Prior to the year 2000, DVRPC's aerial imagery consisted of mylar aerial photo enlargements or "atlas sheets". These atlas sheets were produced from 9x9" aerial photos. The imagery dates from the years 1959, 1965, 1970, 1975, 1980, 1985, 1990, & 1995. The 1959s and 1965s primarily cover the urbanized portion of the DVRPC region (the DVRPC region is made up of nine counties: Bucks, Chester, Delaware, Montgomery, and Philadelphia in Pennsylvania; Burlington, Camden, Gloucester, and Mercer in New Jersey). Subsequent years provide full coverage of the region, minus the occasional missing scan. In order to increase the efficiency of using the historical aerial imagery, the sheets were scanned into TIFF (Tagged Image File Format) files. Each TIFF file ranges between 35- 40MB in size. Unlike DVRPC's more recent aerial imagery (2000 and later), the historical aerials are not "orthorectified" or "orthocorrected". In other words, they are simply aerial images with no spatial reference or uniform scale.Through the process of georeferencing, Montgomery County GIS assigned a spatial reference which will enabled them to be used more readily in a GIS environment. That said, georeferencing is not orthorectifying or orthocorrecting. What it does allow is for the scan to be displayed relative to other spatially referenced GIS layers. A georeferenced scan does not have the properties of an actual orthoimage. Whereas an orthoimage can be used for making accurate measurements, a georeferenced image cannot, as it does not have the spatial accuracy and uniform scale of an orthoimage.
Montgomery County provides this data as a free and open resource.While every reasonable effort is made to ensure the accuracy and completeness of the data, Montgomery County makes no representations about the suitability of this data for any purpose. All data is provided "as is" without warranties of any kind, either express or implied, and it should not be construed or used as a legal description.