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.

Formula

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%	=	Ats / At * 100
where:
	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

Example

Consider the following coverages:

Source polygons	Target polygons
Target-Source intersections

Assuming V_s A_ts and A_t shown below, PARS calculates V_% as follows:

Source values (Vs)	Target values (Vt)

target polygon	portion	Vs	Ats area (ha)	At area (ha)	V%
Polygon 1	1A	low	600	750	80 %
	1B	high	150		20 %
Polygon 2	2A	low	400	1250	32 %
	2B	high	850		68 %

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Source: modified from Ballard and Schut, 1995 by Peter Schut
Contact: Head, CanSIS