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CS 128/ES 228 - Lecture 9b 1
Photogrammetry & Image Analysis
CS 128/ES 228 - Lecture 9b 2
Photogrammetry
Originally, the science (or art?) of interpreting aerial photographs
Stress on quantitative measurements
Now includes analysis of digital images from many sources
Image from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 3
A hierarchy of remote sensing
Satellite sensing
Aerial photography
Ground-truthing
Image from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 4
Perspectives
Vertical:- orthogonal perspective- planimetric map data
Oblique: - high oblique
(includes horizon) - low oblique (no horizon)
Image from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 5
Scale
Determine from: Plane altitude
RF = lens focal length altitude of plane
Known ground features
Top image from Avery. Interpretation of Aerial Photographs.Bottom images from Ben Meadows catalog (L), Olean NW DOQQ ®
CS 128/ES 228 - Lecture 9b 6
Problems
Plane altitude determining altitude (barometer, radar altimeter) variation among photos uneven terrain
Known ground features: need objects of known size & large enough for accurate
measurement, or pair of points for distance measure
CS 128/ES 228 - Lecture 9b 7
Planimetric view
Perfectly vertical (orthogonal) perspective
All features in correct horizontal positions
Impossible unless at infinite height
CS 128/ES 228 - Lecture 9b 8
The principle point
Point directly under camera lens (‘nadir’)
Elevated objects lean away from PP
Depressed objects lean toward PP
Causes image displacement
Images from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 9
Vertical relief causes displacement
Transmission line is straight - why does the line appear straight in one photo and jagged in second?
In left stereogram, line is ~ on nadir; in right stereogram, far from nadir
Image from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 10
Image displacement:
Source of error in horizontal locations, but
Permits estimation of feature elevations
stereoscopic parallax
Image from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 11
Stereoscopic photo pairs
Image from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 12
Stereoscopes
need pair of overlapping photos
different principle points results in parallax
used to create topographic contours
CS 128/ES 228 - Lecture 9b 13
Shadows
Need sun angle
Object must be vertical
Shadow must come from top and fall on level ground
H = L x tan(α)
H = L x tan(α)
Image from Avery. Interpretation of Aerial Photographs.\
CS 128/ES 228 - Lecture 9b 14
Rectification of aerial photographs
Rectification: process of geometric correction that turns an aerial photograph into a planimetric (map-like) image
Problems: lens distortion Earth curvature camera tilt terrain relief
CS 128/ES 228 - Lecture 9b 15
Rectification process
1. Scan aerial photograph at high resolution
2. Locate ground control points on scanned image: ≥3 for affine transformation ≥5 for rubbersheeting
3. Combine with DEM to correct relief displacement
4. Rectify to a ground coordinate system
CS 128/ES 228 - Lecture 9b 16
Relief distortion
Objects at different distances form the lens will be distorted
CS 128/ES 228 - Lecture 9b 17
Result: digital orthophotograph
USGS supplies in DOQ format
NYS GIS site provides freecolored infrared DOQQs
CS 128/ES 228 - Lecture 9b 18
Urban areas: building tilt
In urban areas, tall buildings seem to lean toward the principal point of the photograph
Corrected by building a DTM of each building
Permits virtual reality “flyovers”
Thorpe, A. Digital orthophotography in New York City. www.sanborn.com/Pdfs/Article_DOI_Thorpe.pdf
CS 128/ES 228 - Lecture 9b 19
Image Analysis
Identification of objects
Assigning attributes to objects or areas
Assessing the significance of patterns
Can be: Visual interpretation
Computer-assisted image analysis
CS 128/ES 228 - Lecture 9b 20
Landsat Images
Landsat 1-4 launched 1972 – ’82; expired
Landsat 5 & 7 launched 1985 & 1999; both operational
TM: thematic mapper. - 7 spectral bands- designed primarily for ES themes
http://landsat.gsfc.nasa.gov/project/L7images.html
CS 128/ES 228 - Lecture 9b 21
TM Applications
Band Spectral range (µm)
“Color” Application
1 0.45 – 0.52 Blue-green Soil/vegetation separation
2 0.52 – 0.60 Green Reflection from vegetation
3 0.63 – 0.69 Red Chlorophyll absorption
4 0.76 – 0.90 Near IR Delineation of water bodies
5 1.55 – 1.75 Mid IR Vegetative moisture
6 10.4 – 12.5 Far IR Hydrothermal mapping
7 2.08 – 2.35 Mid IR Plant heat stress
CS 128/ES 228 - Lecture 9b 22
Hydrology example
Images from Avery. Interpretation of Aerial Photographs.
CS 128/ES 228 - Lecture 9b 23
Terra (and EOS)
Terra launched
Carries 5 instruments; the MSS imager is called ASTER (from Japan)
14 spectral bands:- 3 VIS/near IR (15 m)- 6 short IR (30 m)- 5 thermal IR (90 m)
Images from www.nasa.gov
CS 128/ES 228 - Lecture 9b 24
ASTER spectral signature library
“Welcome to the ASTER spectral library, a compilation of almost 2000 spectra of natural and man made materials.”
http://speclib.jpl.nasa.gov
CS 128/ES 228 - Lecture 9b 25
Classification schemes
1.a Unsupervised: raw data analyzed for clusters
1.b Supervised: prior categories imposed
2. Classification of new data
3. Ground truthing … Lo & Yeung. Concepts and Techniques of Geographic Information Systems
CS 128/ES 228 - Lecture 9b 26
And that’s the fun part …