Areal Weights with Thiessen Polygons
In the Thiessen method station weights are determined in proportion to their representative areas defined by a polygon. The polygons are formed from bisectors of the station interconnections around the stations and/or the catchment boundary. Let the area of the polygon around station X~i~ be denoted by A~i~ and let the total catchment area be A~c~ , then the station weight is: A~i~ /A~c~ . The sum of station weights do necessarily add up to 1.
The polygons are computed based on the catchment boundary contour and the station locations. The catchment boundary is a characteristic of a Catchment which is selected from the catchment list box. The catchment boundary data can be entered via <Catchment data> under <Entry & editing>. Note that the geographical, or local meter system, co-ordinates of the location of the stations have to be available or have to added to the station characteristics.
If for a time step at one or more of the point rainfall stations an observation is missing then automatically a missing value will be entered for the areal rainfall.
The combination of point rainfall stations and weights to compute the areal rainfall are stored in the database to be used at a later stage for different time periods.
Series Codes
Area Series can be selected by selecting a series in the series list box and pressing the Select button. Point Series are selected by selecting a series in the series list box and pressing the '>>' button. To deselect a point series, select the series in the spreadsheet and press '<<'. By selecting more series at a time, multiple series can be added to the point series list box. By clicking your right mouse button on top of the Series list box you can select or de-select all series.
Select Catchment
The Catchment listbox contains all catchments in the database. A catchment can be selected by clicking on the catchment name in the list box. The Reference co-ordinates in latitude/longitude co-ordinates are given in the appropriate boxes.
The co-ordinates of the catchment will be used in the computation, make sure the selected catchment has co-ordinates stored in the database. A maximum of 10.000 co-ordinate points can be used in the computation.
Projection of Map-layer
When executing the Thiessen function a choice must be made to make a Thiessen polygon in local meter co-ordinates or Latitude/Longitude co-ordinates. If your background map is in a local meter co-ordinate system make sure the Local meter co-ordinate system option button is selected.
Save Relation
When pressing <Execute> the relation is saved in the database. For the Thiessen function the first two characters of the saved relation are 'TH'. For Areal Weights these are 'AW'.