Photogrammetry
Digital Elevation Models
Orthophotographs
Topographic Mapping – Old School
Surveying Instruments
• Stadia Rod
– Distance and elevation measurement
– Interval between crosshairs gives distance
– Elevation on rod gives elevation relative to
observer
Stadia Rod
Surveying Instruments
• Transit
– Record data in field for later analysis
• Alidade
– Used for direct plotting in field
• Plane Table
– Used in conjunction with Alidade
– Plot distance and elevation directly on rough map
Laser Ranger
Retroreflector
View Through Sight
Installing Bench Marks
Aerial Photographs
• Altitude variation during flight
• Camera tilt (Doesn’t always point straight
down)
• Scale varies across photograph
• Scale varies with elevation
• Foreshortening toward edge of picture
• Parallax shift with altitude
• Lens distortion
• Atmospheric distortion
Photogrammetry
• Use overlap of aerial photos to view photos in
stereo
• Correct photos for camera angle and altitude
• Parallax shift determines altitude
Kelsh Stereoplotter: Concept
Kelsh Stereoplotter in Practice
Analog Stereoplotter
Analytical Stereoplotter
• One step below complete automation
• Photos scanned digitally
• Digital images corrected for camera angle and
altitude
• Operator views images through a
stereoviewer
• Joystick used to maneuver
• Results stored directly as digital file
Digital Photogrammetry
• Not feasible until 1980’s when computers had
sufficient speed and memory
• Match features on photos by recognition
routines
• Determine parallax and calculate x,y,z
Digital Elevation Models
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Derive from existing maps and survey data
Derive from radar or laser ranging
All field-derived data are irregular
Need to generate grid of points
Need DEM’s to generate modern
orthophotographs
• DEM coverage of Mars and Venus is as good as
Earth
Orthophotographs
• Aerial photographs with parallax and scale
distortions removed
• Analog methods are modified from
photogrammetry
– Instead of scribing a contour line, expose a patch
of the images on film
– First invented by Germans and French in 1930’s
– Russell Bean of USGS invented a method in 1955
– Systematic production began at USGS in 1965
– Analog methods used until early 1980’s
Digital Orthophotographs
• Begun in 1980’s when computers finally had
enough memory and speed to handle the load
• USDA wanted base maps for soil mapping;
contributed funding for development and
production
• Full scale production began in 1991, peaked in
1999
• Now nearly complete
• DOQ = Digital Orthophoto Quadrangle
How Orthophotos are Made
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Computer recognizes locations on photograph
Control points on ground for location accuracy
Elevation provided by DEM
Not entirely like digital photogrammetry
Image generally overlaps latitude-longitude
bounds by 50-300 meters
• All use NAD 83 and Universal Transverse
Mercator projection