The Indiana Geological Survey (IGS) has created the "Indiana
Historical Aerial Photo Index" (IHAPI) interactive map to facilitate the
identification and retrieval of historical aerial photographs. More than 950
large-format photomosaic index maps in the IGS archive were scanned and georeferenced,
and then mosaicked to produce 466 county-based images dating from the 1930s
to the 1980s. From these images, a point index was created showing the upper-right
corners of 113,035 individual historical aerial photographs. The interactive
map allows users to easily locate a site of interest and determine unique identification
numbers for individual photos. Copies of the photos can then be ordered from
various archival collections, including the IGS archive.
The scale of the photos is approximately 1:21,000 and, for reference, a scale bar of 1 mile has been superimposed on this example.
Beginning in the 1930s, aerial surveys of Indiana counties were conducted about every 10 years; between the 1930s and the 1970s, more than 100,000 aerial photos of the state were taken. Copies of many of these black-and-white photos exist in archives of the Indiana Geological Survey, the Indiana State Archives, and various libraries and governmental agencies. A concise discussion of the uses of aerial photos can be found here.
Every historical aerial photograph has a unique identification number (ID) in its upper right corner as illustrated in the figure on the right (click to enlarge the image). The date that the photo was taken appears in its upper left corner. Anyone interested in retrieving a particular photo from an archival collection must first determine its ID number.
For most counties, aerial surveys were conducted along north-south flight lines, although east-west flight lines were occasionally used. Along each flight line, photographs typically overlap (shown below) by as much as 60 percent, so that overlapping pairs of photographs can be viewed stereoscopically. Adjacent flight lines also overlap.
The graphic illustrates photographic overlap along flight lines and between adjacent flight lines.
Source: Air Photos 101 - Principles of Aerial Photography , Natural Resources, Canada.
For each survey, an index map of the county
was assembled by stapling the individual photos together to create a mosaic.
The mosaic, containing hundreds of individual photographs, was then re-photographed to create a large-format (typically 20 by 24 inch),
low-resolution photomosaic index. In the past, you could obtain
identification numbers of individual photos by physically visiting an archive and locating
a site of
interest on a paper copy of the photomosaic index. The appropriate individual aerial photo could then
be retrieved from the archival collection.
NOTE: The individual photos that are available from archival collections have resolutions that are much superior to that of corresponding photomosaics.
Contact archival collections to determine the availability of your particular photo of interest, as well as fee schedules for obtaining copies of the photo.