1 - NGA

geographic Iatiiudes on the spheroid info
Isometric Isstffudes on a sphere, a conlormal
MSP PmiSCtiOn (the Mercator) may be
celculafed, using spherical formulas, for the
pkoffing of geographic data.
Ieometrlc paraUal-The intersecting fine
befwaen the plane of a photograph and a
horlzonfal pfane having an equal perpendicular
distance from the same perspective center.
MIL-HDBK.850
lSOp@rhetrlC cunre-A lktO on a map
prOjOCflOnabng which lhere is no varlalbn from
exact scale. There are two boperimetric curves
passing through every polnl on an equal-area
map projectbn. This characteristic gives thal
class of pmjectforrs some preference for
engineering maps.
lsopleth-A line of equal or constant value 01 a
gfven quantity, wfth respect to akher space or
lime. Also called fsog ram.
Isoporlc chert-A chart with fines connecting
points of equal magnetic annual change.
lsoDor-A line found on maarretic charis
sho”wirrg pointe 01 equel ann~al change Also
called magnet ic Isoporlc line.
lsopycnlc—A line connecting pints of
density, particularly of ocean water and
at mosphare.
Lsoradlel—A radial from lhe isocenler.
equal
laoatsay—A condition of approximate
equilibrium in the outer part of the Earth, sucn
that the gravitational effect ot masses extending
above the surface of the geoid in continental
areas IS a~roxirnataly counlarbalanced by a
deficiency of density in the malerlal beneath
those masses, while ths effect of deficiency of
density In ocean waters is counterbalanced by
an excess of denstfy In tha material under the
oceans. See also depth of Isostatlc
compermatlcan; Haylord.Bowle method
of Isoststlc raductlon; Hsyrord-Bullard
(or Bullard) mathod of Lsostatlc
rsductlon; Irsostatlc adjustment;
Isostatlc compensetlon; Iaostatlc
corroctlon;
Iaoatrsay.
Pratt. Hayford theory of
laoetstlc adjuatmen!—The natural process
by which the crust ot the Earth adjusts 10 restore
or meinlain Ifs state of equilibrium. See also
lsDataay.
134
Isosiatlc enomaly—Tfr6 ditlerenca batwaen
an obsawed value 01 gravity and a Wworetkal
value al the point ot observation which has
been corrected Ior alevalicrn of me station
alxrve the geoid, and for Ihe effect of
topography over the whole Earfh, and Ior its
iaostatiw compensation.
Ieoatatlc compensation—The departure
trom normal density ot material in the bwer part
of a column of the Earth% crust which balances
(compensates) fend masaea (topography) above
sea level and deficiency of mess in ocean
waters, and which produces the condffion of
approximate equifibr’wm of Earth’s crust. See
also Isostesy; topographic daflectlon.
lSJStatlC corrsctlon—The adjustment made
to values of gravity or to deflections of the
vertical observed al a point to take account ot
the assumed mass deficiency under
topographic features Ior which a topographic
correction is also made.
Iaoatatlc gsold—An ideal geoid derived from
Ihe spheroid of reference by the application of
computed values of the deflection of the vertical
whkh depend upon the topography and
isostatic compensation.
lsoatere-A line connecting points of equal
atmospheric density.
Iaot lmlc-Pertaining to a quality whch has
equal value in space al a paflicular time.