PhotogrammetryDigital 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