Segments to polygons

Functionality

The Segments to Polygons operation automatically polygonizes a supposedly error free segment map. The operation is designed to be used after you have imported vector files from another package. All segments in the segment map must be connected to other segments or to themselves (islands) by nodes; dead ends, self overlap and/or intersections are not allowed. A mask can be specified to polygonize specific segments.

A polygon is a list of a linked segments representing the (complete) boundary of an area. Provinces, soil units, cadastral lots and other mapping units can be stored as polygons. Polygon maps are generally used as a stepping stone to raster maps.

A segment map may contain different classes or IDs representing different 'layers'. From each segment class name/ID and from sets of class names/IDs, you can create different polygon maps. By selecting a mask the user specifies the segment class names/IDs that are to be used to create a polygon map.

Topological polygonization vs. non-topological polygonization:

When using the Segments to Polygons operation, you can choose whether use topological polygonization or non-topological polygonization.

You should select topological polygonization when:

Before the segments will be polygonized, they are first checked on dead ends, self overlap and intersections, see below.

You should select non-topological polygonization when:

Only segments with a single node will be polygonized.

Tip:

You can check the structure of the segment map for instance by:

  1. Open the segment map you wish to check in the Segment editor;
  2. Select a segment:
  3. Check for some other segments as well.

  4. Go to Move Points mode and zoom in a bit.
    Take a node and move it a bit away, then redraw the map:
  5. When the checks and criteria in steps 2 and 3 are true for the same thing (either topological or non-topological), you can draw the conclusion.
  6. When you have the non-topological structure of 1 node per segment, the segment check Closed Segments in the segment editor should not give any errors.
  7. Don't forget to Undo All Changes in the segment editor; then close the editor.

Segment check during topological polygonization:

Prior to the actual topological polygon creation, segments are checked for possible topological errors. Segment checking is carried out in three steps:

Optionally, you can use the Auto Correction option to automatically remove any errors that may be encountered during the Check Segment phase:

Erroneous segments will only be deleted from a temporary copy of the input segment map, not from the input segment map itself.

If Auto Correction is not selected, the program will stop with an error message when a topological error is encountered.

Note:

As the user has no control over the Auto Correction procedure, it is advised to polygonize segments within the Segment editor where you can correct errors yourself.

Naming the polygons:

Polygons can be named using different methods:

Input map requirements:

No special requirements for the input segment map.

If the Label Points option is selected, the point map with the labels and the segment map should use the same or a compatible coordinate system. When the coordinate systems of the input maps are different (e.g. different projections) but compatible, the point coordinates will be transformed to the coordinate system of the segment map.

Domain and coordinate system of output map:

Domain of output polygon map:

The output polygon map will use the same coordinate system as the input segment map.

See also: