Interpolating Nominal and Ordinal Data
In interpolating nominal or ordinal data, no direct single value can be calculated for the target polygons. Rather, for each target polygon, each unique data value from the intersecting source polygons must be recorded, along with the proportion of the target zone that each value occupies. This routine creates a separate data table related to the target polygons, which contains for each intersecting source polygon, the target polygon id, the nominal/ordinal value from the source polygon, and the percentage that source polygon occupies within the target polygon.
The percentage occurrence of each unique nominal/ordinal value within the target polygon is calculated as the area of each target/source intersection summed for each unique value, as a percentage of the whole target polygon.
Thus, for each Vs there is a corresponding Vt with
V% = Σ Ats / At * 100
Vs = value of the source polygon
Vt = value of the target polygon
V% = percentage of the target polygon occupied by a unique value
Ats = area of the intersection of the target and source polygons
At = area of the target polygon
Non-areal proportions require special treatment.
Consider the following coverages:
Assuming Vs Ats and At shown below, PARS calculates V% as follows:
|Polygon 1||1A||low||600||750||80 %|
|Polygon 2||2A||low||400||1250||32 %|
Source: modified from Ballard and Schut, 1995
- Date Modified: