Another interesting extension of this method is to overlay satellite imagery over 3D DEM renderings. This powerful technique can yield impressive images and useful results. However, several problems must be solved for its application. A big problem for the amateur is finding decent satellite imagery of the area of interest at low cost. Remotely sensed data from space has been kept a close secret by the United States and foreign governments for many years. After a trickle of data started to become available within the past two years, we are on the verge of an unprecidented proliferation of satellite data of all kinds. At present, high quality commercial data is mostly too expensive for the amateur, costing hundreds or even thousands of dollars per image.

The second problem is proper alignment of the satellite image with the DEM (georeferencing). Much of what is available for free has no reference data such as latitude and longitude or UTM coordinates to allow for easy registration to the DEM.

Another problem is the obliquity of the images. Usually the camera's focal plane normal and the earth's surface normal do not coincide. If uncorrected (rectified), this introduces distortions when trying to overlay onto a flat DEM base image.

The fourth major problem is lighting. Terrain features in an optical wavelength satellite image throw shadows that only make sense from one viewpoint and with the sun at one position. This must be carefully considered when composing the composite image. One trick is to position the light source in your rendering software in the same position that the sun was in when the photograph was originally taken.

That being said, I obtained an acceptable image from the excellent NASA Gateway to Astronaut Photography of Earth of the Glens Falls, NY region, which includes the target Salem NY area. This picture was taken from the Space Shuttle probably during the late fall, which explains the lack of much green foliage. After rotating, sizing, layering, cropping, and color adjusting, I produced the image shown at right. The technique is identical to that described above for superposition of the topo map, except for the obstacles mentioned above. The target Salem, NY area is enclosed within the white rectangle at left center. Manchester, VT appears at the lower right of the image immediately to the east of Mt. Equinox. The source satellite image is shown below.

Another source to check for satellite imagery is the NGA Raster Roam. Click on the 10m DOIs and check for coverage in your area of interest. More on satellite imagery in subsequent sections.