Aerial Imagery

• Issues
  • a big problem with most 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
  • R2V ($1500) is a package that supports this - see Image Warping and Geo-referencing Using R2V
  • Geographic Transformer ($800) is a lower-cost alternative
• USGS DOQ
  • the aerial image file format is called DOQ (Digital Orthophoto Quadrangle)
  • DOQ description: Digital Orthophoto Quadrangles
  • 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 sixteen DOQQs per USGS quad.
  • You can get DOQ from many places.  See the National Map Seamless Data Distribution System and terraservice.net (TerraServer-USA) 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, looks like this
       • easy to read
    3. ArcView-style header
       • another simple format (example)
• 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
  • you can use the USGS PhotoFinder interface to look for images
• gathering it yourself
  • for small areas, the practice of kite aerial photography might apply
    (see other musings on this subject)