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

• 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• 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