Brunson 771 - Page 9

5 Operation and Adjustment Manual
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The“‐H”inthevariousmodelnumbersreferstoa“Hollow”verti‐
calaxis.Transitsthathavethe‐Hdesignationallowyoutolook
straightdownthroughahollowverticalspindleandtoseetargets
orcollimatorswhicharedirectlybelowthebaseoftheinstrument.
Whydoweusefour“feet”(levelingscrews)insteadofthree?

Weareoftenaskedthisquestion,sincemosttheodoliteshavethreeleveling
screws.Thereasonisthatwhenadjustingfourlevelingscrews,theinstrument
heightisnotaffected.

Whenthreelevelingscrewsareused,twoof
themmustnecessarilyformapivotaxis,and
thethirdisusedtoraiseorloweronesidetotilt
theinstrument.Itgivestheeffectshownatleft.
Thereddotindicatestherelativeinstrument
heightbetweentwopositionsoftheinstrument
(tiltedandnottilted).


However,whenfourlevelingscrewsareused,
theinstrumentposition(again,reddot),effec‐
tively,doesnotchange.Thisgivestheeffect
shownatright.Onesidegoesupbythesame
amountastheothersidegoesdown,andthe
instrumentpivotsatitscenter,notononeside
ortheother.

Thisisjustanotherexampleofthewaythatwehavecraftedourinstrumentsto
bemoreaccurateatshorterranges.Overlongerdistances,minorchangesinin‐
strumentheightarenotimportant.Butwhenmeasuringveryaccuratelyover
shorterdistances,avoidingchangesininstrumentheightcanbecomecrucial.