In this chapter, we will use zone analysis to determine the effect of gas tank leakage. The first analysis is similar to the one we did before, except that we will involve all gas stations this time. This is equivalent to finding affected parcels if the gasoline tanks in all gas stations leak at the same time. In the second analysis, we will find the parcels affected by a leakage from any of them. We will also change the model and assume that a leakage will affect an area of 200 metres around the contamination source.

The first analysis involves the following steps.

1. Retrieve from the database all gas stations.
2. Generate buffer zones around them to simulate the area affected by a leakage.
3. Overlay the zones and the parcels to find the affected parcels.

The second analysis involves the following steps.

1. Generate a buffer zone around each of the gas stations.
2. Find the area at the intersection of the buffers. This represents the area with the highest risk of being affected.
3. Overlay the intersection zone over the parcels.

 

 

 

 

 

The First Analysis

Find land parcels within 200 metres of any gas station.

1. Select the Query > Query Condition ... menu item from the DB Manager window.

2. Retrieve all properties with type equal to 203, which is gas stations.


3. There are three gas stations, and their locations are highlighted in the map below.

4. Select a pixel resolution of five metres.

5. Select the Zone > Create > By Corridor... menu item to create a 200-metre buffer zone, called ZONE_A, around the selected properties.

6. The following map shows the zone and the three gas stations.

7. Select the MapQuery > By Zone ... menu item to overlay ZONE_A upon the polygons.

8. The parcels within 200 metres of any gas station are highlighted.

 

The demonstration Spatial analysis: Finding the lots affected by underground gas tank leakage in Demo 2 shows how to perform a similar analysis.

What is the average assessment value of these properties.

Compute the average assessment value of these properties, excluding those with assessment values of zero dollars.