1 - NGA

eefablishfng new land boundaries and
aubdivislons. See also dependent reeurvey.
Index chart-An outline charl showing the
limits end identifying designations of
navigational charts, volumes of sailing
directions, etc.
Indax contour lln-(JCS) A contour fine
amenfualed by a heavier line weight 10
dstingutsh H from intermediate camrfours. Index
contours are usually shown as every fifth
contour with their ass”~nest values, 10 facifilafa
reading elevations.
Index correction-l. A correction applied 10
the raading from any graduated measuring
device to compensate for a canstant error such
as would be caused by misplacement of the
Scale; lhe reverse 01 Ihe index error. 2.
(leveling) That correction which must be applied
10 an ObSeWed difference Of elevsflon to
effminale lhe error introduced into fhe
observations when lhe zero of the graduations
on one or bofh leveling rods does not coincide
axactly with Ihe actual bottom surface of the
rod.
‘Indax arror-The instrumental error which is
constant and attributable 10 displacement of a
vemler or some analogous ellecf.
Indax ream-A real mark, such as a cross or
dot, lying in the plane or the object apace of a
photograph and used singly as a reference
mark In cerfain fypes of monocular Inswuments,
or as one of a pair to form a floating mark as in
certain types of stereoscopes. In stereoscopic
map plOffing instruments which utilize a stereo
pair of Index marks, each mark is called a haff
mark. see also floatlng merk.
fndex of refracffon-See SnelM faw of
refrecflon.
Index prlsnx-A sextant prism which can be
Krtetad to any angle mrresponding to altitudes
between established WrrUs. It k the b@irle or
pendulum sextant wunterpatl ot the indax
mirror of a marine aexrant.
Index fo photography-see photo Index,
definffion 1.
Indian dalum-The Indian detum is eccepted
as fhe preferred datum for India and several
adjacant countries In Southeast Asia. It is
computed on the Everest ellipeoid with its origin
MIL-HDBK-850
127
at Kafiinpur in Central Indii. Derived in 1S30,
fhe Everest ellipsoid is the ofdeat of the
eilipao”kfs in common use and &smuch too 6rrSS11.
As a resuft of the fatter, the. datum Cennoi be
extended too far from the origkt or very fofuo
geoid aeparafions will mr. For this reason and
Ihe fact that the ties bofween local friarrgulatbn
in southeast Asia are rypbeffy weak, the Indian
datum is probably the least salislacfory of the
preferred datums.
Intffan eprlng fow water—The approximate
mean water faval defermfned from afl bwer bw
watars at spring tkles.
Indian tide plane-The datum of Indian
spring bw watar.
Indicated comer-A Ierm adopfad by the
U.S. Geological Survey to dasignate a mmer of
Ihe pubfii fend wrveye whosa focafion canno!
be varified by the criferfa necessary to clase if as
a found or existenl corner, but which b accepted
locally as the mrrecf mmer and whose bcation
is perpaiusted by such reeks as fence One
intersacfions, piles of rocks, and stakes or pipes
driven info the ground, which have been
recovered by field investigation.
Indlcafed prfnclpal point—See prlnclpal
point.
Indlract effect on the deflectlona—See
topographic deflecllon,
fndirect Ieveflng-see barometric
Ievellng; thermomotrlc leVellng;
wlgonometrlc Ioveflng.
Indirect measurement—Any measurement
secured by determining Ifs quanftfy from tfs
relatlon to some measured quantify. A
!echnique used in aurveylrrg whan ff is
impossible to actually tape e dkitance across a
river or of her such obstruction. See afao dfract
meseuremenf.
Indirect obsewatlon—A measure of a
quantity which is a function of the quantify or
quantities whose value is desired, such as an
obsemed difference in elevation wifh a spirit
level, used to obtain the elevation of a bench
msrk.
Indirect phonography—Photography In
which fhe camera records an Image COaf upon a
screen or similar display surface by elacfronic
[television, radar, etc.) or other means.