Line Simplification Filtering

This reduces detail quickly and efficiently by only using the most extreme points that define a line's shape. The degree of simplification is determined by a user-defined tolerance value.

Can be used in conjunction with Island Size Filtering, but not with Line Delta or Angle Change Filtering.

Parameters:

Tolerance value. Values are entered in map units. These are usually metres or kilometres.

Operations:

This filtering method uses the Douglas Peuker Standard Algorithm for line reduction.

This starts by considering the entirety of the line. The start and end points, termed the anchor and floater points, are treated as defining a straight line. The perpendicular distances of the other points from this line are measured. If all the points lie within a pre-specified tolerance, the end points are taken to represent the whole feature i.e. the line, once simplified, becomes a straight line.

Otherwise the floater point is stored and the most distant point becomes the new floater. The new floater splits the original, unsimplified line into two parts, each of which will be treated in the same way as the original. The algorithm continues to work recursively in this way until the simplified line is made of straight-line segments i.e. the perpendicular distances of all points on the original line fall within the specified tolerance of the simplified line.

To apply Line Simplification Filtering to a Layer:

• Select Layer | Filtering... to bring up the Filtering page
• Tick the Line Simplification Filter Apply checkbox
• Then supply a tolerance value using one of the following methods:
• Enter a value directly into the edit box
• Press Evaluate to scan the currently loaded layer and suggest values for the tolerance. Then press Transfer to transfer the suggested values into the parameter box
• Press Suggest to evaluate a value for the tolerance and transfer it to the parameter box in one operation
• Press OK to accept the changes