Digital Elevation Models
Topographic Mapping – Old School
Surveying Instruments
• Stadia Rod
– Distance and elevation measurement
– Interval between crosshairs gives distance
– Elevation on rod gives elevation relative to
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
View Through Sight
Installing Bench Marks
Aerial Photographs
• Altitude variation during flight
• Camera tilt (Doesn’t always point straight
• Scale varies across photograph
• Scale varies with elevation
• Foreshortening toward edge of picture
• Parallax shift with altitude
• Lens distortion
• Atmospheric distortion
• Use overlap of aerial photos to view photos in
• 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
• Operator views images through a
• 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
• Determine parallax and calculate x,y,z
Digital Elevation Models
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
• DEM coverage of Mars and Venus is as good as
• Aerial photographs with parallax and scale
distortions removed
• Analog methods are modified from
– 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
• Full scale production began in 1991, peaked in
• Now nearly complete
• DOQ = Digital Orthophoto Quadrangle
How Orthophotos are Made
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

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