Geotools: Total area where GridCoverage has value x

Question

I'm working in Geotools with Java. So far, I have a GridCoverage2D and a List of Geometries. The GridCoverage2D is a digital elevation model, originating from a geotiff. Everything works fine till here.

Now I want to get the area for each polygon where the elevation has a certain value. For example for this Geometry, I want to know the total area where the elevation is 27 m. How can I achieve this?

I have no clue how to start :(

Two options I have in mind:

• Splitting the geometries in small parts (How do I do this), for each point get the center point (I can do this) and then evaluate this GridCoverage2D. That way I have a list with very small geometries, and an elevation corresponding to each geometry. Some array magic is all that's needed further. Is this a good plan, and how do I quickly split the geometry in small parts?
• Using a filter/query. Yet I don't know how that works and the tutorial isn't helping. Is it even possible to achieve what I want with a filter?

Answer 1

This turned out to be harder than I expected, which probably means I've missed something obvious. You can solve the question by creating a histogram of the coverage and then selecting the bin which contains your value. The all you need to do is multiply the total number of cells by the area of a cell.

So assuming you've read in or created a coverage:

 private double selectCells(GridCoverage2D cov, int value) {
    GridGeometry2D geom = cov.getGridGeometry();
    // cov.show();
    final OperationJAI op = new OperationJAI("Histogram");
    ParameterValueGroup params = op.getParameters();
    GridCoverage2D coverage;
    params.parameter("Source").setValue(cov);
    coverage = (GridCoverage2D) op.doOperation(params, null);
    javax.media.jai.Histogram hist = (Histogram) coverage
            .getProperty("histogram");

    int total = hist.getSubTotal(0, value, value);
    double area = calcAreaOfCell(geom);
    Unit<?> unit = cov.getCoordinateReferenceSystem().getCoordinateSystem()
            .getAxis(0).getUnit();
    System.out.println("which gives " + (area * total) + " " + unit
            + "^2 area with value " + value);
    return area * total;
}

private double calcAreaOfCell(GridGeometry2D geom) {
    GridEnvelope gridRange = geom.getGridRange();
    int width = gridRange.getHigh(0) - gridRange.getLow(0) + 1; // allow for the
    int height = gridRange.getHigh(1) - gridRange.getLow(1) + 1;// 0th row/col
    Envelope envelope = geom.getEnvelope();
    double dWidth = envelope.getMaximum(0) - envelope.getMinimum(0);
    double dHeight = envelope.getMaximum(1) - envelope.getMinimum(1);
    double cellWidth = dWidth / width;
    double cellHeight = dHeight / height;

    return cellWidth * cellHeight;
}

Obviously if you plan on calling it more than once you can cache the histogram and the cellsize.