Aerial Imagery

• see also: Commercial Sources of Aerial/Satellite Imagery with Global Coverage
• see also: Collecting Your Own Aerials

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 DigitalGlobe which handle both aerial and space imagery.

Georectification

• The biggest challenge with most historical aerial photos is that they lack georegistration.
• 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.
• Global Mapper ($400) is even more affordable.

USGS DOQ

• the older USGS aerial image file format is called DOQ ("Digital Orthophoto Quadrangle")
• format documentation: Digital 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 National 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 "header file"
• there are several kinds of header files for DOQ, all plain ASCII, all with a ".hdr" file extension
  1. "old header format"
     • usually around 25k in size
     • no newlines, so it's hard to read
     • contains the crosshair corner locations as pixel values (!)
  2. "new header format"
     • much shorter, less information, easy to read
  3. ArcView-style header
     • another simple format

USGS NAPP (National Aerial Photography Program) and NAHP

• a browsable mono and color infrared image database for the continental US
• 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

Recent USGS

• 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

• gathering it yourself
  • for small areas, the practice of kite aerial photography might apply, or RC minihelicopters..
    (see other musings on this subject)