Using source and target masks

PARS has the facility to use source and target masks to identify polygons of either coverage that should be excluded from the attribute reaggregation procedure. For example, when the target coverage contains water bodies as separate polygons, it may be appropriate to exclude those polygons when reaggregating attributes that are restricted to dry land, such as the number of hectares of farmland.


  1. The filter field should be a one character field in order to avoid potential problems with the polygon selection process.
  2. The use of masks increases the likelyhood of abandoned polygons.
  3. When a separate filter coverage will be used to determine the target area, it should be unioned with the target coverage before PARS is run.  If it is unioned with the source coverage, polygons may be split, and this will result in double counting of count values.

Source: modified from Ballard and Schut, 1995