Nadir: (1) That point on the celestial sphere vertically below the observer,
or 180° from the zenith. (2) That point on the ground vertically beneath
the perspective centre of the camera lens.
nano: Standard prefix indicating 10^-9.
NASA: National Aeronautics and Space Administration.
NASDA: National Space Development Agency, Japan.
NASIS: Northeast Australia Satellite Imagery System.
Nautical mile (knot): A unit of distance used principally in navigation.
For practical navigation it is usually considered the length of one minute
of any great circle of the Earth, the meridian being the great circle most
commonly used; also called sea mile. 1 nautical mile equals approximately
1.8 kilometres or 1.6 statute miles.
NDVI: Normalised Difference Vegetation Index.
Nearest neighbour resampling: A resampling technique which selects
the nearest pixel value in the input image to write to each output location.
Near infrared: The preferred term for the shorter wavelengths in
the infrared region extending from about 0.7 µm (visible red), to
around 2 or 3 µm (varying with the author). The longer wavelength
end grades into the middle infrared. The term really emphasises the radiation
reflected from plant materials, which peaks around 0.85 µm. It is
also called solar infrared, as it is only available for use during the daylight
hours.
Neat lines: The boundary grid lines on a map along which adjacent
map sheets align and abut exactly.
Negative: (1) A photographic image on film, plate, or paper, in which
the tones are reversed. (2) A film, plate, or paper containing such a reversed
image.
NESDIS: National Environmental Satellite, Data and Information Service;
part of NOAA.
Nimbus: US series of polar-orbiting, meteorological satellites.
NIR: Near Infrared.
nm: Abbreviation for nanometer (1 nm equals 1 x 10^-9 m).
NOAA: (1) National Oceanic and Atmospheric Administration. (2) Series
of US polar-orbiting, meteorological satellites.
Noise: Random or regular interfering effects in the data which degrade
its information-bearing quality.
Non-imaging sensors: Sensors that give quantitative measures of the
integrated intensity of electromagnetic energy from all objects within their
field of view; included are radiometers and spectrometers.
Non-selective scattering: The scattering of electromagnetic energy
by particles in the atmosphere much larger than the wavelengths of the energy,
causing all wavelengths to be scattered equally.
Non-spectral hue: A hue which is not present in the spectrum of colours
produced by the analysis of white light by a prism or diffraction grating.
Examples are brown, magenta, and pastel shades.
Non-standard colour composite: A composite image formed by assigning
image channels to 'non-standard' primaries during image display or hardcopy.
For example, a standard false colour composite of Landsat MSS data would
involve displaying band 4 as blue, band 5 as green and band 6 or 7 as red;
a non- standard composite may be band 5 as blue, the inverse of band 6 as
green and band 7 as red.
Normalised Difference Vegetation Index (NDVI): Vegetation greenness
index computed from NIR and red channels as (NIR-red) / (NIR+red)
NRSC: National Remote Sensing Centre, UK.