diff --git a/lib/map_boundary.rb b/lib/map_boundary.rb
new file mode 100644
index 000000000..f337c12f1
--- /dev/null
+++ b/lib/map_boundary.rb
@@ -0,0 +1,21 @@
+module MapBoundary
+   def check_boundaries(min_lon, min_lat, max_lon, max_lat)
+      # check the bbox is sane
+      unless min_lon <= max_lon
+        raise("The minimum longitude must be less than the maximum longitude, but it wasn't")
+      end
+      unless min_lat <= max_lat
+        raise("The minimum latitude must be less than the maximum latitude, but it wasn't")
+      end
+      unless min_lon >= -180 && min_lat >= -90 && max_lon <= 180 && max_lat <= 90
+        raise("The latitudes must be between -90 and 90, and longitudes between -180 and 180")
+      end
+
+      # check the bbox isn't too large
+      requested_area = (max_lat-min_lat)*(max_lon-min_lon)
+      if requested_area > APP_CONFIG['max_request_area']
+        raise("The maximum bbox size is " + APP_CONFIG['max_request_area'].to_s + 
+          ", and your request was too large. Either request a smaller area, or use planet.osm")
+      end
+    end
+end