Resections and their accuracy

[kampho]

Howdy all

I am an municipal engineer that does a lot of site and set-out surveying with a Set 5 Sokkia instrument and SDR 33 data recorder. It is often impossible for me to set up over a known point during construction and I am wondering what the accuracies of doing a resection are. I generally only do a 2 point resection but am beginning to think that I should be using at least 3 points. The way our boys off-set the points I give them probably doesn't warrant me being overly accurate but it would be nice to know if i am doing the right thing!

Cheers

 

[KRSServices]

I think I understand your question. The solution is relatively simple in that you must establish a reference elevation, or Bench Mark which is relatively stable and will be unaffected by influences such as compaction disturbances or obstruction of lines of sight.

The best benchmarks can be the top of existing manhole covers, hydrants or survey monuments (in an urban area) and trees, power poles (spiked) or other immobile objects in open fields.

With these benchmarks, the original topo must be referenced and shot and then the resections or cross sections of construction can then be shot, plotted and compared. Any benchmark placed MUST be protected from disturbance. I hope I understood your query correctly.

KRS Services

http://www.krs-services.com

 

[tim216]

When you resect with an sdr33 you can use as many control points as you can physicaly see.I have used an SDR for about 10 years and my experience has taught me that if you control or traverse is good 2 points is fine since it requires you to take a foward and reverse reading, however if you end up resecting between control points that you established by resection than I would suggest using no less than 3 but 4 or 5 tops then have them stake out points that were set from a different setup as a check on your resection.

 

[sc]

KRS,

I tend to avoid manhole covers, hydrants, water valves, etc. Even though they are relatively solid, they can be removed and replaced without to much disturbance to the surrounds, making it difficult to see if the level has changed. They are also susceptible to moving when ran over by trucks (generally rotation).

kampho,

Act like a surveyor, check, check and recheck your work. Do your normal 2 shot method, then do an independant measure between points that have been placed by resection. This need only be a quick tape measure check to make sure distances are OK.


regards
sc

 

[KRSServices]

y'know sc, in theory you are absolutely correct, but in reality in the 10 or so years of urban and rural construction surveying, I have never lost a level to someone replacing a hydrant or having the manhole frame shift, particularly when I was on-site resident, but that's certainly not to say that it cannot. I too do not use valve covers either. I always establish my benchmarks from a known CCM (survey monument) if possible. Otherwise I run and balance my traverse points, and verify before using to ensure disturbance has not occurred. And you are absolutely correct when you stated check, re-check and check once more. I have found so many young inspectors and field people do not check their numbers and end up a dec. high or low for grades, etc. Costly, costly and stupid mistakes.

KRS Services

http://www.krs-services.com

 

[sc]

Another good thing to do on a long job is to establish your own concrete and steel bench marks (survey monument?). I have done this a few times and generally use a 300mm square by 500mm deep belled concrete pad with a steel rod protruding 10mm from the centre. It is not permanent, but it is reliable if placed in a safe location that you can setup over. I generally create a minimum of 3 and try to align them on complete lengths and bearings, ie 33degrees and 155m to simplify checking.


regards
sc

 

[carlorom]

Hi,

I generally agree with all your comments but warn against using two points only as you will get a seemngly valid result from two points but if one has been tampered with the consequences can be dire so check and check again three undisturbed points or benchmarks are advisable. Manholes have been rebuilt and shifted in the past so I avoid them ton sites that need frequent revisits I have now resorted to use adhesive targets - see elsewhere on this forum.If you have enough of them on site they can be very useful to establish position from any convenient setup.

 

[RDK]

I’ll typically use two points for the resection and then shoot a third to verify the setup. This has always produced acceptable results for me.

I also use a Set 5 but only the on board data collection.


Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com