Aerial Photographs and Remote Sensing Imagery

For much of the twentieth century, aerial photography has been an essential tool for mapmakers.  While maps had previously been made through ground surveys and painstaking calculations, the ability to view the landscape from the air revolutionized the craft.  The overhead perspective allows for the correction of errors made in ground measurements as well as being a way to observe landscape changes over time. 

Aerial photography is any photograph of the ground taken from the air.  This definition encompasses everything from nineteenth century photographs taken from balloons to modern satellite photography.   This guide focuses on photographs taken from airplanes for surveying purposes.  While satellite images of the globe are ubiquitous in Internet applications such as Google Maps and Bing Maps, historical aerial photograph collections are useful for  comparing how land has changed over time.

The most apparent advantage to aerial photographs is that they provide a different perspective from what we see on the ground. 

There are many potential uses for Aerial Photographs.  According to The Aerial Photo Sourcebook by Mary Rose Collins, uses for aerial photographs include agriculture, archaeology, banking, biology, cartography, government, economics, education, engineering, environment, forestry, geneaology, geology, history, insurance, law enforcement, legal research, military, planning, real estate, sports, transportation, and utilities. 

In a vertical aerial photograph, the camera is nearly perpendicular to the horizon.  Most survey photographs will be vertical.

Here is an example of a vertical aerial photograph of Washington, DC:

Image from the USGS Earth Observation and Science Center (http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/DOQs).

An oblique aerial photograph is any view of the ground other than a vertical view.

This example of an oblique aerial photograph shows Galveston, TX in September 2008, shortly after Hurricane Ike:

Taken from USGS Extreme Storms Oblique Photo and Video Surveys (https://maps.google.com/maps?q=http://coastal.er.usgs.gov/hurricanes/ike/post-storm-photos/20080915_Ike.kmz).

DOQs and DOQQs- Many of the aerial photos that you will work with are called DOQs and DOQQs, which stand for Digital Orthophoto Quadrangle and Digital Orthophoto Quarter Quads, respectively.  These are digital orthorectified photos produced by the USGS.  DOQs cover an area approximately 7.5 degrees of latitude by 7.5 degrees of longitude, and DOQQs cover an area approximately 3.75 degrees of latitude by 3.75 degrees of longitude.

A primer on working with DOQQs.

http://spatialnews.geocomm.com/education/tutorials/doq1/

Orthorectification- Orthorectification is the process of removing distortions in an aerial photograph by adjusting for topography, lens distortion, and camera tilt.