Creating Cannibalised Shapes (removing areas of overlap)

Are you interested in seeing and storing a catchment that has been cannibalised by another catchment? This tip will help you to create a new shape that can be stored and analysed. This is useful when wanting to investigate an area, without data from another catchment.

To create this new shape, you will need to know the schema name and the names of the overlapping shapes.

Note: To have the two overlapping drivetimes appear on your map, make sure you have them tagged in a browse before dragging onto your map.

To create the cannibalised shape:

Open a map and find the schema that contains at least one of your two shapes on the left-hand data tab

• Right click on the schema and select Add Map Layer | Add Layer…

• Use the drop down next to Layer Type to select Area

• Click on the Select… button next to Geometry Expression

• Delete the current expression from the Expression box at the top of the window.

• In the Expression box use the following:

SUBTRACT(SHAPE1.'KEY1':MASTER.BOUNDARY, SHAPE2.'KEY2':MASTER.BOUNDARY)

Where:

SHAPE1 = The name of the Schema that holds Shape1

KEY1 & KEY2 = The names of the overlapping shapes. These can be found by opening a browse on the schema that contains the saved DriveTimes

SHAPE2 = The name of the Schema that holds Shape2

MASTER.BOUNDARY = Is the geographical boundary column. This does not usually require any change

Note: When typing the shape name, it needs to match exactly. To find the name of a schema, right click on it in the Data view and select Show Names

• Select OK, and then click Create. This will add a layer to the map of the residual shape of the first catchment after the second catchment’s overlap has been cut away

When using this expression, the resulting shape will show the effect of cannibalization on the first shape. The order that the shapes are referenced is key, the second shape referenced will be subtracted from the first.

For example:

SUBTRACT(SHAPE_1.'CHIS030':MASTER.BOUNDARY, SHAPE_1.'BRIX030':MASTER.BOUNDARY)

Map Description automatically generated

Storing the new shape:

• Set the layer you just added as the Focus Layer

• Click on the shape on the map, select Copy within the Selection Shape section of the Maps tab

• Select Store - found in the same location.

• Give the new shape a Key and a Description

• Choose the Schema in which to store the new shape & select Ok

The area will now appear as a row in the schema you saved it to. If you already had a browse of this schema open when you saved the new shape, you will need to refresh it (from the Home ribbon).

Data can be added to this browse in the usual way.