Good morning.
Have a client that is remodeling an existing 17 story building and wants us to stakeout a new proposed stairwell on each floor as it is cleared for remodeling.?ÿ No clear hole from top to bottom, except the elevator shafts, to transfer control.?ÿ We could traverse up the existing stairs I suppose, but those short shots don't give me warm and fuzzies.?ÿ Already suggested drilling a couple holes in the top floor in the proposed stairwell area, dropping a plumbob and marking the next floor down, etc...?ÿ ?ÿThey are looking for a quicker, ore cost effective way than setting up a drill rig 32 times.?ÿ
I could resect from 2 or 3 control points down the streets, but that would require shooting thru existing glass windows.?ÿ Searched a couple of times on the www, but haven't found anything yet indicating if this is a good idea or not.?ÿ I'm concerned about refraction affecting line of sight and distances especially as the angle thru the glass changes with each floor.?ÿ?ÿ
Opinions and suggestions from those that may have encountered this before welcome.
Thanks,
Greg
I am thinking that if you got the same distance shooting through the window from both directions you would be fine. It seem that coatings and the incidental angle could be factors.?ÿ
Shooting through glass will give you a faulty position due to refraction.?ÿ While it is routinely done on monitoring project, it works because we are looking for relative positions and not absolutes.?ÿ The monitoring TS is always in the same location and shooting through the glass at the same point and angle of incident thus the refraction is constant resulting in correct relative positions.?ÿ If you remove or change the glass you get different values.
When it is so important to be precise, the glass can be removed.
If you can get permission and access, any of the surround buildings could be used to set some control onto in places you can access with your station. The remodel part is the pain, as most new construction provides portals for control or elevations to be carried.?ÿ I've done the level loop up and down a parking garage ramp to carry an elevation and was able to keep it inside of 0.01 with out cheating, not easy, but doable.
Good Luck!
We ran a closed loop through a college library prior to construction so we could locate the existing columns for design.?ÿ We shot through glass, but kept ourselves fairly perpendicular, both vertically and horizontally, figuring that would cut down on the effects of refraction.?ÿ We closed less than 0.015', which made me think we were fine.?ÿ The addition went up without a problem.
If the elevator shafts can be used this would be the easiest. Are they already removed??ÿ
Otherwise a service duct or have to core some holes. Coring is not expensive here. Can be difficult if the slab is post temsioned.
IF the windows can be opened at all, then one way I have worked round the refraction problem is to fix large paper targets on the outside of three windows on the same floor and shoot them non-contact from the external control. That gives an accurate position on the outside of the glass. You can measure the thickness of the glazing units. Inside set up more or less square to each target (hence no refraction problem). Measure to a sheet of paper inside the window, measure to the target through the glass (square on). If the distances differ by the glazing thickness use the direct observation, otherwise use the distance from the paper and add the thickness of the glazing. Traverse between the three stations and adjust that onto the external coordinates - don't try and resect inside from two targets as refraction will come into play.
17 storeys may be too high to get the non-contact, but if you can get a good way up at least you will have a solid baseline to use for any further traverse upwards.
Could you stick a target on the outside of 2 windows on each floor shoot them from the outside. Then set a point on the inside perpendicular to each target . Sight the center hold a target In front of the shot flush with the window and add the glass thickness
In these situations it's time to test. It seems trivial to do some testing, grab a window and hold it in front of a tribrach with a prism. Hold it normal to the line and tilt it different ways to test how it works with an acute angle to line. Move the window and watch what happens. Sounds like a couple of minutes of fun to me. Remember what our instruments are constructed with. The lens is made of glass and works by refraction.