Simplify

Simplify simplifies the geometry of the selected items.

The OpenGIS Simple Features Specification contains rules for determining what can be classed as simple geometry (see below).

When SIS simplifies geometry it follows the OGC rules changing the original geometry as little as possible. As much as feasible, it also retains any data associated with the item.

Examples of when simple geometry may be required:

• When exporting data for use in other programs. For example, LineString items with self-intersections can cause problems in other systems.
  
• When carrying out Geometry Tests. Many of these (for example finding items which intersect with the selected item) rely on the items tested being simple.
  

First select the items to simplify; now choose Simplify (Edit > Clean/Simplify). This converts the selected item(s) to the OGC “simple” form (e.g by removing self-intersections).

Select Edit > Clean/Simplify.

According to the OpenGIS Simple Features Specification, the rules for simple geometry are:

• A Point item is always simple.
  
• A LineString item is simple if it does not cross or touch itself (except possibly at its end-points).
  
• A Polygon item is simple if its loops do not cross, and its interior is a single connected region. So a doughnut is simple but a Polygon with disconnected islands is not.

The rules for MultiLineString items are:

• All the components of the multi-item must be simple.
  
• The interiors of each component must not intersect any of the other components.
  
• In a simple MultiLineString every junction between component lines must have at least three paths leading out of it.
  
• All the Polygons of a simple MultiPolygon must only touch at points.
  

When you simplify an item, Cadcorp SIS Desktop 9.1 modifies its geometry to follow all the above rules changing it as little as possible.

Note: Using this command before exporting data which is to be used in other software programs will increase the chance of the data being read correctly by the other system.

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