Transcript Chap 10 - Faculty

Visual interpretation
Chapter 10
• Visual interpretation in important to GIS
development and application.
• It is the interpretation of aerial
photography by a photo interpreter.
– Regarded as somewhat useful but oldfashioned likely to be replaced by digital
analyses
• Visual interpretation is now seen in the
new context.
– Skills that a photo interpreter uses are applied
equally to photographs as well as digital
images
– So a new name has come into use that of
image analyst or image interpreter
• In addition, remote sensing data has
become more detailed, much like
photography
• Finally, computer science is not evolved to
the point where image analysis can be
done completely automatically
Characteristics of aerial imagery
• Visual interpretation is used to extract
information from fine resolution imagery at
scales of 1:40,000 or larger
– And includes almost all aerial photography
and fine scale satellite imagery (e.g. IKONOS)
– Excludes coarse resolution imagery produced
by satellites such as Landsat
Characteristics of aerial imagery
• GIS requires the following conditions be met
– Geometric errors removed
– Detailed spatial information is contained
– Can provide taxonomic details such as separation of
corn from wheat
– Spatial relationships can be examined such as
vegetation distribution and water bodies
– Patterns of change from one day to the next are
monitored
– Equipment and experienced staff are available
– Historical phenomena can be examined such as landuse change
Sources of imagery
• Two common sources
– Archival imagery previously acquired for
another purpose.
– Imagery acquired specifically for the project at
hand
Archival imagery
• Several sources are available.
– USGS
– US National Archives and Records
Administration
– State agencies
• Department of Transportation
• Department of natural resources.
• Such imagery must often be bought
Archival imagery
• In most cases, computerized databases
permit online searches.
• Many states update photography on a
regular basis.
– These are useful in examining change
Custom acquisition
• Imagery is acquired, according to the user
specifications.
– The date scale emulsion coverage and other
characteristics are determined by the user.
• The disadvantages include
– high cost
– requirement for planning well in advance.
– effort devoted to preparing and monitoring the
specifications
– delays and cancellations due to weather or equipment
malfunctions
Custom acquisition
• Users should prepare a contract or
statement of work
– Specifies in detail the products and services
required from the firm with respect to cost,
deadlines, and products to be prepared
• Contract is a means for controlling the
completeness and quality of the imagery
Custom acquisition
• The photo interpreter may participate in
making decisions about scale, date, time
of day, deadlines, choice of film, and
coverage
Custom acquisition
• Photo interpreter should ask:
– Will imagery be adequate for the purpose of
the project?
– Is the film emulsion satisfactory for the task?
– Is the season suitable?
– Will planned time of day provide appropriate
shadowing?
– In stereophotography, what will the overlap
be?
Elements of image interpretation
• Image analysis requires explicit recognition of
eight elements of image interpretation that
form the framework and understanding of an
image
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Shape
Size
Tone
Texture
Shadow
Site
Association
pattern
Shape
• The outline of a feature
– Important to note that shape depends on
perspective
– Overhead perspective, introduces scale effect
Size
• The first to the dimensions of a feature
• Relative size determined by comparing
the object with familiar nearby features
• Absolute size refers to the use of the
aerial image to derive measurements
Tone
• Refers to the average brightness of an area or,
in the case of color imagery, to the dominant
color of the region
– Depends on the nature of the surface in the ankles of
observation and illumination.
– Smooth surfaces behave like specular reflectors,
they tend to reflect radiation in a single direction
• These features may appear bright or dark
– Rough surfaces behave this diffuse reflectors.
• Scatter radiation in all directions.
• A peer is medium gray tones
Texture
• Refers to the variation in tone over a
surface or the apparent roughness of the
surface as seen in the photo
• Created by micro shadows in small
irregularities in the surface.
Shadow
• Refers to large distinctive shadows that
revealed the outline of a future as
projected onto a flat surface.
– Depends on the nature of the object, angle of
illumination, perspective, and slope of the
ground surface
Site
• Refers to a futures position with respect to
topography and drainage.
– Some things occupy a distinctive topographic
position because of their function
• Sewage treatment facilities at the lowest feasible
topographic position.
• Power plants located adjacent to water for cooling
Association
• Association refers to the distinctive spatial
interrelationships between features
– Schools often associated with athletic fields.
– Large parking lots often associated with malls
Pattern
• Refers to distinctive arrangement of
features
– Orchards have trees plant can rows
– Mobile home parks have rectangular buildings
arranged in rows
Image interpretation tasks
• Classification - Assign objects to classes
– Detection, recognition, identification.
– Interpreter confidence is ranked as possible,
probable
• Enumeration - Count items
• Measurement- mensuration
– Uses image scale to derive measurements
such as length with distance and volume
• Delineation – demarcation of regions
Interpretation strategies
• Field observation.
– Required when the image and its relationship to
ground conditions are imperfectly understood
• Direct recognition.
– Interpreter derives information directly from inspection
of the image
• Inference
– Based on visible features can derive information
about invisible features
– Certain relationships and identify features
Interpretation strategies
• Interpretive overlays.
– Useful when relationships between visible patterns
are used to reveal patterns not directly visible.
