Fixed interpolation error.

rugged-core/src/main/java/org/orekit/rugged/core/dem/SimpleTile.java

@@ -188,8 +188,8 @@ public class SimpleTile implements Tile {
     public double interpolateElevation(double latitude, double longitude)
         throws RuggedException {
 
-        final double doubleLatitudeIndex  = getLatitudeIndex(latitude);
-        final double doubleLongitudeIndex = getLontitudeIndex(longitude);
+        final double doubleLatitudeIndex  = getDoubleLatitudeIndex(latitude);
+        final double doubleLongitudeIndex = getDoubleLontitudeIndex(longitude);
         final int    latitudeIndex        = (int) FastMath.floor(doubleLatitudeIndex);
         final int    longitudeIndex       = (int) FastMath.floor(doubleLongitudeIndex);
         if (latitudeIndex  < 0 || latitudeIndex  >= (latitudeRows - 2) ||

rugged-core/src/test/java/org/orekit/rugged/core/dem/SimpleTileTest.java

@@ -94,6 +94,19 @@ public class SimpleTileTest {
 
     }
 
+    @Test
+    public void testInterpolation() throws RuggedException {
+        SimpleTile tile = new SimpleTileFactory().createTile();
+        tile.setGeometry(0.0, 0.0, 1.0, 1.0, 50, 50);
+        tile.setElevation(20, 14,  91.0);
+        tile.setElevation(20, 15, 210.0);
+        tile.setElevation(21, 14, 162.0);
+        tile.setElevation(21, 15,  95.0);
+        Assert.assertEquals(150.5, tile.interpolateElevation(20.0, 14.5), 1.0e-10);
+        Assert.assertEquals(128.5, tile.interpolateElevation(21.0, 14.5), 1.0e-10);
+        Assert.assertEquals(146.1, tile.interpolateElevation(20.2, 14.5), 1.0e-10);
+    }
+
     @Test
     public void testOutOfBounds() throws RuggedException {