P band: Radar bandwidth for 0.225-0.39 GHz (300-1000 mm).
Panchromatic: Used for films that are sensitive to broadband (eg,
entire visible part of the spectrum) electromagnetic radiation, and for
broadband photographs.
Parallax: The apparent change in the position of one object, or point,
with respect to another, when viewed from different angles. As applied to
aerial photographs, the term refers to the apparent displacement of two
points along the same vertical line when viewed from a point (the exposure
station) not on the same vertical line.
Parallax difference: The difference in the distances separating complementary
image points for two ground points of different elevation, as measured on
a stereo pair of photographs correctly oriented with respect to one another.
Parallelepiped: (1) A four-sided rectilinear shape bounded by two
sets of parallel lines. (2) A classification approach which sub-divides
the image data space into categories defined by channel themes. These themes
may be represented as parallelepipeds on image crossplot diagrams.
Passive system: A sensing system that detects or measures radiation
emitted by the target. Contrasts with active system.
Pattern: (1) In a photo-image, the regularity and characteristic
placement of tones or textures. Some descriptive adjectives for patterns
are regular, irregular, random, concentric, radial, and rectangular. (2)
The relations between any more-or-less independent parameters of a response,
such as the pattern in the frequency domain of the response from an object.
PCA: Principal Component Analysis.
Perigee: The orbital point where a satellite is closest to the body
around which it is orbiting. Contrasts with Apogee.
Perihelion: The point closest to the sun in an elliptical orbit around
it.
Perspective: Representation, on a plane or curved surface, of natural
objects as they appear to the eye.
Photogrammetry: The art or science of obtaining reliable measurements
by means of photography.
Photograph: A picture formed by the action of light on a base material
coated with a sensitised solution that is chemically treated to fix the
image points at the desired density. Usually now taken to mean the direct
action of electromagnetic radiation on the sensitised material.
Photographic interpretation: The act of examining photographic images
for the purpose of identifying objects and judging their significance.
Photomap: A single photograph, composite, or mosaic showing co-ordinates
and marginal information; normally reproduced in quantity.
Photometer: An instrument for measuring the intensity of light or
the relative intensity of a pair of lights; also called illuminometer. If
the instrument is designed to measure the intensity of light as a function
of wavelength, it is called a spectrophotometer.
Photon: Quantum of electromagnetic radiation.
pico: Standard prefix indicating 10^-12.
Pitch: Rotation of an aircraft about the horizontal axis normal to
its longitudinal axis, which causes a nose-up nose-down attitude.
Pixel: 'Picture element'. An image data element having both spatial
and spectral aspects. The spatial variable defines the apparent size of
the resolution cell (that is, the area on the ground represented by the
data values), and the spectral variable defines the intensity of the spectral
response for that cell in a particular image channel (see resolution
cell, instantaneous field of view).
Planck's constant (h): Constant used to relate the frequency (n)
or wavelength (l) of electromagnetic radiation to energy. (h = 6.62 x 10-^34
J sec.)
Platform: A vehicle or object used to take remote sensing measurements,
such as satellites, aeroplanes, balloons, kites, or rockets.
Plotter: A graphic hard copy output device which can use any number
of technologies to 'plot' an image. Pen plotters, electrostatic plotters,
photo-plotters, ink-jet plotters and laser plotters are some examples.
Polarisation: The direction of vibration of the electrical field
vector of electromagnetic radiation.
Polar orbit: The orbit of a satellite whose path takes it over the
poles of the Earth.
Polar Platform program: Set of unmanned sun-synchronous remote sensing
satellites planned in conjunction with the International Space Station program;
also known as Earth Observing System (EOS).
Polygon: A plane figure consisting of three or more vertices (points)
connected by line segments or sides.
Polynomial model: A model which uses linear algebra to convert from
one co-ordinate system to another. These models relate specific features
of the two co-ordinate systems and may be derived empirically or by using
control points.
Positive: (1) A photographic image having approximately the same
rendition of light and shade as the original subject. (2) A film, plate,
or paper containing such an image.
Precision: A measure of the dispersion of the values observed when
measuring a characteristic of elements of a population. The clustering of
sample values about their own average.
Preprocessing: In a remote sensing system, the processing of data
received from the sensor to a form acceptable by the data bank and subsequent
processing functions. This may also include geometric and radiometric calibration,
enhancements, and other transformations.
Primary colours: A set of colours from which all other colours can
be derived, but which cannot be produced from each other. The additive primaries
(light) are red, green and blue. The subtractive primaries (colorant) are
yellow, magenta (a deep pink) and cyan (a blue-green). The psychological
primaries (perceived as basic and unmixed) are the pairs red/green, yellow/blue
and black/white.
Primary sampling unit (PSU): Groups of image pixels which are relatively
homogeneous with respect to spectral variations and are large enough to
allow for registration errors to the ground, map or photograph.
Principal component analysis (PCA): The analysis of covariance in
a multiple data set so that the data can be projected as additive combinations
onto new axes (principal components). The first principal component is defined
to be aligned with the direction of maximum variance in the original image
data.
Processing: (1) The operation necessary to produce negatives, diapositives,
or prints from exposed film, plates, or papers. (2) The manipulation of
data by means of computer or other device.
Program: An item-by-item series of statements in programming language
designed to solve a particular problem that has been isolated; also referred
to as computer software.
Projection, map: A systematic drawing of lines on a plane surface
to represent the parallels of latitude and the meridians of longitude of
the Earth or a section of the Earth. A map projection may be established
by analytical computation or may be constructed geometrically. A map projection
is frequently referred to as a projection but the complete term should be
used unless the context clearly indicates the meaning.
Pseudo-colours: Also called false colours. Colours arbitrarily assigned
to an image to represent data values, rather than natural likeness. Often
used in satellite imagery.
Pulse: (1) A variation of a quantity whose value is normally constant;
this variation is characterised by a rise and a decay, and has a finite
duration. (2) A short burst of electromagnetic radiation transmitted by
the radar.
Pulse length: The length or duration of each pulse in a radar transmission.
Pushbroom system: An imaging device consisting of a fixed linear
array of many sensors which is swept across an area by the motion of the
platform thereby building up an image. It relies on sensors whose response
and reading is nearly instantaneous, so that the image swath can be segmented
into pixels representing small dimensions on the ground.