>> Whenever you deal with national scale data for any country with coastline, >> you frequently end up with an absolutely gigantic and horrifically complex >> single polygon which depicts the coastline and all the rivers throughout the >> country as a single continuous edge. This mega-polygon, so often present and >> so often necessary, is very time-consuming for gdal_polygonise to produce >> and the result is very painful for every GIS geometry package to handle. >> >> It would be great if the people behind gdal_polygonise could put some >> thought into this extremely common situation for anyone working with country >> or continent scale rasters to make sure that it is handled well. It has >> certainly affected us a great deal when working with data at up to 2m >> resolution for a country larger than the UK... >> > > This second use case is a very bad mismatch to the design of the polygonize > algorithm, as you have discovered. If this is indeed a common use case (I > have no basis to judge one way or the other), a very different algorithm > would be far more appropriate. In many respects, the desired algorithm would > also be easier to implement if the nature of the data is basically binary - > in the country/region/polygon, or out. Matters get more complicated if we > allow holes, disconnected regions, or have multiple regions to identify (but > a small number). > > Perhaps one of the core project members could describe how to build a case to > support this need and submit an enhancement request.
Hi David, thanks for the reply. It's probably worth noting that there are analogous situations for countries without coastline. Consider a national map raster for a landlocked country, which has been masked to remove neighbours. You could get a giant polygon surrounding the country representing the complexity of all the border as a single polygon. It's not quite so fiendish as the example I gave above, but it can happen pretty easily. Also, while I appreciate the value of checking commonality of use cases, there is also an alternative way of viewing the issue: Does GDAL have the features and behaviour that are needed for national scale map work, or is it a 'town and county' type of project? There is a hidden danger in relying on how commonly something occurs as a use case: if a piece of software currently can't support a use case or supports it incredibly poorly, then by definition there will be few (likely zero) people using it for that purpose. Graeme. _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
