Interpolating Measure Data

In interpolating measure data, the area of the source polygon is irrelevant, but the area of the target polygon is used to arrive at the target value.

Formula

For measure data, the value for each target polygon is estimated by converting the measure values to their equivalent count values for each target/source intersection. These values are summed for each target polygon, and then converted back to a measure value by dividing by the area of the target polygon.

Vt = ( Σ Vs * Ats ) / At

where:

Vt = value of the target polygon

Vs = value of the source polygon

Ats = area of the intersection of the target and source polygons

At = area of the target polygon

Non-areal proportions require special treatment.

Example

Consider the following coverages:

Source polygons

Plygons A and B
Target polygons

Plygons 1 and 2

Target-Source intersections

Target-Source intersections 1A, 1B, 2A and 2B

Assuming As Ats and Vs shown below, PARS calculates Vt as follows:

Source values

Source values 1 m and 2 m
Target values

Target values 1.20 m and 1.68 m

target polygon portion Vs
(depth) (m)
Ats
area (ha)
Vs * Ats
(m * ha)
At
area (ha)
Vt
depth (m)
Polygon 1 1A 1 600 600 750 1.20
1B 2 150 300
Polygon 2 2A 1 400 400 1250 1.68
2B 2 850 1700

Source: modified from Ballard and Schut, 1995