Tribrach

GDF 22

In 1924 Heinrich Wild brought the T2 theodolite into the world: it was small, light, precise and had optics without parallel. It was supplied with a simple device to let the target and instrument be swapped over from one levelled set up to another. It created its own standard, it became the core of survey methods for years to come, it is in daily use all over the world and will continue to be so for years to come: the interchangeable tribrach. At the time of its introduction there was no ‘standard’ tripod mounting  for survey equipment. Instruments were available with a variety of mounting systems; 4 foot-screws, dome heads and fixed plate to name but a few. The ‘railway’ theodolites of the 19th cent. tended to be used as single piece with the tripod; the Wild tribrach changed all that. Surveying with a theodolite became mobile.

So what makes it so special and why did it become a standard?

1. It makes the 3 tripod traversing method work, the interchangeability between target and instrument means that the surveyor only has to centre and level the tribrach once for each station set up; a big time saving and an elimination of a source of error.

2. The tribrach used a simple (5/11th inch thread, the same as the standard scaffold fixing in construction) screw fixing to the tripod plate; it’s relatively easy to replace and the plate can be modified to fit other mountings such as scaffold tube, railway lines etc.

3. It’s small and light and makes cheap tripods work harder. A good tribrach will get precise results from a poor tripod but not the other way around.

4. It has become universal through use. Once EDM became a common method of traversing, cheap copies (famously by MOM in Hungary) of the Wild pattern tribrach sprang up. Described as ‘Swiss pattern’ they were made to meet the post war demand for 3 tripod traversing. Rival systems (notably the Kern ‘dome’) were costly.

My traversing set

So in this neat box I have all I need to get the maximum precision from a total station, 2 prisms and 3 tripods. When this box goes in the boot I’m off on a day chasing millimeters between nails or pegs. Control is not my favorite job but having the right tools for it is a huge comfort!  It’s a mixed bag of blagged kit (a Hungarian GDF 122, a GDF 111-1 A and the classic GDF 22 from the 1980s) but they have been calibrated as a set and my traverse results are fine with them.

So now I’ll get to the point and show how to set up over a point, this is a fundamental skill that makes survey work!

3 steps to set up tribrach over a point

1. Fix the tribrach securely to the tripod stage, tighten firmly!

2. Get the thing roughly level and over the point by eye then bring the plummet crosshair over the mark using the foot screws (this is the counter-intuitive bit; you are using a ‘level’ adjustment to ‘centre’ the set up). Always work on 2 or 1 foot-screw, never one at a time. Watch how the plummet cross hair moves. Always move your thumbs in opposite directions (thumbs in or thumbs out) to move the 2 axes of the centre mark over the point.

3. Use the tripod leg adjustment to level up over the point, again this is counter-intuitive, you will be pivoting the tribrach over the point until it is level. Repeat the sequence from the step 2: foot-screw adjustment to remove the small error created by the leg adjustment and fine level.

So there you have it, the procedure is the same for laser plummet tribrachs, the squinting through the optical plummet may become a thing of the past but I’ll be keeping mine: they work without batteries and don’t trip you up as you move targets from one tribrach to another!

Setting up over a point is a practical skill, it takes a bit of practice but once mastered you will be able to position your instrument where you want it and move it around the survey without grief.

More on Wild history is at:http://www.wild-heerbrugg.com/milestones_of_technology.htm

mmmmmmmm

Survey hardware supply & calibration:

http://www.sccssurvey.co.uk/s/home

Klaus on the road with kubit:

http://klaus-kaiser-kubit.blogspot.com/

About billboyheritagesurvey

Heritage worker
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4 Responses to Tribrach

  1. David Andrews says:

    May I suggest a Step 4
    If necessary un-clamp the tribrach and slide (don’t rotate) it so that the crosshair is aligned with the marked point. Re-check the level.

    • Thanks for the comment David. I consider the ‘slide the stage’ step something of a last resort and don’t teach it as it can be taken by some as a ‘fix all measure’..which it isn’t! Provided you slide the tribrach in one direction ( it’s working like a wedge with a level top at this point- so there is only one direction of slide that works with this step) only and not 2 at once…AND check the bubble afterwards is a nice way to ‘sharpen up’ the set up if you need to.

  2. Great for being going to your website again, it has been months personally. I require these postings to comprehensive my assignment inside the college, and it also has identical topic with your posting. Many thanks, great share.

  3. Pingback: Traverse with TheoLt pro | Billboyheritagesurvey's Blog

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