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Copyright © John Lindsay, 2015

Readings

Jensen Chapter 4, pg. 124-125

Image Acquistion

• There exists an enormous archive of earth imagery both with respect to historical aerial photography and satellite imagery.
• Usually held by federal/provincial/state repositories.
• In Canada, the record extends 80 yrs back.
• Hard-copy vs. scanned digital vs. digital
• Varying cost of purchase.

SWOOP Images

• Southwestern Ontario Orthophotography Project (SWOOP) 2010
• 0.3 m pixel resolution orthophotos & 2-10 m DEM
• Access through the Data Resource Centre
• Free access for UofG

National Aerial Photo Library

• Other Canadian AP’s through the National Aerial Photo Library of Canada, Ottawa
• http://airphotos.nrcan.gc.ca/prod_e.php
• About $15 for a contact print and $30 for a pan scanned digital image (600 dpi)
• A typical project area could be 30 photos in size = $900.

When is archive data unsuitable?

• If existing data is out of date
• Wrong season
• The wrong scale for use
• Poor quality
  • Might be due to degradation of film, poor weather/atmospheric conditions, unsuitable film type/spectral bands, etc.

When is archive data unsuitable?

• In these situations, you must contract the acquisition of new imagery


• This is always more expensive than purchasing archived data

New image acquisition—things to remember

• Timing—usually a very small window
• Scale (flying height and focal length)
• Required percent overlap between photos
• Flightline (number, orientation)
• Film type
• Camera type and characteristics
• Aircraft capabilities

Time of Day Considerations

• When the sun angle is less than 30° long shadows can obscure valuable information.


• Can be very useful for interpreting terrain though.


• When the sun angle is quite high hot spots can be present...

Hot Spots

• Unusually bright spot on photo.
• Pronounced over water and forest.
• Occurs when sun angle is greater than about 52°.

Weather Considerations

• Excessive humidity can cause image degradation due to scattering.
• Cloud cover also degrades images.
• Best to acquire after a front goes through.
• Rain washes the particulates out of the atmosphere
• Strong winds cause problems too...

Drift and Crab

Flightlines over a project area

Flightlines over a project area

• Must start by determining the mission constraints:
  • Photo scale
  • Camera focal length
  • Film format (e.g. Large format)
  • Image overlap

Photo Mosaic

• Multiple overlapping air photos are 'stitched' together
• Traditionally done by hand now usually done using software

Photo Mosaic

Types of Mosaics

• Index mosaics
  • Usually prepared for the purpose of providing an index to individual photos in a series
• Uncontrolled mosaics
  • Uses best fit but because of tilt of aircraft between exposures and topographic displacement, these mosaics are not reliable
• Controlled mosaics
  • Use ground control points and rectified imagery