Hi guy
I traversed a construction site a while back put in stations on the perimeter that I'd use later for resection. I always use 3 stations to resect.
Now that construction is underway I've got one small piece of setting out to do in a corner of the site where I can only see 2 of my traversed stations. So what do I do? In the past I've banged extra nails in the ground and then re-done the traverse.
But I'm wondering if there is an alternative solution for the odd scenario like this when I need an extra station to perform my resection.?ÿ
Can I set over one station traversed station, backsight another traversed station, shoot in a station on a tripod (F1 and F2), and then use that station as part of my resection?
Clearly it won't be as good as traversing the extra station in but if I use a long backsight and it's not too far from the set on station when I shoot it might be okay.
When I perform the resection the onboard software delta results will act as a check that the extra station was shot okay.
Kind regards, Andrew
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So what do I do?
What are you setting out? Would a 2-point resection suffice?
I??ve done many 2 point resections. If you know the 2 points are good, no problem depending on what you are staking out.?ÿ
Classical resection was done using three stations (minimum) and angles only. That was before total stations. it could not be done with only two stations, as there was only one observation (the angle between them) and two unknowns (N and E of the occupied station). Now that you can do angles and distances, there is sufficient redundancy to do resections with only two points (four observations, still two unknowns, or six observations and three unknowns for 3D). Of course, more is better, but if you know the stations are good, you can predict your accuracy using the simulation function in a least squares program. Geometry matters but not as much as it did when we were doing angles only.
Do you need the instant gratification of a resection?
I'd probably set two new random points where I could collectively shoot 3 or 4 of my old control points, if possible, and bring them into Carlson and rotate them into place.
What you are proposing to do is to merely extend/densify the resection control. No problem with that. But when you stub out control you should tie it redundantly in some fashion or another. On way to accomplish that would be to tie it a second time using a different backsight.?ÿ
OK. Do this.
Use the data collector to do a 2 pt. free station from two. Make sure you get them in the right order and don't flip your world backwards. Set another point at 90 degrees, even if it is short, then shoot it twice in each face. Data collector should have a happy average for it now. Break setup and move. Now shoot all three. Problem solved, and you have data for the office where they could put it all in an entirely different least squares package and make sure of what you did. For bonus points, if you have enough tripods, leave a glass at the first resected point and shoot it as a 4th control.
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Geometry matters
Avoid angles approaching 180?ø when using 2-point resection!
@field-dog With modern instruments and calculators an angle near 180 should be no problem. We aren't restricted by verniers and interpolated functions...
I'll take your word on that. I've been influenced by the warning in the old SMI User's Manual in regards to their 2-point resection program. I'm too tired to dig up the manual right now.
@field-dog With modern instruments and calculators an angle near 180 should be no problem. We aren't restricted by verniers and interpolated functions...
The key is that nowadays you measure both angle and distance.?ÿ In a direction where an angle solution is weak, distance solution is strong, and vice versa, so the combination is nearly always good except with really bad geometry like the known points too close together.
Occupy pt2 BS pt1, trav to new pt3. Occupy pt3 BS pt2, shoot pt1 as a check. Start staking.
@bill93 we began adding distances with the advent of affordable EDM's. It was still tough to get a good solution near weaker angles without a really good instrument and calculator. As verniers gave way to plates and scientific calculators showed up in trucks those problems faded away. Unfortunately it's like everything else. We quit teaching the why and people hang onto the rules when they no longer apply...
Occupy pt2 BS pt1, trav to new pt3. Occupy pt3 BS pt2, shoot pt1 as a check. Start staking.
I agree, why not just traverse from these two points back to another point for a check?
It's called work for a reason. No easy button.?ÿ BTW: resections from two points work just fine. However, you always want to check into a third point. I have seen instances where two control points 150ft apart moved 0.05' in the same direction due to excavations and compaction.
