Thiessen weighting method for calculating estuary report card grades
The 2015 Tamar Estuary Report Card uses the Thiessen polygon method for deriving the grades for the five zones within the Tamar River estuary as shown in the report card.
The Thiessen polygon method is frequently used for creating an environmental data time series for a spatial area where a set of point measurements are available at non-uniform distances from one another. One example of its application is in calculating a ‘catchment rainfall’ time series based on a set of observation points both within and just outside the catchment. The approach creates a weighted average of the observation points where weights are calculated based on a ‘region of influence’ for each point. This region of influence is essentially the area falling closer to that point than any other.
In the case of the Tamar Estuary report card the ‘polygon’ calculation is undertaken in one dimension only. The estuary zones are conceptualised as a portion of a straight line based on distance from the estuary mouth as shown in the Figure below.
T1 to T5 are the observation points, with T1 to T4 inside the zone and T5 falling outside the zone, but in this case closer to some parts of the zone that T4. The red lines indicate the midpoints between the observation points (numbered M1 to M4). These lines indicate ‘region of influence’ of each of the observation points. Note that the region of influence of T5 extends into Zone 1 (see M4) even though this observation lies outside the zone.
The value for Zone 1 is then calculated as a weighted average of values from T1 to T5 based on these ‘regions of influence’ as follows: