NOTE :  We have created several zones over the same area. The first one selected might not be PARCEL. Keep clicking on OK until you have reached the proper zone.

Notice that the area is smaller than 7.806 hectares as computed previously using the vector polygons. This is due to the coarse resolution of five metres used for this zone.

4. Use the Zones > Operation ... menu item to find the intersection between the high-risk zone AIBIC and PARCEL to produce zone AIBICIP. The relation between the resulting zone and the parcels is shown below.

5. Use the Zone > Information ... menu item to compute the area of this zone. You will find that the area of the parcels within the high risk zone is 4.088 hectares.

What is the difference between zones AIBICIP and AIBIC ?