For land trusts, adding an aerial photo image to a project can dramatically improve the usefulness of GIS data.  Good aerial photos can provide details about structures, vegetation and other features that can be crucial in understanding a property or a landscape.

UNDERSTANDING AIR PHOTOGRAPHS AND GIS

For use in GIS, a photograph must be registered to specific places on the earth, a process called georeferencing.  In addition, to create accurate imagery - images that truly represent the surface of the earth - a photograph must be orthorectified.  Orthorectification is a process where an image is adjusted to correct for terrain displacement and camera tilt - the resulting photographs are called orthophotographs, or orthoimagery.  Orthophotographs have real-world coordinates and have a uniform scale, so they are good base images to use in a GIS.

Aerial photographs can be created from airplanes flying set routes, or from satellites orbiting the earth. Generally, photographs from airplanes are higher resolution, but for land trusts satellite photos are becoming very good now and are usually cheaper.  Some photos are simply pictures, others have "bands" of data that can be analyzed in special software - for land trusts, basic photo images are usually sufficient.

For creating maps that show a somewhat larger area (one square mile or more), photography that has a 1-meter (3-foot) pixel resolution is usually sufficient. For a close up view of a parcel, one-foot or even six-inch pixel data may be needed. Prices go up as resolution increases.

Once you have an orthophoto, you can usually just add it as a layer in GIS software.  However aerial photographs can sometimes be troublesome and may require more advanced knowledge to use effectively.

ACQUIRING AERIAL PHOTOGRAPHY

The major options for most land trusts are as follows:

• Many local and state public agencies have low cost or free aerial photographs available for download or purchase on a DVD - check your local government.
  
• The USGS has a large store of black and white and color aerial images (DOQs, Digital Orthophoto Quadrangles) – these are one-meter resolution images, often a number of years old (up to 10 or more), but they are inexpensive and widely available. Each covers about 5 miles on a side, or about a quarter of a USGS quad map and is about 50MB in size. They can be placed in GIS software and overlaid with data, or they can be printed directly on large or small paper sizes and used for display or mark up.  You can see online versions at TerraServer USA, and you can order them as GIS data on CD/DVDs at GIS Data Depot. 
  
• For more general information about aerial imagery see the USGS National Map site, a terrific resource.
  
• Commercial imagery is available from a number of vendors including AirphotoUSA, Globe Explorer and SpaceImaging. 
  

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