Geometry Collections

A Geometry Collection is a collection of Point, LineString, Polygon and Text items. All the components are drawn with the same style and can be selected and manipulated as a single item.

If the Geometry Collection is a collection of Point items, it is called a Multi-point item.

If it is a collection of LineString items, it is a MultiLineString item. MultiLineString items can have branches and need not be connected.

If the Geometry Collection is a collection of Polygon items, it is a MultiPolygon item.

Creating Geometry Collections

Geometry Collections are created by merging the geometry of two existing items, by adding them together or by combining them using Boolean editing.

You can merge together Point items, Text items, LineString items and/or Polygon items. Existing Geometry Collections can be merged with other items, to create more complex Geometry Collections.

You cannot create Geometry Collections from topology items.

The 3D items TINs, surfaces, solids and grids can be used to make Geometry Collections, but in doing so, the original items lose their Z height.

Creating Geometry Collections using the Add Geometry command

  1. Select one of the items to be merged.
  2. Select Add Geometry (Create > Edit).
  3. Click on the item to merge with it.
  4. Continue clicking items to add.
  5. Press Escape when you want to end the command.

Creating Geometry Collections using the Boolean commands

The Analysis commands can be used to create new multi-geometry items by merging existing items in one of the following ways:

Union

Replaces the selected items with an item containing common geometry.

Intersect

Replaces the selected items with an item containing common geometry.

Subtract

Subtracts the geometry of the other selected items from one of the selected items.

Symmetric Difference 

Replaces the selected items with an item containing alternating geometry.

Dissolve 

Merges items based on attribute values.

Editing Geometry Collections

You can select a Geometry Collection by clicking on any of its components. The component items of a Geometry Collection can all be edited using the normal commands on the Alter menu.

Removing an item from a Geometry Collection

  1. Select the Geometry Collection.
  2. Select Remove Geometry (Create > Edit)
  3. Click on the component item to remove.

This command is still active until you press Escape, so you can remove further geometry from the Geometry Collection.

Breaking a Geometry Collection into its component items

  1. Select the Geometry Collection.
  2. Select Decompose (Create > Edit).
  3. The items remain selected but become individual items.