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Photo Interpretation (Visual)
Overview
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Data that can be derived from Aerial Photo
Photo interpretation (Visual)
Difference
Examples of Difference
Interpretive elements
3D
Types of interpretation
Guides to interpretation
Summary of Photo Interpretation
Data that can be derived from Aerial Photo
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Distance (using ruler and scale of the photo)
Angel (using protractor - Direction)
Areas
Height (3D – contour using stereoscopy)
Extraction of natural and artificial features
Photo Interpretation
• Interpretation is based upon the concept of
differentiation - the ability to identify a single
element due to its difference with respect to
background and/or surroundings
• Difference can be "measured" in three responses:
spatial, spectral, and temporal
• Visual interpretation is dependent on the skill of
the operator, experience, and knowledge of the
area
• Automatic classification (Digital) is based on
using computers instead of human eye
Difference
• Spatial: related to dimension or location
(includes size, shape, position, association)
• Spectral: EM response (tone, colour as a result of
reflection and emitted EM energy)
• Temporal: change in spatial or spectral
characteristics with time. Can be cyclic (daily,
seasonal, annual) or "permanent" (eg. clearing,
construction)
Examples of Difference
• Object is big in size spatial Suitable image scale ?
• Water is similar to vegetation in Black & White
imagery from the visible band (dark tone) but
different in the Near Infra Red spectral Portion
EM spectrum to sense in ?
• In late summer, crop based agricultural activity is
minimal due to the dry conditions. By spring crops
are reaching their prime temporal what time scale
/ point is best to identify a specific feature ?
Key interpretive elements
Pattern, Texture, Shadow
Tone, Colour
Association , relationships
Change in characteristics with time
Size, Shape, Site / Location
(stereoscopic appearance)
TEMPORAL
SPECTRAL
SPATIAL
• Primary: Size, Shape, Tone, Texture, Pattern
• Additional: Site / Location, Association, Shadows,
Stereoscopic Appearance
Interpretive elements
• Shape: General form, configuration or outline of
individual objects, eg. regular, irregular, linear,
curved, rounded, triangular etc. e.g. House, Round
About
• Size: The physical extent or dimension of a
feature. May be comparative (relative) (eg. a big
bird is still small compared to an elephant).
– Comparable: Large, small, massive, tiny
– Magnitude: kms, metres, microscopic, macroscopic
• Tone: Relative brightness or colour of a feature
– Gray scale:very dark tone, dark tone, dark gray, mid
tone, light tone, white
– Colour: Blue, Green, Red, etc.
Interpretive elements-continue
• Pattern: Relates to the spatial arrangement of
objects. Repetition of form or spectral response
(tone) eg. gridded, regular (plantation), linear,
irregular, ordered (hierarchal), symmetric,
geometric etc. e.g. Agriculture, Orchard, Forest
• Texture: Frequency of tonal change of the object
eg. smooth, coarse, speckled, fine, fuzzy, soft.e.g.
water appears as smooth while forest as coarse
• Site/Location: physical position of object feature
(topographic or geographic), hill slopes, ocean,
land, mountains etc.
3D
• Monoscopic (easier and quicker, flat area) versus
Stereoscopic (number of photos, cost, purpose)
• Shadow: Used to determine 3D information from
a 2D image, shape and height of object via shadow
• Shadows can enable objects to be "seen" when the
natural conditions may not be favourable eg.
power poles (small planimetric extent, can only be
identified by shadow), sand dunes (shadows result
in contrast, can see topographic outline)
• Appearance: How the surface appears in 3D,
generally related to surface consistency and
change in height, eg. Object (Tall, spherical) and
Area (Undulating, flat, rugged)
Types of interpretation
• Generally there are two types of interpretation:
– Selective (Descriptive, graphical, pictorial, sample
images)
– Elimination (Dichotomous)
• The selective method provides the interpreter with a range
of choices with accompanying information or descriptions
(eg. feature characteristics such as tone, shape, texture etc).
The interpreter simply refers to the object and then selects
the choice that best matches that seen (eg.
pictorial/graphical, sample images, descriptive text)
• The elimination method uses a structured "routing" process
to reduce the classification to a minimum number of
options (ideally only one). Usually simple 2 path decision
making tree with only minor decisions to be made at each
step
Guides to interpretation
• Define the objective of the interpretation:Simple (feature
recognition) and complex (modeling)
• Identify the type of photos needed (B/W, Color_IR,
Stereoscopic vs Monoscopic coverage, Temporal, Scale,
Calibration / enhancement / rectification)
• Prepare an equipment that aid in interpretation e.g.
magnifying lens, stereoscope - mirror stereo, pocket stereo,
and digital visual system
• Have a good eyes, lateral thinking, and patience
• Acquaint yourself with the area: Knowledge of
phenomenon and area: general, local, specific
• Follow a deductive process: general examination of the
image and concentration on specific features
• Check the consistency and reliability of interpretation by
maps and ground truth data (Accuracy)
Summary of Photo Interpretation
• Features on a photo can be delineated based on the
following characteristics:
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Tone (B/W -gray shade), color and brightness)
Size (depend on scale e.g. size of house)
Shape (geometrical arrangement, road/house)
Texture (smoothness or roughness,water, grass)
Pattern (spatial arrangement, network of streets)
Shadow (height of object)
Site (geographic location, features on rivers)
• Photo interpretation can be supported by
magnifying lens and stereoscope (pocket, mirror)
Photogrammetric Applications
• 3D analysis for environment
3D analysis enables users to create, analyse, and display surface
data, which is important in many applications in the urban areas, for
example, road, canal, dam, and simulation of air and water pollution.
3D data can be extracted from aerial photo directly (stereoplotters)
which allow Photorealistic presentation with possibilities for
navigation through the 3D city model (fly through) and animate onscreen designs
• Urban Growth
Tracing socio-economic and physical pattern of urban development
overt space and time is a key element for heritage and future
development. Historical aerial photographs imagery can be classified
and integrated with GIS to show the historical development of a city
Aaerial photo: Kuala Lumpur