National Ecological Framework (19 of 23)

Background

One of the principal functions of a GIS is spatial overlay, where information from one map is overlaid onto another map coverage, thus allowing the analysis of spatial relationships. Using this spatial linkage, it is possible to transfer information from one map coverage to another. This process is referred to as polygon to polygon interpretation.

Polygon to polygon interpretation starts with a set of known values for an attribute in one polygon coverage (the SOURCE coverage), and approximates these values for each of the polygons in another coverage (the TARGET coverage). Usually, the attribute values from each source polygon are assigned to a set of target polygons proportionate to their relative areas of intersection with the source polygon. This type of polygon to polygon interpolation is referred to as volume preserving polygon interpretation. The Polygon Attribute Reaggregation System (PARS) automates this process. PARS was developed for CanSIS in cooperation with Polestar Geomatics Inc. (Ballard and Schut, 1995).

The potential applications of PARS are many. It is useful wherever information has been mapped based on one set of polygons, and summaries or analyses are desired based on another set of polygons. For example, population stastiistics may be reported and mapped using census enumeration areas, but a summary of population is desired for a set of physical regions whose boundaries bear no relation to the census enumeration areas. Alternatively, the ranges and population densities of animal species may be mapped with one set of polygons, but summaries are desired by major ecological zones whose boundaries are not councident with the species maps. PARS can be used to re-aggregate these population sources within the selected target polygon coverages.

One important feature of PARS is the attention given to different types of source data: NONINAL, ORDINAL, INTERVAL and RATIO. These different data types require separate formulas for polygon attribute reaggregation. PARS handles all four data types.