Determining whether geographic point is within X meters of a state border (using shapefile for border data)

Question

So I'm writing a Java app, and I've got an ESRI Shapefile which contains the borders of all the U.S. states. What I need is to be able to determine whether any given lat/lon point is within a specified distance from ANY state border line - i.e., I will not be specifying a particular border line, just need to see whether the point is close to any of them.

The solution does NOT have to be very precise at all; e.g. I don't need to be dealing with measuring perpendicular to the border, or whatever. Just checking to see if going X meters north, south, east or west would result in crossing a border would be more than sufficient. The solution DOES have to be computationally efficient, as I'll be performing a huge number of these calculations.

I'm planning to use the GeoTools library (though if there's a simpler option, I'm all for it) with the Shapefile plugin. What I don't really understand is: Once I've got the shapefile loaded into memory, how do I check to see whether I'm near a border?

Thanks! -Dan

Answer 1

Assuming JTS for Geometry which is what is included in GeoTools:

public boolean pointIsClose( File file, Point targetPoint,double distance) {


  boolean ret = false;
  Map connect = new HashMap();
  connect.put("url", file.toURL());
  DataStore dataStore = DataStoreFinder.getDataStore(connect);


  FeatureSource featureSource = dataStore.getFeatureSource(typeName);
  FeatureCollection collection = featureSource.getFeatures();
  FeatureIterator iterator = collection.features();



  try {
    while (iterator.hasNext()) {
      Feature feature = iterator.next();
      Geometry sourceGeometry = feature.getDefaultGeometry();
      ret= sourceGeometry.isWithinDistance(targetPoint, distance );
    }
  } finally {
    iterator.close();
  }
  return ret;
}

The double number will have to come from the CRS which will define the units in which the calculation will be performed.

These are the geotools imports:

import org.geotools.data.DataStore;
import org.geotools.data.DataStoreFinder;
import org.geotools.data.FeatureSource;
import org.geotools.feature.Feature;
import org.geotools.feature.FeatureCollection;
import org.geotools.feature.FeatureIterator;
import org.geotools.geometry.jts.JTS;
import org.geotools.referencing.CRS;
import org.opengis.referencing.crs.CoordinateReferenceSystem;