– Soils may be revealed by relationships with
vegetation patterns, slope, and drainage
• Photomorphic regions
– Identify regions of uniform appearance on an image.
– Does not attempt to resolve individual landscape
components.
• Mostly used with small-scale imagery in which coarse
resolutions tend to average separate components
Interpretation strategies
• Image interpretation keys.
– Keys are reference information designed to
enable rapid identification of features.
– Usually a collection of annotated images or
stereograms and a description, which may
include sketches or diagrams
Aerial mosaics and image maps
• IMAP is characterized by the
planimetrically accurate representation of
your surface and uses symbols to
represent select features
• An aerial image is not really a map
because Tilton relief displacement
introduced your metric errors
– Its features are represented without selection
or symbolization
Aerial mosaics and image maps
• A aerial images form the basis of useful
map-like representation of the Earth’s
surface such as mosaics and orthophotos
• Aerial mosaics are produced by
assembling adjacent aerial photographs to
form a single image
Aerial mosaics and image maps
• Uncontrolled mosaics performed by
assembling adjacent photographs without
strict concern for geometric integrity
Aerial mosaics and image maps
• A controlled mosaic presents the detail
of an aerial photograph in planimetrically
correct position
Aerial mosaics and image maps
• An aerial index is created by placing
adjacent area photographs together in
their approximate relative position without
represent econometric relationships
– These are to be used as a guide for
identifying those photographs that represent
an area that is needed without searching the
entire collection
Aerial mosaics and image maps
• Orthophotos are aerial photographs
prepared using stereoscopic parallax and
photogrammetric principles to remove the
effects of relief displacement and tilt to
provide a planimetrically correct aerial
image
Aerial mosaics and image maps
• And orthophoto quadrangle is an
orthophoto that represents the same
region as the USGS topographic
quadrangle
– These can be thought of as image maps
• The digital orthophoto quadrangle
(DOQ) is a digital version
Field observations and accuracy
assessment
• Field observations are necessary because
aerial imagery is not infallible or a total
source of information
– The information is derived through
interpretation
Field observations and accuracy
assessment
• Think of a map or GIS as a statement
about conditions of a certain place
– The more precise the statement, the more
useful it is to the user
• Delineate small parcels, specific labels, etc.
– The less precise the statement, the less
useful
• The larger the parcels, more general labels, etc.
Field observations and accuracy
assessment
• For example, a broad area labeled as a
forest
– Versus
• Smaller areas each individually labeled as
oak, pine, and fir.
Field observations and accuracy
assessment
• Field data are first-hand observations
collected on the ground
– Often use copies of images or maps to
annotate in the field
• Field work can create delays, increased
expense, and other difficulties
– But it is essential for establishing the validity
of the work
On-screen digitizing
• Traditionally, photointerpreters used
transparent (mylar) overlays to show
outlines, mark features, and make
annotations
– These then had to be digitized, transferred
and registered to a planimetrically correct
base layer
Manual
Digitizing
Most common form of
coordinate data input
Requires a digitizing table
– Ranging in size (25x25
cm to 150x200cm)
• Ideally the map should
be flat and not torn or
folded
• Cost: hundreds (300) to
thousands (5000)
On-screen digitizing
• In digital imagery it requires taking a raster
image and putting a drawing a vector layer
on top of it.
– This all can be saved as one file
– Can then be transferred directly to a GIS
• This is called heads-up digitizing
Heads-Up
Digitizing II
• Raster-scanned image
on the computer
screen
• Operator follows lines
on-screen in vector
mode
Digitizing Errors
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Undershoots
•
Dangles
•
Spurious Polygons
On-screen digitizing
• Not inherently difficult
• Requires consistency and attention to
detail
• Can be challenging
– Edges can be difficult to identify
– Shadows
– Low image contrast
Accuracy Assessment
• Procedures well established to evaluate the
accuracy of interpretation
– For maps with polygons of specific types, an overall
map accuracy and individual class accuracies can be
determined
• Requires a set of reference data points that
have known characteristics
– Ideally should be random
– That may miss some categories, so, often random
selection combined with collecting across all classes
and across the entire study area
Accuracy Assessment
• Should keep field collected data for input
separate from, field data used for accuracy
assessment
• Assessment is a table called the error
matrix
– Each reference point and its matching map
label are compared
The kappa statistic
• There are so many values in the error
matrix that it becomes confusing to
people.
– Want a single number that encapsulates the
significance of the error matrix.
– This is the kappa (κ) statistic
• Difference between the observed agreement
reported by the diagonal and the agreement that
might be due solely to chance
The kappa statistic
• Kappa is estimated by k hat
observed  expected

1  expected
• Observed = accuracy reported in error matrix
– Sum correct / total
• Expected = correct classification due to
chance
The kappa statistic
• The statistic adjusts the percentage
correct by subtracting the estimated
contribution of chance agreement
– A κ=0.83 means that the classification is 83%
better than would be expected from chance
assignment of pixels to categories
– Kappa ranges between +1 and -1
Interpretation Equipment
• Paper prints or transparencies
• Magnification
• Flatbed scanners
– Dpi – up to about 3500
• Light tables
• Densitometers
• Stereo imagery equipment