Optical Instruments – Terminology?

[Swarf, Mostly!Participant @swarfmostly]

[Swarf, Mostly!Participant @swarfmostly]

Hi there, all,

I hope that my question won't be ruled as outside the scope (no pun intended!) of this site – I know that at least one member will have the answer.

On that well-known auction site there are often listings for engineer's (maybe it would be better to say 'surveyors' ) levels, the tripod-mounted telescope with a bubble and a cross-hair reticle.

They are often described as 'Dumpy' levels but I thought that was a trade name, not a generic one. Am I mistaken?

They are also sometimes (often!) described as 'theodolites' which used to annoy me but nowadays I just shrug and move on.

Best regards,

Swarf, Mostly!

 

Edited By Swarf, Mostly! on 17/10/2012 15:15:03

[Martin KyteParticipant @martinkyte99762]

I am happy to bow to superior notice but I believe the Dumpy bit indicates that the instrument auto levels itself when set close to level. It does this by use of a compensating thin prism which is suspended by its upper edge. Small errors in setting the instument level on the tripod are compensated for by the attitude of the prism.

regards Martin

[David LittlewoodParticipant @davidlittlewood51847]

Google is your friend…. **LINK** and **LINK**

David

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

A Dumpy level is one axis – just basically angle in a vertical plane . Usually used in the null mode ie telescope set level and used to read off markings on surveyors staff . Traditionally used with a Chain measure for horizontal distances as staff is moved around . The simplest version of a dumpy level is just literally a level with a telescope on top .

A Theodolite is two axis – angle in vertical and horizontal planes . Often used in semi null mode ie telescope set level and used to identify horizontal plane angular locations of objects as read off on a scale . This is the instrument used for full surveys and map making . The vertical plane settings are used primarily for measuring relative heights and heights of mountains . This is the instrument needed for full triangulation surveying .

Both instruments always have sophisticated levelling system built in . Old ones had plumb bobs and just occassionally mercury flotation levelling systems .

Modern laser instruments are essentially the same but much easier to use and often have GPS built in .

You will often see building sites being laid out now with just a GPS box on a pole .

Michael Williams .

[Clive HartlandParticipant @clivehartland94829]

A Dumpy Level is a level that has a bubble in tandem with the telescope,and when levelled over the three footscrews will determine a back to back level around a project. It usually has a circular bubble that allows coarse levelling of the instrument before critical levelling of the tubular bubble.

It is used in conjunction with a surveyors stave and needs two operatives. Accuracy will be about 2.5mm at a 100mtrs.

An Automatic level is quite a sophisticated Level, it has a damped pendulum that has about 15' of swing back to front and sometimes has a flag comes into view to show it is out of level. Again it has a circular bubble of 20" accuracy for course levelling for setting up.

These levels nominally are about 8" accuracy and some less than 1" and to 100ths with an attached plan Plate over the OG. EG. An N3(70) from Leica.

Theodolites are now being designed and built with so many functions that we workers who repair them have a job to keep up with the technology. The latest is fitted with a TV camera that allows realtime viewing of the target and has X hairs in the pic.

Laser measuring is now the Norm with reflectorless working and accuracy to sub 1mm.

Most are now automatic and can seek out a special target, the Theodolite is left in one position and the operator walks about with the target on a pole that also determines the height of the ground at that point. As stated variations exist with GPS which is used for larger surveys and you may see them used by Police at road accidents also.

Clive

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