I validated the Google Street View based viewshed and found the viewsheds were correct to about 350-425 meters. After that point the images in Google Street View became too pixilated to reliably view the location where the image terminated. In the left image, the area in red highlights the area I could not see in the Goolgle Images but could when in person.
In the example to the right I could see to about 425 meters to the north and the south, beyond that point it became too blurry in the images. When I went to the location, the terrain was very flat and I was able to see approximately 2,000 meters to the south and make out the highway underpass and overpass with the aid of ten power binoculars. Looking to the north the image probably extended at least 2,000 meters but became too hazy to tell where the view terminated.
It should be noted that the distance limitation attributed to the Google Street View images may be too conservative but it was furthest I felt comfortable extending the viewshed based on the pixilation and discernable features in the imagery. Also for the purpose of these viewsheds I am most concerned about being able to discern person or vehicle sized objects on the ground, so I ignore if I can see the tops of trees or buildings.
Most viewsheds utilize digital terrain models (DTM) as the basis for what can and cannot be seen from any one location. DTMs are usually synonymous with a digital elevation model (DEM) and show the bare earth of an area. I have heard these viewsheds be described as “showing the best possible circumstances.” However, because the DTM does not take into account vegetation or man made structures the results can lead viewers to believe more is visible than reality.
If possible a digital surface model (DSM) should be used, which shows elevations on the top surfaces of buildings, vegetation, man made structures and any other object elevated above the bare earth. However, DSMs are usually collected with specialized equipment such as light ranging and detection (lidar) systems or interferometric synthetic aperture radar (IFSAR) systems.
If these options are not available to a user, manual viewsheds can be created by either going out to the field or by examining panoramic photos to see what is visible. This information is then saved to a GIS for later use. The purpose of the manual viewshed is to find the best locations to place potential sensors, collect the capabilities of existing sensors, verify the quality of viewsheds created otherwise and/or to validate conceptual models.
By using Google Street View I was able to create a manual viewshed. While viewing the panoramic view I switched back and forth between Google Earth and ArcGIS which utilized the 2008 NAIP imagery as a baselayer to assist in the digitization of the viewshed. Because Google Earth does not give dates of its imageryit is beneficial to view the location with multiple sources of imagery such as local.live.com‘s bird’s eye view in case the panoramic imagery and the GIS base layers have significant differences.