jeospatial

Jeospatial is a simple geospatial point database library for Java. It aims to provide an easy-to-use and reasonably-high-performance set of tools for solving the k-nearest-neighbor problem on the earth's surface.

Geospatial point databases in this library are implemented using vantage point trees (or vp-trees), which are a data structure that performs binary space partitioning on a metric space. Construction of a vp-tree executes in O(n log(n)) time and searches against the tree execute in O(log(n)) time. As a practical point of reference, it takes about two seconds to construct a vp-tree that contains roughly 30,000 geospatial points on a 2007 MacBook Pro, and about two seconds to execute 10,000 searches against that tree (for a search throughput of about 5,000 searches/second). By contrast, it takes between 300 and 400 milliseconds to sort a list of those 30,000 points by distance from a query point (for a search throughput of 2-3 searches/second).

Major concepts

The two most important interfaces in the jeospatial library are in the com.eatthepath.jeospatial package.

The GeospatialPoint interface defines a single point on the earth's surface; concrete implementations of the GeospatialPoint interface can be found in the com.eatthepath.jeospatial.util package.

The GeospatialPointDatabase interface defines the behavioral contract for classes that index collections of GeospatialPoints and provide facilities for performing nearest-neighbor searches among those points. The VPTree and LockingVPTree classes are both concrete implementations of the GeospatialPointDatabase class and can be found in the com.eatthepath.jeospatial.vptree package.

Examples

Finding nearest neighbors

Let's say we have a list of all of the zip codes in the United States and we want to find the ten closest zip codes to some point in the world (let's say Davis Square in Somerville, MA, USA). We might do something like this:

// Load a bunch of zip codes from a file and construct a vp-tree from // those points List<ZipCode> zipCodes = ZipCode.loadAllFromCsvFile(); VPTree<ZipCode> pointDatabase = new VPTree<ZipCode>(zipCodes); // Pick a query point (Davis Square in Somerville, MA, USA) SimpleGeospatialPoint davisSquare = new SimpleGeospatialPoint(42.396745, -71.122479); // Find the ten nearest zip codes to Davis Square List<ZipCode> nearestZipCodes = pointDatabase.getNearestNeighbors(davisSquare, 10);

The nearestZipCodes list will have ten elements; the first will be the closest zip code to Davis Square, the second will be the second closest, and so on.

Finding all neighbors within a certain radius

Assuming we have the same set of zip codes, we might want to find all of the zip codes that are within a fixed distance Davis Square. For example:

// Find all zip codes within ten kilometers of Davis Square List<ZipCode> zipCodesWithinRange = database.getAllNeighborsWithinDistance(davisSquare, 10 * 1000);

The zipCodesWithinRange list will contain all of the zip codes -- sorted in order of increasing distance from Davis Square -- that are within ten kilometers of Davis Square.

Finding neighbors by other search criteria

Assuming we still have the now-familiar list of zip codes, we might want to find the closest zip codes to Davis Square that are outside of the state of Massachusetts. We could do that using the SearchCriteria class:

// Specify search criteria that matches anything outside of the state of // Massachusetts SearchCriteria<ZipCode> searchCriteria = new SearchCriteria<ZipCode>() { @Override public boolean matches(ZipCode zipCode) { return !zipCode.getState().equals("MA"); } }; // Find the ten closest zip codes to Davis Square that are outside of // Massachusetts List<ZipCode> closestOutsideMA = database.getNearestNeighbors(davisSquare, 10, searchCriteria);

The closestOutsideMA list will contain the ten closest zip codes to Davis Square sorted in order of increasing distance.

Finding points inside a bounding box

If you're working with a section of a map with a cylindrical projection (e.g. a Google or Bing map), you might want to find all of the zip codes that are visible in that section of the map. A set of bounding box search methods comes in handy here:

// Find all of the zip codes in a bounding "box" List inBoundingBox = database.getAllPointsInBoundingBox(-75, -70, 43, 42);

As might expected, the inBoundingBox list contains all of the zip codes that fall between the longitude lines of -75 and -70 degrees and the latitude lines of 42 and 43 degrees. Other variants of the getAllPointsInBoundingBox method allow for sorting the results by proximity to some point and applying additional search criteria.

Acknowledgements

License

jeospatial is an open-source project provided under the BSD License.