1 - NGA

porspectlvo chm’t-A chari on a perapacfive
Pr@actiin,
perspective grid-(JCS) A network of firms,
drawn or superimposed on e Photograph. 10
represent the pers+ecfive of a syafernrdic
natwwk d fines on the ground or datum plane.
~~h~~ad Canadian grid. Sea al= grid
parapectlvs map projoctlon-A map
project’km produced ~ efrdght fines radiiting
from a aslacfad point and passing lhreugh
poirrfs on the earth’s surface to the projection
surface. Alee called geomctrlc map
pro)actlon.
parepactlvo plane-Any ptane ccmlaining
the perspective canter. The intersection ef a
perspective pfana and the gmurrd will ahvays
-r as a straight fins on an ae~I
photograph.
MIL-HDBK-850
parspectlva proJactlon-The projection of
points by sfreighl firma drawn through them from
aoma given peint to an intamaction with the
plans et projection, Unless etharw.iea indicaIad,
tha point of pjection is understood 10 be at a
finite distarwe from the plane of projection,
parspectlva ray—A line joining a perspactiva
canter end e peirrt objact. See else Image ray.
perapectlve apatlal modat-Crptical
raconatrucfion of an area of tarmin shcwing
depth by viawing a pair of aarial photographs
through a atersoacapa.
parspactlva view—A three-dimensional
representation generated with a reference to a
app~iawer location on or abeve Ore surface
perspectlva-l%a appearatwe to the eye of
objects in rasped to their relative distance and
pesifion.
perapectlvity—The corraepondence batwaen
the points, lines, or pkwras of two geemelric
configurations in perspective. Usually raf erred to
as linear pe-ivii because the true
parapdive center must be recoverable befora
angular percpedvhy can be included.
perturbation-in celastial mechanics,
dtierencas of the actual orbii from a central
188
fercs odid, arising from some axlar’rd fo=
such es a thkd @ altrac4ing the other two; a
rasisling nrerlum (atmosphere): failure of the
Parerrt bodytoad aaapdrrtmaaa, and so
forth. See alse gravitational pwturbatlons;
long period parkurbatlons; Iunlsolar
porfurbatlone; nongravltatlonal
perturbatlona; perlodlc perturbations;
●ocular perturbatlona; short parlod
perturbations: torrastrisl parturbetiorm.
perturbed orbff-Tfra otbit of a aatelfiie
differing fmm Ms norrnaf orbii due to various
d=turblng affacfa such as nonaymmefrical
gravltstkmal affecls, atmoapherk drag, radiil’on
pressure, acd se forth. Sea alao perturbation.
perfurtrlng factom (forces)-in celestial
mschanii, any force that ads on lha orbi!ing
body to change its ortit from a cen!ral ferce
orbit.
phase age—Saa ago of phase
Inoquallty.
phaao angle-l. Tha phase difference of two
periodically recurring phenomena of the sama
fr~uency, expressed in angular maasura. 2.
The angle at a celestial body between the Sun
and Earth.
phase inequality—Variations in U-m tide or
tidal currents a-iated with changes in the
phase of the Meon. At new and full Mom
(springs) the tide-prcducing forces ef the Sun
and Meon ad in ccmjuncfion, reaulthg in greater
Ihan average tide end tidal currents. At first and
teat quarters of the Moon (naepa) the tideprcducing
fore= cppoaa each other, resufting in
smaller than everaga tide and tidal curranm.
phase-l. (general) Of e petiedic quanlify, for
a ~flicubr Valua of the indapendenf variable,
the fradional part of a period through which the
independent variebfa has advanced, measured
from an arbiinwy reference. 2. (surveying) The
apparent dkplacernanl of en ob)~t or s’wnal
caused by ona aide being rrwre atrengfy
illuminated than the other. The resuftant error in
pointing is aim’iar 10 the armr cauaed by
observing an accantric signaf. 3. (astronomy) A
atege in e cycle of r-rring aspects, =u6A by
a ~~e~tb Varieticm of the illumination of an
object. The Moon passes through ifs phases,
new Meon to full Moon and back to naw Moon,
as its position relative to the Sun and Earth
changes.