1 - NGA

measurement 01 film speed.
tYlopter-A unit of rnaaauremenf of the power
01 a lens, especially a spectacle type lens. The
power In diopters equals the reciprocal of the
focal length In meters: thus, a lens whose local
length Is 20 cm has a power 015 dbpters.
dloptrfc ey$tern+optica) An optical system
comairting only refradlve elernams (lenses).
MIL-HDBK-850
dlp angle-l. (surveying) The vertical angle O!
the obeervefion point between the plane of the
true horfzon and a sfght line to the Spparem
horizon. 2. (phologrammelry) The vertical
angf% at the air station, between the true and
the apparent horizon, which is due to ffiihl
he)ght, Earth curvature, and refraction.
dip clrcla-An instrument for measuring
me9nel~ dfP. ii consists essentially of a dip
needfe, or magnetic needle, suspended In such
a manner ae to be free to rotate about a
horizomal ezis
dlp correction-See trelght-ot-eye
correction.
dlp eqrsator-Se6” megnetlc equator.
dlp needle—See magrrellc dlp needle,
definition 1.
dip of the horizon—See dip, definition 1.
dlp pola-See ma~netlc pole.
dip-l. The vertical angle, at the eye of an
observer, between the horizontal and the line 01
sight to the visible horizon. Also called dlp of
the horfzon. 2. Tha angle between the
horizontal and the fines of force of the Earth’s
magnetic field at any poim. Afso celled
Incllnatlon; magnatic dip; magnetic
Incllna:lon; magnetic Iatltude. 3. The first
detecfabla decrease In the attitude of a celestial
body after reaching its maximum altitude on or
near meridian transit.
direct angla-An angle measured directly
between two lines, aa distinguished in tranait
traverse frmm a deflection angle.
dkect levellng—The determination of
differences of elevatbn by means of a
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continuous series of short horizontal lines
Venical distances from thesa tines to adjacem
ground marks are determined by direct
observations on gresiualad rods with a leveling
irrstrumam equippad wilh a qzirif level. Sea aLso
difterentlal Ievellng.
direct meaeurement—Any measurement
obtained by applyhg a tape to a fine or a
protrador to an angle, or by turning en angle
with a transit; espcially af@icebie to surveying.
See also Indirect measurement.
direct motion-The appsrent motion of a
planet or other objeci eastward among the
stars.
direct obaarvatfon-A measure of the
quantify whose value is deeired, such as a
single measure of a horizontal angle.
direct photography-Phonography In which
the image of a subject is recorded directly by Iha
camera in the conventional manner.
direct posltlva-A positive image obtained
directly without the use ot a negative.
rtlrect problem—The datermlnatlon of the
geodelic position of the end point and the back
azimuth at position two when the given
information IS the Iatkude and longitude of
position one, along with the torward azimuth
and the distance between the Iwo points.
direct redlal plot-See dlrecf radial
triangulation.
direct radial trlangulatton—A graphic radial
triangulation made by tracing the direcliarrs from
successive radial cenlers directly onto a
Iransparenl plotting sheet rather than laying \he
triangulation by the templet method. Also celled
direct radial plol.
direct acannlng camerss-A type of
panoramic camera wherein the tenses awing or
relate about the rear nodal point at a given rate.
direct telescopa-A telescope Is aald to be
direct when it Is in ifs normal position.
direcf vernfer—A vernier scale which has
spaces or divisions slightly shorter than those o!
the primary scale. The numbers on the vernier
scale coincide with the numbers on the p!tmary
scale,