Terrain amplification factors were measured for a variety of source positions (locations and heights) both upstream and downstream of two model hills, an axisymmetric hill and a two-dimensional ridge. The spatial variation of these terrain amplification factors was used to delineate the vertical and longitudinal extent of the areas where excess concentrations (terrain amplification factors greater than 1.0) occurred. For the axisymmetric hill, a region of 40% excess concentration was found to extend a maximum of 1.8 hill heights in the vertical, 14 hill heights upstream, and 10 hill heights downstream. For the two-dimensional ridge, this region of 40% excess concentration extended 2.2 hill heights in the vertical, 8 hill heights upstream, and 15 hill heights downstream. Maximum terrain amplification factors for both the axisymmetric hill and the two-dimensional ridge were found on the downstream side of the hills and had values of approximately 5.6 and 6.8 respectively.