从 GADM 获取国家级几何图形

Question

I'm using geotools to open gadm36.shp from gadm.org which shapefile containing worldwide administrative areas. I'm trying to get a single Geometry (eg org.locationtech.jts.geom.MultiPolygon) for each country. So if for example, there were 195 countries in this shapefile, I would have 195 Geometries. As a side note, I also have the GPKG file for the world so if using that is simpler, I'm happy to use that instead.

// load collection from shapefile
File file = new File("<PATH>/gadm36.shp");
Map<String, Object> map = new HashMap<>();
map.put("url", file.toURI().toURL());
DataStore dataStore = DataStoreFinder.getDataStore(map);
String typeName = dataStore.getTypeNames()[0];
FeatureSource<SimpleFeatureType, SimpleFeature> source = dataStore.getFeatureSource(typeName);

FeatureCollection<SimpleFeatureType, SimpleFeature> collection = source.getFeatures(Filter.INCLUDE);

That last line collection has a size of 339127. It seems to contain every sate, country, town, village, etc. How do I get a smaller list of just the countries?

Answer 1

Instead of Filter.INCLUDE you need to provide a filter that selects the features that you want. You'll need to play about a bit but if you just want countries then something like ENGTYPE_1 = 'Kingdom' or ENGTYPE_1 = 'Country' might do it.

The easiest way to build a filter is to add the gt-cql module to your project and use

Filter f = ECQL.toFilter("ENGTYPE_1 = 'Kingdom' or ENGTYPE_1 = 'Country'");
FeatureCollection<SimpleFeatureType, SimpleFeature> collection = source.getFeatures(f);

I'd also recommend using the geopackage rather than the shapefile as your filter will be passed to the database and you'll get the features back more quickly.

And as a final style note URLs.fileToUrl(file) is preferred over file.toURI().toURL() as it handles windows files and spaces much better.