8 Digital Boundaries with multiple parts

Another situation that you will come across when using digital boundary datasets is when you have a single geographical region that is made up from 2 or more individual geometry parts.

Geometry Parts

Image 20: Geometry Parts

You can see this above. Here we have a single census Output Area 35UCGE0005, the geographical extent of which is held as multiple individual polygons. This is an area of the Northumbrian coastline in north-east England with the small polygons to the top right of the map being small sparsely populated islands in the North Sea. Rather than each island being assigned an Output Area of it`s own the islands are grouped together with a larger Output Area located on the mainland.

As a single multi-part geometry record:

Image 21: As a single multi-part geometry record:

Or alternatively each part can form a record in its own right.

Or alternatively each part can form a record in its own right.

Image 22: Or alternatively each part can form a record in its own right.

This situation where a single geographic region has multiple geometry occurs frequently within UK digital boundary datasets. You will often see this happening around the coastline – Scotland with all of it`s islands is particularly good for this. The same thing will happen when geographical regions are split by other natural features such as rivers. Sometimes though (particularly in statistical boundaries) the regions are deliberately split. This occurs throughout census Output Areas where a single Output Area may be broken into a number disjoint parts.