Free and Open Source Software for Geomatics Conference FOSS4G 2010 Barcelona

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Currently a new regulation for nature preservation subsidy is implemented in the Netherlands. The new regulation involves the definition of “subsidy packages”, polygons indicating the agricultural preservation type. The subsidy calculation is based on both polygon area and preservation type. Subsidy polygons are defined using an underlying (crop)parcel basemap. A subsidy polygon can be equal in size and shape as a basemap parcel, but can also be a part of it.

Small water bodies were part of the parcel basemap, but according to EU law these small water bodies should not be part of the subsidy polygons. Unfortunately the subsidy parcels were already digitized when these new EU regulations were issued. The parcel basemap was corrected only recently and because of this, parcel areas became smaller.

So the challenge was how to adjust approximately 35000 digitized subsidy polygons. The areas of the subsidy polygons had to be moved, in such a way that shape and size remained the same as much as possible. Borders had to keep their border form. Because of the number of polygons, this movement had to be automated.

An iterative procedure has been developed for this problem. Each iteration involves applying a buffer, followed by an intersection with the underlying parcel, and a check on resulting area. The goal is that the resulting area is the close to the original area. After some iterations, making the buffer distances larger or smaller if necessary, the area will be about the same as the original area (provided that the original was not larger than the target parcel). Using a buffer with so-called miter joins, the shape of the digitized polygon also remains, more or less, the same.

This iterative process is library independant can be implemented using PostGis or a Geometry Library of choice.



Barend Gehrels - Geodan
Erik Van Der Zee - Geodan