We often need to use editing tools for digitising a new object such as a point, line or polygon feature. These are fairly straightforward to create, but one of the problems with polygons is that we sometimes need to create a new one alongside an existing one. Let us suppose we have a scanned map with two polygons we want to digitise.
If we were to digitise the new polygon (B) using the boundary of the existing one (A), this can sometimes produce areas of overlap from inaccurate digitising of the shared boundary. This could lead to problems where we have unwanted, additional sliver polygons, as shown in the image below.
An easy solution to avoid creating overlapping polygons is to use the Auto-Complete Polygon tool from the Editor Toolbar. To do this follow the simple process below:
- In the Editor Toolbar, set Task to ‘Auto-Complete Polygon’ (from Topology Tasks in the dropdown menu) and Target to the polygon layer that you want to add polygons to.
- Click once inside the existing polygon then use the Sketch tool to digitise the non-shared boundary (new Polygon B)
- Double click back inside the existing polygon (Polygon A) to finish. Alternatively, snap the edit sketch to the edge of the existing polygon.
The newly created polygon should now share the exact boundary of the first polygon.
It’s easy when you know how!
Posted by Mike Phillips, Senior Consultant, Exprodat.