Use NaN to signal lower level error

src/Terrain/TerrainQuery.cc

@@ -412,7 +412,7 @@ bool TerrainTileManager::_getAltitudesForCoordinates(const QList<QGeoCoordinate>
         if (_tiles.contains(tileHash)) {
             if (_tiles[tileHash].isIn(coordinate)) {
                 double elevation = _tiles[tileHash].elevation(coordinate);
-                if (elevation < 0.0) {
+                if (qIsNaN(elevation)) {
                     error = true;
                     qCWarning(TerrainQueryLog) << "TerrainTileManager::_getAltitudesForCoordinates Internal Error: negative elevation in tile cache";
                 } else {
@@ -421,7 +421,7 @@ bool TerrainTileManager::_getAltitudesForCoordinates(const QList<QGeoCoordinate>
                 altitudes.push_back(elevation);
             } else {
                 qCWarning(TerrainQueryLog) << "TerrainTileManager::_getAltitudesForCoordinates Internal Error: coordinate not in tile region";
-                altitudes.push_back(-1.0);
+                altitudes.push_back(qQNaN());
                 error = true;
             }
         } else {

src/TerrainTile.cc

@@ -121,13 +121,13 @@ double TerrainTile::elevation(const QGeoCoordinate& coordinate) const
         int indexLon = _lonToDataIndex(coordinate.longitude());
         if (indexLat == -1 || indexLon == -1) {
             qCWarning(TerrainTileLog) << "Internal error indexLat:indexLon == -1" << indexLat << indexLon;
-            return -1.0;
+            return qQNaN();
         }
         qCDebug(TerrainTileLog) << "indexLat:indexLon" << indexLat << indexLon << "elevation" << _data[indexLat][indexLon];
         return static_cast<double>(_data[indexLat][indexLon]);
     } else {
         qCWarning(TerrainTileLog) << "Asking for elevation, but no valid data.";
-        return -1.0;
+        return qQNaN();
     }
 }