Trip Discovery - Equal-Angle Grid

TripCellRed.setup.java

    @Override
    public void setup(Context context)
    {
        // first pull values from the configuration     
        Configuration config = context.getConfiguration();

        int minutes = config.getInt("com.esri.trip.threshold", 15);  //minutes stoppage delineating trips
        threshold = minutes * 60;  // minutes -> seconds

        double gridSide = 1000.;   // Nominal/average/target length of side of grid cell (meters)
        String sizeArg = config.get("com.esri.trip.cellsize", "1000");
        if (sizeArg.length() > 0 && sizeArg.charAt(0) != '-') {
            double trySize = Double.parseDouble(sizeArg);
            if (trySize >= 100)  //likely unrealistic smaller than about 200m to 500m
                gridSide = trySize;  // input as meters
            else if (trySize > 0)
                gridSide = 1000 * trySize;  // input as km
        }

        String featuresPath = config.get("com.esri.trip.input");
        FSDataInputStream iStream = null;
        spatialReference = SpatialReference.create(4301);  //  GCS_Tokyo

        try {
            // load the JSON file provided as argument
            FileSystem hdfs = FileSystem.get(config);
            iStream = hdfs.open(new Path(featuresPath));
            country = EsriFeatureClass.fromJson(iStream);
        } 
        catch (Exception e)
        {
            e.printStackTrace();
        }
        finally
        {
            if (iStream != null)
            {
                try {
                    iStream.close();
                } catch (IOException e) { }
            }
        }

        // build the grid of cells
        if (country != null) {
            envelope = new Envelope();
            country.features[0].geometry.queryEnvelope(envelope);
            buildGrid(gridSide);
        }
    }

    private void buildGrid(double gridSide) {   // Nominal length of side of grid cell (meters)
        double cellArea = gridSide*gridSide;
        latMax = envelope.getYMax() + .005;  // +.005 to catch nearby outliers
        latMin = envelope.getYMin() - .005;  // -.005 to catch nearby outliers
        final double latMid = (latMax + latMin) / 2;  // idea: latitude of half area (ctry or envp?)
        latExtent = latMax-latMin;
        lonMin = envelope.getXMin() - .005;  // -.005 to catch nearby outliers (approx. 500m)
        lonMax = envelope.getXMax() + .005;  // +.005 to catch nearby outliers
        final double lonOneDeg = lonMin + 1;  // arbitrary longitude for establishing lenOneDegBaseline
        Point fromPt = new Point(lonMin, latMid),
            toPt = new Point(lonOneDeg, latMid);
        // geodesicDistanceOnWGS84 is an approximation as we are using a different GCS, but expect it
        // to be a good approximation as we are using proportions only, not positions, with it.
        final double lenOneDegBaseline = GeometryEngine.geodesicDistanceOnWGS84(fromPt, toPt);
        // GeometryEngine.distance "Calculates the 2D planar distance between two geometries."
        //angle// final double lenOneDegBaseline = GeometryEngine.distance(fromPt, toPt, spatialReference);
        arcLon = gridSide / lenOneDegBaseline;  // longitude arc of grid cell
        final double latOneDeg = latMid + 1;
        toPt.setXY(lonMin, latOneDeg);
        final double htOneDeg = GeometryEngine.geodesicDistanceOnWGS84(fromPt, toPt);

        int enough = (int)(Math.ceil(.000001 + (lonMax-lonMin)*lenOneDegBaseline/gridSide)) *
            (int)(Math.ceil(.000001 + latExtent*htOneDeg/gridSide));
        grid = new ArrayList<double[]>(enough);
        double xlon, ylat;
        // If using quadtree, could filter out cells that do not overlap country polygon
        for (ylat = latMin, yCount = 0;  ylat < latMax;  yCount++) {
            fromPt.setXY(lonMin, ylat);
            toPt.setXY(lonMin+arcLon, ylat);
            double xlen = GeometryEngine.geodesicDistanceOnWGS84(fromPt, toPt);
            double height = cellArea/xlen;  // meters
            double arcLat = height / htOneDeg;
            for (xlon = lonMin, xCount = 0;  xlon < lonMax;  xlon += arcLon, xCount++) {
                double[] tmp = {xlon, ylat, xlon+arcLon, ylat+arcLat};
                grid.add(tmp);
            }
            ylat += arcLat;
        }
    }