Introduction to Polygon Attribute Reaggregation System (PARS)
One of the principal functions of a GIS is spatial overlay, where information from one map coverage 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 interpolation.
Polygon to polygon interpolation 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 interpolation. The Polygon Attribute Reaggregation System (PARS) automates this process.
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 statistics 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 coincident 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: NOMINAL, ORDINAL, MEASURE and COUNT. These different data types require separate formulas for polygon attribute reaggregation. PARS can handle all four data types.
Another important feature of PARS is the function which permits the selection of a subset of polygons within the target map coverage. By default, PARS interpolates new values for every polygon in the target map coverage that has an intersecting source polygon. However, in some cases it may be desirable to assign new values only to a specific area within the target coverage. For example, with a source coverage of census enumeration areas containing total population counts, and an ecological zone map as the target map coverage, it makes sense to assign population numbers to only the land based ecological zones, and not to the lake or ocean ecological zones. PARS has a function which adjusts the way in which values are interpolated depending on whether a subset of target polygons is selected. A similar filter can be used to restrict which part of the source coverage is used by PARS.
Source: Ballard and Schut., 1995
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