How to detect changes between two GeoMedia polygon features

Detecting changes between two polygon vector features is a common workflow in GIS especially for forestry, and land use planning. For example, it may be useful to quantify the amount of forest cover changes (loss of vegetation, or vegetation growth) between two periods of time. GeoMedia provides the spatial analysis functions that can help to detect the changes provided the polygon vegetation features for the two periods of time are available. The example below illustrates how to use GeoMedia's Spatial Difference command to identify the loss of vegetation and vegetation growth.


Identify the loss of vegetation

1. In GeoMedia, select Analysis | Spatial Difference.
   
   The Spatial Difference dialog box appears.
   
   
2. In the From features in combo box, select the later vegetation polygon feature e.g. Vegetation2012.
   
3. In the Subtract features in combo box, select the earlier vegetation polygon feature e.g. Vegetation2002.
   
4. In the Output difference as Query name field, type in a meaningful string e.g. Loss of Vegetation since 2002.
   
5. Optional. Click the Style button and set a thick border color e.g. red.
   
6. Click OK.
   
   The areas representing the loss of vegetation since 2002 are displayed in red.
   
   

Identify the new vegetation growth

1. Select Analysis | Spatial Difference.
   
   The Spatial Difference dialog box appears.
   
   
2. In the From features in combo box, select the later vegetation polygon feature e.g. Vegetation2012.
   
3. In the Subtract features in combo box, select the earlier vegetation polygon feature e.g. Vegetation2002.
   
4. In the Output differences as Query name field, type in a meaningful string e.g. New vegetation growth since 2002.
   
5. Optional. Click Style and set a thick green border color.
   
6. Click OK.
   
   The new vegetation growth areas are displayed in dark green.