Overlay shows the relationships between two existing features. OpenCities Map provides tools to perform a variety of overlay operations –you can find points that fall within polygons, lines that overlap polygons, and so forth. The result is a new layer in the Map Manager; a new Feature. When creating an overlay on two layers, a new layer is created containing the resulting graphical elements. In addition, the properties of both contributing layers are copied to the overlay layer. This allows you to edit and report on the overlay features using OpenCities Map tools like the Data Browser.

Only two layers can be overlaid at one time.

When one of the layers to be overlaid is a point layer or a linear layer, the Linear / Point operation section of the Topology Analysis dialog box is activated, as-shown below.

In the Linear / Point operation section, you must select the feature you want to keep in the overlay output layer: the points or lines, or the polygons.

There are three selection criteria for point/ line‐to‐area overlays: Inside, Outside, and Overlap.                                      

                                                     

These criteria may be enabled individually or in combination. Each type of operation will have different results.

                                    

Criteria	When Keeping Point/Line Layer	When Keeping Area
Inside	When keeping the points or lines, it returns all points or lines in the point/line layer that are contained entirely within the areas in the polygon layer.	When keeping the areas, it returns all areas in the polygon layer that contain at least one of the points or lines in the point/line layer.
Outside	When keeping the points or lines, this returns all points or lines in the point/line layer that are entirely outside the areas in the polygon layer.	When keeping the areas, it returns all areas in the polygon layer that do not contain any points or lines in the point/line layer.
Overlap For Points           For Lines	When keeping the points, it returns all points in the point layer that lie exactly on the boundaries of the areas in the polygon layer   When keeping the lines, it returns all lines in the line layer that overlap the boundaries of the areas in the polygon layer.	When keeping the areas, it returns all areas in the polygon layer with boundaries that lie on points in the point layer.     When keeping the areas, it returns all areas in the polygon layer with boundaries that overlap lines in the line layer.

 

Steps to perform Point/Line to Area overlay analysis:

 1) In the Map Manager, Right-click on the point layer and select Overlay

                                        

2) The Topology Analysis dialog opens, and the selected layer is automatically added to the ‘Layers to be overlaid’ list on the right side of the dialog box. Now use the arrow buttons to select the Polygon layer.

3) Once the Point and Polygon layers are selected, the Linear/Point operation will be activated. In the example shown below, the following options were set:

Keep: Lot_Shapes (Area Layer) – Select the layer for output geometry

 Inside: enabled

 Outside and Overlap: disabled

                                           

4) Type a name for the overlay in the Result field, and select the Ok button. The new Feature is added to the Map Manager Layers list.

                                  

Results are shown in this example – the Overlay Output layer in Grey has returned all the Lot_Shape containing the Block_Centroids.

                                                                          Data Browsers of individual layers                   

                                       

                                               Block_Centroid                                  Lot_Shapes Feature Properties

                                      

                                       After Overlay- Data Browser of the Output Overlay Feature with combined properties