Surveying

Instrument Basics©2004 Dr. B. C. Paul

Getting Over Your Point

• Implied in saying you know one point is the idea that you can set up over the top of it.

• Exact Point of instrument set up is determined with a plumb bob– Common to have a stake in the ground with a

nail in the stake– Instruments have a place to hang a plumb

bob down till almost touches the point

• Need to set up above the point

Setting Up Above A Point

• Tricks– Have to set legs up level and have plumb bob

hang down above the point

• Reality Adjustments– After getting “in the ball park” still have to get

things level• Leveling will normally move you off the point.

– Most instruments allow the head to shift• Tightness of leveling knobs free the head to shift or

hold it in place

Variations on a Theme

• Theodolites may have “optical plumb bob”, little view finder and scope that allows you to look at exactly what is under you.– Large objects have enough attraction to shift a plumb

bob• Drove the early British Surveyors nuts when they went

to India and the Himalayan Mountains pulled the plumb bobs to the side.

• Suggested the point was under the instrument– Underground if put the point on the ground some

equipment operator is guaranteed to take it out– Put the points on the roof and instrument under the

point

The Telescope

• Scope is equipped with cross hairs

TelescopeSunshade

Eye Piece

Focus

Eye Piece generally turns clockwiseAnd counter clockwise to focusThe cross hairs for your eye

Focus knob generally focuses the scopeOn the field of view

Proficiency Time Again

• Go Out and Set Up Over a Point with a Plumb Bob– Level the instrument– Sight something with the telescope and focus

both the object and cross hairs

• Go Set Up Over a Point with an Optical Plumb Bob– Level and Sight