Glossary

A | B | C | D | E | F | G | I | L | M | N | O | P | R | S | T | V | W
A
a.g.l
Above ground level. The height above the ground of a plane carrying an air photo camera
a.s.l.
Above sea level. Flight heights for planes carrying air photo cameras are usually given a.s.l. in metres or feet.
B
Band
A wavelength interval in the electromagnetic spectrum.
Brightness
Magnitude of the response produced in the eye by light.
C
CN (R)
Canadian National (Railway).
Corner reflector
Cavity formed by two or three smooth planar surfaces intersecting at right angles. Electromagnetic waves entering a corner reflector are reflected back toward the source.
CP (R)
Canadian Pacific (Railway).
D
Dielectric constant
Electrical property of matter that influences radar returns. Also referred to as "complex dielectric constant."
DLS system
Dominion Lands Survey system of land division. Land is divided into square blocks that are nominally one square mile in area.
E
Electromagnetic spectrum
Continuous sequence of electromagnetic energy arranged according to wavelength or frequency.
F
False colour photograph
Another term for "infrared colour photograph."
Film
Light sensitive photographic emulsion and its base.
Flight height
Usually given for planes carrying cameras. The height, usually given above sea level, at which the plane was flying when a photograph was taken.
Focal length
In cameras, the distance from the optical centre of the lens to the plane at which the image of a very distant object is brought into focus.
G
Geostationary
Refers to satellites traveling at the angular velocity at which the earth rotates; as a result they remain above the same point at all times.
I
Image
Pictorial representation of a scene recorded by a remote sensing system.
Interpretation
The process by which a person extracts information from an image. A series of clues is assembled in an orderly fashion and a solution arrived at that best fits the clues. The interpreter uses seven criteria in interpretation of photo images: tone, texture, pattern, shape, shadow, size, and location (or association).
IR
Infrared region of the electromagnetic spectrum that includes wavelengths from 0.7 micrometres to one milimetre.
IR colour photograph
Colour photograph in which the red-imaging layer is sensitive to photographic IR wavelengths, the green-imaging layer is sensitive to red light, and the blue-imaging layer is sensitive to green light.
L
Landsat
A series of unmanned earth-orbiting satellites, a joint initiative of the United States Geological Survey (USGS) and the National Aeronautics and Space Administration (NASA). Landsats 1-7 have aquired over 2 million images of the earth's surface since 1972.
Lens
One or more pieces of glass or other transparent material shaped to form an image by refraction of light.
Light
Electromagnetic radiation ranging from 0.4 to 0.7 micrometres in wavelength that is detectible to the human eye.
Location
One of the seven air photo interpretation criteria. The interpreter is concerned with the position of an object with respect to other things (hence it is sometimes referred to as association).
M
Microwave
Region of the electromagnetic spectrum in the wavelength range 0.1 to 30 cm.
Mosaic
Composite image or photograph made by piecing together individual images or photographs covering adjacent areas.
Multispectral scanner (MSS)
Scanner system that simultaneously acquires images of the same scene at different wavelengths.
N
NASA
National Aeronautical and Space Administration.
O
Oblique photograph
Photograph acquired with the camera intentionally directed at some angle between horizontal and vertical orientations.
Overlap
Extent to which adjacent images or photographs cover the same terrain expressed as a percentage.
P
Palaeochannel
A former river channel, now largely devoid of running water.
Panchromatic image
Referred to as a "black and white image;" objects are imaged as shades of grey.
Pattern
One of the air photo interpretation criteria. Regular repetitions of tonal variations on an image or photograph. Some elements of the natural landscape form patterns, for example, river channels may be arranged into distinctive drainage patterns. The human landscape also exhibits patterns, for example the squares of the DLS system.
Photographic IR
Short wavelength portion (0.7 to 0.9 micrometres) of the IR band that is detectable by IR colour film or IR black and white film.
PR
Provincial Road
Pseudoscopic vision
An optical illusion that may occur on images with extensive shadows. Ridges appear to be valleys and valleys appear to be ridges. The illusion can be corrected by arranging the image so that the shadows trend from the top margin of the image to the bottom
PTH
Provincial Trunk Highway
R
Radar
Acronym for "radio detection and ranging." Radar is an active form of remote sensing that operates in the microwave and radio wavelength regions.
Radiation
Propagation of energy in the form of electromagnetic waves.
Relief
Vertical irregularities of the earth's surface.
Relief displacement
Geometric distortion on vertical air photographs. The tops of objects appear in the photograph to be radially displaced from their bases outward from the photograph's centre point.
Remote sensing
Collection and interpretation of information about an object without coming into physical contact with the object.
Resolution
Ability to separate closely spaced objects on an image.
S
Satellite
An object in orbit around the earth.
Scale
Ratio of distance on an image to the equivalent distance on the ground. It is usually expressed in the form 1:20,000, meaning that one unit of measurement on the photo represents 20,000 of the same units on the ground.
Sensor
Device that receives electromagnetic radiation and converts it into a signal that can be recorded and displayed as an image.
Sequential air photos
Photos taken of the same area at different times, useful for tracking changes over the years.
Shadow
One of the air photo interpretation criteria. As photographs are usually taken when the sky is cloud-free, ground objects will throw shadows. Some shadows are a distinctive shape enabling identification of the ground object.
Shape
One of the air photo interpretation criteria. Some landforms and objects of the human landscape have distinctive shapes that enable their identification.
Size
One of the air photo interpretation criteria. The size of an object, both actual and relative to other objects, aids with identification.
Specular reflectance
Reflectance of electromagnetic energy at specified wavelengths.
Stereopair
Two overlapping photographs that may be viewed stereoscopically.
Stereoscopic vision
People see things in three dimensions (i.e., they see stereoscopically) because they see objects from two different viewpoints – the right eye and the left eye. The two sight lines converge on the object being viewed and the two images are processed in the brain to give a three dimensional impression.
Stereotriplet
Three overlapping photographs that may be viewed stereoscopically, two at a time.
Strandline
Impression of the former higher position of a lake or the sea, often taking the form of a beach deposit.
T
Terrain
Surface of the earth.
Texture
One of the air photo interpretation criteria. Technically it is the frequency of change and arrangement of tones on an air photograph. It is best described by reference to the texture of cloth, so photo surfaces may be rough, mottled, silky, etc.
Tone
One of the air photo interpretation criteria. A measure of the amount of light reflected by an object and recorded on a panchromatic (black and white) photograph. It is recorded in shades of grey from almost black to almost white.
V
Vertical exaggeration
In a stereoscopic model, the extent to which the vertical scale appears larger than the horizontal scale.
W
Wavelength
Distance between successive wave crests or other equivalent points in a harmonic wave.