Interpolating Count Data

In interpolating count data, the area of the target polygon is irrelevant, but the area of the source polygon is used in the calculation of the target value.

Formula

For count data, the value for each target polygon is estimated from the value of each intersecting source polygon multiplied by the area of intersection as a fraction of the area of the source polygon.

Vt = Σ Vs * Ats / As

where:

Vt = value of the target polygon

Vs = value of the source polygon

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

As = area of the source polygon

Example

Consider the following coverages:

Source polygons (As)

Plygons A and B
Target polygons .

Plygons 1 and 2

Target-Source intersections (Ats)

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

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

Source values (Vs)

Source values 100 cows and 200 cows
Target values (Vt)

Target values 90 cows and 210 cows

target polygon portion Vs
(cows)
Ats
area (ha)
As
area (ha)
Vs * Ats / As
(cows)
Vt
(cows)
Polygon 1 1A 100 600 1000 60 90
1B 200 150 1000 30
Polygon 2 2A 100 400 1000 40 210
2B 200 850 1000 170

Source: modified from Ballard and Schut, 1995