1 - NGA

I
Hormbow Irwol attachment-A level used
In rnn~nction wilh a micrometer in a Ielesmpe
whereby Ietiude observations by the Horrebow-
Talcoff method can be made.
Horrebow.Talcott method of Iatltude
deWrmlrtatlon—See latitude
delarmlnsition; zenith. telescope
method.
hot spot+pholography) A small area 01
undesired brilliancy o! Illumination in the image
projected by a prfmer or a reader.
Hotlne Obllque Mercator—A mapping
prmjecfion devebped spacilicaliy for LANDSAT
Imagery, dividing the Earth info five zones o!
Iafiiude, Wtihin each zone, oblique strips
cm’respondng to indwktual LANDSAT spacecraft
paths are projected onto a plane. The axk of
projection surresponds approximately 10 the
path of scene cemers.
hour angle difference—See merldlan
angle dltrerence.
hour angle system (of coordinates)—A n
equatorial system 0! curvilinear celestial
coordinates which has the Equaior and the local
meridian as primafy and sewndary reference
planas, respectively. The position of a celestial
body iS Qfven by ifs hour angle end declination.
hour angle-The hour angle of a celestial
body la the time elapsed since ifs upper transit.
II k tfre engle between the observers
(eslronomic) maridlan and the declination circle
01 the body, measured positive westward from
the meridian. See also Greenwlcn hour
anQle; IOcal hour angle; sidereal hour
angle.
hour circle-Any great circle on the celestial
sphere whose plane {s perpendkwlar 10 the
plane o! the celestial equator. Also called circle
of decllnatlon; circle of rlghl aacanalon.
See also celestlal merldlen; coluras.
hub-A temporary traverse station marker,,
usualfy 01 wood, The stake is driven flush with
ihe ground with a tack or small nail on top 10
mark the exact point of reference for angular
and linear measurements,
Hull Integrlly Test Site Charfe (HITS)—
Charfs which provide detailed bathymakic and
environmental data for submarine test dive
MIL-HDBK-850
areas.
Huygen’s prlnclple-A general princlpla
applying to all Iorma of wave motion which
states that every point on the instantaneous
position of an advarcirq phasa fmnf (wave
front) may be regarded as a source of
secondary spherical wavelets. The poeiffon of
the phase front a momam Ialar Is then
determined as the envelops ot all of the
secondary wavelets (ad Inflrrlfum). This principle
is extremely useful In undarsfanding effecls due
to refraction, reflecfbn, dfiracsion, and
SCaltWin!J of all types 0( radiatiin, including
sonic radiadon as well as electromagnetic
radiation and applying aven lo ocean wava
propagation.
Hydrographlc Information Handllng
System (HIHAN)—HfHAN is a
sottware/hardware sysfem under development
by Ihe Datacfrment Stennis Spaca Center (NRL)
which will aufomate the preprocessing, merging,
integration and preparation of hydrogrsphic data
from a variety of sources.
h ydrog rsphlc charf-(JCS) A nautical chart
showing depths of water, nature of bottom,
COnlOUrs,Of bottom and Wasliine, and tidasand
currents m a given eea or sea and iand srea.
Also called marine map; nautlcssl cfrarf.
hydrographlc datum—The plane of
reference of soundings, depth surves, and
elevations of foreshore and offshora featuras.
Also called charl datum. See also
International low watar; low water
datum; National Geodetic Vertical
datum of 1929; eoundlng detum; tidal
darum.
hycfrographlc detail—Tfse featuras elong lhe
shore and the submerged parts of bodies of
water. Also called Irydrographlc teature.
hydrographlc dlgltal data—Dafa In dlscrate
numericaf form derfvad from tha measurements
and descrfpflon of the physical features of the
oceans, seas, lakes, rivers and ofhar wafers,
and their adjoining coastal araas (with patt@ular
reference to navigational ussge.)
hydrographlc feature—See hydrographlc
datall.
hydrographlc reconnalssanca-A
reconnaissance of an area of wafer to
defermine depths, baach gradients, the nature
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