Photogrammetry . Lecture 12

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• Lecture 12 Content

• Photo Interpretation

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• In aerial photography everything can be seen but
  nothing is recognizable unless you are familiar
  with the area
• Main aim of photogrammetry is to have minimum
  fieldwork (which is slow and expensive)
• Information from aerial photos are classified by
• Metric information
• Concern with the dimensions and the position of
  objects ?photogrammetry
• Semantic information
• Concern with the nature or identity of objects ?
  photographic interpretation

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• Origin of the use of aerial photos
• Military purposes (1st and 2nd world wars)
• Nowadays widely used by civil fields geography,
  geology, engineering, erosion studies, forestry,
  wildlife management, hydrology and watershed
  management, agriculture, etc.
• Definition
• Photo Interpretation is the act of examining
  photographic images for the purpose of
  identifying objects and deducing their
  significance
• Two points of view
• Identification
• Deduction

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• Identification
• Leads to the enumeration of classification of
  objects according to given themes
• The identified objects can be used as items for
  inventory which can be exhaustive (eg. Land-use
  mapping) or statistical (eg. Agricultural or
  forest surveys)
• Deduction
• Allows research concerning various relationships
  between objects, either directly identified or
  indirectly by inference
• The research are tools for explanation (eg.
  Geological surveys)

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• Photo interpretation is used in the preparation
  of maps
• Is used for the extraction of semantic
  information necessary for the compilation of
  topographic maps ? process is called topographic
  photo-interpretation
• Photographic interpretation is not the single
  source of information in topographic mapping
  (field work is necessary)

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• Principles of object recognition used in photo
  interpretation
• Photo interpretation is best appreciated by
  practical work
• Pictorial clues generally known as image
  characteristics that are useful
• Tone
• Texture
• Pattern
• Shape
• Size
• Shadow
• Situation
• Association

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• Tone
• Refers to brightness of the images on the photo
• Is the black and white range of a photograph
• The greater the light reflected from an object
  into the camera lens, the lighter will be the
  tone on the photograph
• The variations in tone (shades of grey) form the
  shape, pattern and texture of the image
  (discussed later)

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• The following factors affect an image tone
• The nature of the surface of an object
• Smooth surfaces usually reflect more light than
  rough surfaces, thereby creating a lighter,
  smoother image
• The weather conditions at the instant of exposure
• Overcast, dark skies reduce the incident light to
  objects thereby greatly reducing the range of
  tone, particularly the lighter shades
• The effect is a dark, flat photograph, lacking in
  contrast, which will be difficult to interpret

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• The relative positions of the camera and sun
• The amount of light reflected by an object into a
  camera lens will vary with the angle formed by
  the sun, camera, and that object
• With the sun located at the horizon, there will
  be shadow and little light will enter the camera,
  therefore, a dark image is created
• Sun at an angle, smooth objects (glass, railway
  lines, still water) will reflect large quantities
  of light which will give a false indication of
  its size (halation), and the glaze will obscure
  nearby detail
• The exposure setting and film processing
• Processing and incorrect setting of the film
  directly affect the tone of an image usually by
  loss of contrast, which is consistent throughout
  the entire photo

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• The photographic emulsion
• Panchromatic emulsion normally used
• General rules for using this film are
• Water surfaces usually have dark tone, but if the
  water contains much sediment, the tone becomes
  lighter
• Un-vegetated dry sand is usually light
• Surface roads are generally light
• Railways are normally dark
• Roofs and buildings facing the sun is light
• Bare earth, rock surfaces, young grass and corn
  field are usually light
• Deciduous or broad leaved trees are light
• Evergreen or narrow leaved trees are usually dark

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• Texture
• Is the product of an aggregate of uniform
  features which are too small to be clearly
  discerned individually
• It is the appearance of the photographic image as
  it is made up of dots
• The aggregation of the dots forms the textural
  elements
• Often related to the scale of the photograph.
  For example
• At a large scale (14,000) leaves of a tree
  contribute to the texture of the branches
• At a smaller scale (120,000) the entire crown of
  trees contribute to the texture of the forested
  area

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• Pattern
• Refers to the spatial arrangement of objects in a
  repeated sequence and/or in a characteristic
  order
• Made up of the disposition of textures according
  to their density, their geometrical lay-out, and
  the frequency of their distribution
• Maybe be regular or irregular and may be found in
  both natural or man-made feature

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• Examples
• Trees in a cultural landscape may have a pattern,
  eg. An orchard in which trees are planted in
  lines, at regular intervals
• Drainage can be classified according to certain
  characteristic patterns, eg. Radial, parallel,
  etc.
• Cultural features often have a pattern, eg. A
  town dating from medieval times with a star
  shaped wall or canal pattern
• In agricultural areas we often see different
  types of field patterns
• When the photographic scale decreases
  sufficiently (that is larger scale), pattern
  turns to texture (which is an aggregate of
  individual elements)

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• Shape
• Is the form or topographic expression of an
  object as observed in a two dimensional
  photographic image
• It is the most important single factor which is
  used in the identification of objects
• Most man-made features consists of straight
  lines, sharp angles and regular curves, unlike
  natural features which tend to have irregular
  shapes

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• Examples
• Houses and other buildings are normally
  characterized by their rectangular shapes and are
  typically the less expensive homes
• Military barracks, commercial buildings,
  supermarkets, warehouses, greenhouses, etc. is
  often recognized by their arrangements of
  rectangular shapes
• Circular objects are usually identified
  immediately because there are few in numbers and
  stand out in contrast to the much more common
  rectangular or irregular shapes. Examples
• Fuel and water tanks, traffic circles, sport
  tracks, etc
• Roads are identified by their width and curvature

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• Size
• Is the volume dimension of an object as can be
  observed in the three-dimensional (stereo) photo
  model (eg when stereoscopes are used)
• Size of an image is related to its volume and
  surface dimensions
• Classification of image in terms of size is
  achieved by comparison with nearby known feature
  (eg. car, football field)
• Example
• If a building measuring 5mm square on the
  photograph is known to be a dwelling house, then
  a building measuring 30mm2 is unlikely to be a
  normal school, and is more likely to be a factory
  or school

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• Shadow
• Is the obscurity within an area from which direct
  rays from a source of light (normally the sun)
  are excluded by an interposed opaque body
• May either be a help or a hindrance depending on
  whether the feature casts the shadow or is itself
  in the shadow
• Shadow casts by the object may give the more
  familiar profile view, giving clues as the
  profile or shape of an object (bridge, water
  tower, a tree, etc.)
• May also give an indication of the relative
  height when considered with adjacent shadows
• Image points that are within a shadow are
  difficult to see because they reflect little light

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• Situation
• Is the location of objects with respect to
  terrain features
• Helpful to relate an object to its immediate
  vicinity
• Example
• Thermal or nuclear power plants need abundant
  supply of coolant water and are found near rivers
• Telecommunication stations are placed on high
  locations to minimize interference with the line
  of sight

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• Association
• Is the combination and arrangement of objects,
  caused by their relationship with each other
• Example
• Hydroelectric power plants must have water under
  pressure and are always associated with a natural
  or manmade water supply
• A building next to a railway may be identified as
  a railway station
• Schools are associated with playgrounds

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• The End