Historically, aerial photos were captured by either large organization or
small companies with access to an airplane. The US government made many of
its aerials freely available, both georeference (DOQ) and not georeferenced
(NAPP). There were large companies like AirPhotoUSA that gathered
and licensed aerials. Since around 2005, most aerial companies have been
merged into global providers like
which handle both aerial and space imagery.
- The biggest challenge with most historical aerial photos is that they
- Also, they often have non-uniform warping due to
perspective and elevation effects.
- The photos have to be warped to match a known,
georeferenced basis, a process often called "rubber sheeting" because
the image is stretched like a sheet of rubber.
- Full rectification, on terrain that isn't flat, requires a DEM to
account for the elevation parallax. This is a catch-22 if you are
trying to create a DEM at the same time you are trying to rectify an aerial.
- R2V ($1500)
is a package that supports this - see
Image Warping and
Geo-referencing Using R2V
- Geographic Transformer ($800) is a lower-cost alternative.
Mapper ($400) is even more affordable.
- the older USGS aerial image file format is called DOQ ("Digital
- format documentation:
Orthophoto Standards (pdf)
- sometimes they are in GeoTIFF format, but more commonly, they are
enormous grayscale JPEG files with a .DOQ extension (or .COQ,
"compressed ortho quad")
- In general, each 7.5" USGS quad is covered by 4 3.75" DOQs at around
1 m/pixel. A DOQQ is one quarter of a DOQ, which in turn covers one quarter of a
USGS quad. So there are usually sixteen DOQQs per USGS quad.
- You can get DOQ from many places. See the
Map described on the page
Getting USGS Data.
- there is a small amount of overlap between each DOQ
- for georeferencing purposes, the 4 corners of the image are
indicated with small crosshairs
- a solid crosshair for NAD83 and a dotted crosshair for NAD27
- geographical information for each DOQ is usually contained in a
- there are several kinds of header files for DOQ, all plain ASCII,
all with a ".hdr" file extension
- "old header format"
- usually around 25k in size
- no newlines, so it's hard to read
- contains the crosshair corner locations as pixel values (!)
- "new header format"
- much shorter, less information, easy to read
- ArcView-style header
Aerial Photography Program) and NAHP
- a browsable mono and color infrared image database for the
- the images are not online and not digitized, you have
to order print/film copies of them
- coverage is not good, e.g. it's missing the entire state of Hawai`i
- More recently (2009-) the USGS has made
some aerial imagery available via its
NationalMap, but the
interface is difficult and only recently high-resolution orthoimagery is
available for a very few areas