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material.
Iloetlng mark-( photogrammelry) A mark
seen as occupying a poaifion in the threedimensbnal
apaca formed by the stereoscopic
fusion of a pair of photographs and used as a
rafarenca mark in examining or measuri~ the
stereoscopic mcdel.
lloetlng-(cartography) The technique of
making minor adjuatnwnrs of detail in order 10
maintain their pruper relativa position.
flood control map-A special map, or set of
msP, *ai!3@ for study and planning the
control of areas subject to inu ndafiin.
flood tide-The portion of the tide cycle
between low water and the following high water.
Also called rlslng tlda.
IIOWllnS-The slope extending from the
heighte abng the neatline to the model datum,
al en angle no greater than 45°, to preclude
Iorming the plaslfc shee! at a 90” angle at the
neatiine of a plasfic relief map.
fluorescent map-A map reproduced wllh
fluorescent ink or on fluorescerrl paper, which
enables the user to read the map in darkness
undar ulfravlofaf light.
flux-gate magnatometar—An instrument
designed to measure the Earth’s magnetic field.
Also called aaturabla reactor.
ffuxmefer-An krsfmmenf for measuring the
infeneity of a magnatlc field.
fly lavaling—See flylng Iavala.
fly-by mathod-(surveying) A technique of
datermhrlng approximate elevations where
extremely rugged terrain is enmuntered. The
principle Is Idarrtkal to the two-base method
except the roving barometers are air transported
and read in the aircraft as it passes on a level
with the topographic teature whoee elevation is
required.
Itying Ievale-1. A level tine run at the close
of a working day fo chack the results of an
extended fine run In one direction only. Longer
slghfs and fewer setups are used as fhe
pur’pese is 10 deteci large miefakes. Also callad
fly Iavalhrg. 2. Level Ilnes run with the
engineer’s ordinary leveling equipment buf wllh
—
MIL-HDBK-850
a distinctly bw order of acsuracy. Enor of cbsure
may be perhaps one foot [or more) fintas the
square root of the dstanm in mifee.
focal len@r-A general farm for tha distance
between the center, vertex, or rear rmda of a
lens (or the vartex of a mirror) amf the point al
which tha Image of an Infkrffefy distant object
mmas into critical fores. The term must be
preceded by an adjective such as %qulvalen!or
%slibrated- 10 have a precise rrwankrg. See
also back total Iangth; callbratad local
length; ef Iectlve local Iangth;
equivalent focal Iangth; nominal focaf
Iangth.
focal plane--(pholography) The plane,
perpendicular to the axis of fhe lens in which
images 01 points in the object tieki 01 the lens
are focused.
focal point—See focus.
focal renge—See depth of Iocua.
focal.plane plateA glass plate set in the
camera so that the surface away from the lens
mincides wifh the focal plane. Its puqmse is to
position the emulsion of the film in fhe focal
plane when fhe film is pnye.icelly pressed Into
confad with Ihe glass plate. Afso called
contact glaaa; contact pteta.
tocua-The point toward which raye of light
mnverge to form an image aftar passing
through a WM. Also defhmd as tha condMon of
sharpest imagery, Also called fOCaf point;
prlnclpal focus. See a160 hyperfocal
dlstanca; sideraal focus.
folded optlca—(photogrammetry) Any optical
or lens system containing reflecting ccmnponams
which reduces the phyalcal iengfh of a
photographic or sensing system, or changas the
path ot an optkal axis.
foldlng varnler—A single vernier so
mnstmcted and numbered that it may be raad
in eilher direction.
toot.meter rod—A stadia rod, marked in feet
and tenths on one side, and meters and
hundredths on the other aide, used to
determine distances and elevatbns in one unit
of maasuremenf and to check them by readings
in a different unit,
torca tunction—See potential.
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