src/main/java/com/conveyal/gtfs/storage/BooleanAsciiGrid.java
public static BooleanAsciiGrid forEarthPopulation() {
        try {
            InputStream gridStream = BooleanAsciiGrid.class.getResourceAsStream("gpwv3-quarter-boolean.asc");
            return new BooleanAsciiGrid(gridStream, false);
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }
public static BooleanAsciiGrid forEarthPopulation() {
        try {
            InputStream gridStream = BooleanAsciiGrid.class.getResourceAsStream("gpwv3-quarter-boolean.asc");
            return new BooleanAsciiGrid(gridStream, false);
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }
Get a grid for places where population density is over 5 people per square kilometer.
We use the Gridded Population of the World v3 data set for 2015 UN-adjusted population density.
This data set was downloaded at 1/4 degree resolution in ESRI ASCII grid format. The grid file was edited
manually to eliminate the no-data value header, since I could not find a way to operate on no-value cells in the
QGIS raster calculator. Then the raster calculator in QGIS was used with the formula ("glds00ag15@1" > 5),
which makes all cells with population density above the threshold have a value of one,
and all others a value of zero (since the no data value in the grid is -9999). This was then exported as another
ASCII grid file, which zips well. The license for this data set is Creative Commons Attribution.
See http://sedac.ciesin.columbia.edu/data/collection/gpw-v3