Check population density around stops

src/main/java/com/conveyal/gtfs/error/SuspectStopLocationError.java

@@ -0,0 +1,31 @@
+package com.conveyal.gtfs.error;
+
+import com.conveyal.gtfs.validator.model.Priority;
+
+import java.io.Serializable;
+
+/**
+ * Indicates that a stop is in a suspect location, for example in a place like a desert where there are not enough
+ * people to support public transit. This can be the result of incorrect coordinate transforms into WGS84.
+ * Stops are often located on "null island" at (0,0). This can also happen in other coordinate systems before they
+ * are transformed to WGS84: the origin of a common French coordinate system is in the Sahara.
+ */
+public class SuspectStopLocationError extends GTFSError implements Serializable {
+    public static final long serialVersionUID = 1L;
+
+    public SuspectStopLocationError(String stopId, long line) {
+        super("stops", line, "stop_id", stopId);
+    }
+
+    @Override public String getMessage() {
+        return String.format(
+            "Stop with ID %s is in a sparsely populated area (fewer than 5 inhabitants per square km in any " +
+                    "neighboring 1/4 degree cell)",
+            affectedEntityId
+        );
+    }
+
+    @Override public Priority getPriority() {
+        return Priority.MEDIUM;
+    }
+}

src/main/java/com/conveyal/gtfs/storage/BooleanAsciiGrid.java

@@ -9,7 +9,7 @@
 
 /**
  * Loads an ESRI ASCII grid containing integers and allows looking up values as booleans (where > 0).
- * This is used for
+ * This can be used for a basic stop location test using a simple population raster.
  */
 public class BooleanAsciiGrid {
 
@@ -51,19 +51,22 @@ public BooleanAsciiGrid (InputStream inputStream, boolean gzipped) {
     }
 
     /**
-     * Get a grid for places where population density is over 5 people per square kilometer.
+     * Get a grid for places where population density is over 5 people per square kilometer in any neighboring cell.
      * 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.
+     * and all others a value of zero (since the no data value in the grid is -9999). Next, the GRASS r.neighbors tool
+     * in QGIS was used to blur this layer, adding to each cell the sum of the neighboring eight cells ("neighborhood
+     * size" of 3, the dimension in each direction). A final logical operation ("blurred@1" > 0) was applied, with the
+     * result 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
      */
     public static BooleanAsciiGrid forEarthPopulation() {
         try {
-            InputStream gridStream = BooleanAsciiGrid.class.getResourceAsStream("gpwv3-quarter-boolean.asc");
+            InputStream gridStream = BooleanAsciiGrid.class.getResourceAsStream("gpwv3-quarter-buffer-boolean.asc");
             return new BooleanAsciiGrid(gridStream, false);
         } catch (Exception ex) {
             throw new RuntimeException(ex);

src/main/java/com/conveyal/gtfs/validator/PostLoadValidator.java

@@ -6,7 +6,9 @@
 import com.conveyal.gtfs.error.GeneralError;
 import com.conveyal.gtfs.error.RangeError;
 import com.conveyal.gtfs.error.ReferentialIntegrityError;
+import com.conveyal.gtfs.error.SuspectStopLocationError;
 import com.conveyal.gtfs.model.Stop;
+import com.conveyal.gtfs.storage.BooleanAsciiGrid;
 
 import java.util.List;
 
@@ -33,6 +35,7 @@ public PostLoadValidator (GTFSFeed feed) {
     public void validate () {
         validateCalendarServices();
         validateParentStations();
+        validateStopPopulationDensity();
     }
 
     /**
@@ -46,6 +49,20 @@ private void validateCalendarServices () {
         }
     }
 
+    /**
+     * Validate that stops are not in locations with no people. This can happen from incorrect coordinate transforms
+     * into WGS84. Stops are often located on "null island" at (0,0). This can also happen in other coordinate systems
+     * before they are transformed to WGS84: the origin of a common French coordinate system is in the Sahara.
+     */
+    private void validateStopPopulationDensity () {
+        BooleanAsciiGrid popGrid = BooleanAsciiGrid.forEarthPopulation();
+        for (Stop stop : feed.stops.values()) {
+            if (!(popGrid.getValueForCoords(stop.stop_lon, stop.stop_lat))) {
+                feed.errors.add(new SuspectStopLocationError(stop.stop_id, stop.sourceFileLine));
+            }
+        }
+    }
+
     /**
      * Validate location_type and parent_station constraints as well as referential integrity.
      * Individual validation actions like this could be factored out into separate classes (PostLoadValidators)