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prqectivety refeled figures poaaeas ce~in
metric chsractetistics whii are Invariant under
projection. Also called thaory of cross ratio.
thaory of cross ratl~e theory of
nrrharmonlc retlo.
thormel imagery-(JCS) Imagery produced
by sensing arwt mcotding the thermel energy
emitted or reflected from the objects whmh are
imaged.
thormomatrlc levellrrg-The determination
of elwalbne above sea level from observed
values C4 the boiling point of wafer. A type of
indirect favefing.
thick lens-A term usad in geometrical optics
10 indicata Ihsl the thtinesa of a tans Ls
considered and that afl distances are being
massurad fmm the nodal points instead of tha
lens canter.
thin lana-A term used in geometrical optics to
indicate thet the th~knass ef a lens is ignored
and thet all dstances ere measured from the
Iems centec usad for eppmximate
computations...
thlnnhrg-The process whereby a Ikrasr
feetura is ganeralizsd through ths use of e
series of rules thst reduces the number of data
points while maintaining the bssic shspe of the
feeture.
third-ordar Iavellng-spirit leveling which
does not attain Ihe quality of ~-order
leveling, but doas conform to the current
epacifiitkms fcx third-order Iavefirrg per
“Clsesitiaition Stenderds of Accuracy and
General Spacifiitiorw of Geodetic Cairn]
Surveys.” Recommerdad for most general
vertical control purposes within a fimited eree.
third-order traveree-A survey treverse
which extends between adjusfed positions of
other control sunmys wfkh conform 10 the
current .mecifiifiins for third-order [class I or
class 11)iriangulafii per “Ctseeikeiiin
Sbmdaida of Accurscy and Ganarrzl
Spaciiitiene of Gaodefii Control Surveys.”
ffaoommendad for meat generel horizontal
control purposea within a fimifed area.
Ihlrd-ordsr trlangulatlorr-Formerly known
as terliery thrtgulalin, these survays conform
10 current specifiitiona for Ihird-erder (cfesa I or
class 11)triangulation per “Ckrssifiition
MIL-HDBK-850
257
Stmdatds of Accuracy and General
Spaciiit”m d Geodatii Control Surveys.’
Ractrrnmerdedfor rrmaf ganeral horizurtal
control purposes withn a limited ersa.
third-ordar work—This is Ihe bwesf order d
control surveys fer which monumentafiort b
authorized.
threa ●lgma(30)-The 99.73 percant
wrrfiderrce interval C4 a dkfribution. See afse
standard wror.
thraa-amr proWastor+ full-circle protractor,
equ@ad with three arms, the fiiucfal edgaa
(astarrdad) d whbh peas through the carder of
the circle. Ths middle arm is freed end raatfs 0°
on the graduetsd circle. The other arms ere
rnevsble, and thair pesitions on the circle ere
read with the aid of verniers. The two movsble
arms are equipped with clamps and msy be se!
at any angle with raspd to the fiisd arm, withjn
the limits of the inetmman!. It is used for firrdtrg
a (ship’s) posiiion graphiity when the angles
between three known fried points are available.
Alse celled station pointer.
three-body problem—That problem in
classical Celestiil mechenics whiih Wets the
motion et a small body, usually of negligible
ins=, relative to and under the gravitational
influence of two other finite point messes.
thrsa-dlmanslonel (3-D) date—Volumetric
data representing maesuremerrls in three
dimensions, as angular or linear measures such
as phHembda-kspps or latitude-longitudaalevation.
thrae-point mathod—See raasctlon.
thraa-point problem-Tha determination of
the horizontal poeitiin of e point of observation
fmm data compriakrg two cbsawed horfzorrtel
angles betwean three objects of known position.
The problem k. solved graphtcalfy by fhe use of
a Ihre-ann protector, and anefytikdy by
Irigormmetricel computation. Sea also
resection; trlanglo-of-error method.
lhree-wlra lovellng-A method of Ievefing
applied when the reticle of the level hss three
Iirras. The rod la reed al each of the Ihree fine=s
and the avenzge is used for the final reeuft with
an accuracy as great as it three lines of Iavefs
had been run and the results avereged.
llcks—Sae raglster marks.