So there's no need to set a point in the ground for a resection, unless you need it in your control network and you will be coming back to it regularly. If you do set a point and measure up, you are negating the benefits of the zero-height setup.
For the last couple of years I've been doing monthly (wet months) and quarterly (dry months) settlement monitoring on a storm drain pump station.?ÿ With all the site obstructions there's only 1 spot from which I can see all of the monitoring stations with the same rod height, with only a couple of tenths of leeway in the setup location.?ÿ Rather than try to find that spot each time by estimation, on Day 1 I set a point in the ground, and I set up over it each time.?ÿ I do measure the HI with a tape, but I don't hold the original point coordinates, I assign it a new point number and let the subsequent resection adjustment determine its position.
Having a consistent height comparison for the control point gives me one more bit of reality check on each monitoring event.?ÿ If I were to use the zero-HI method I wouldn't have any basis for height comparison between occupations.
Also, on topo surveys I always include a realistic HI (measured or estimated) on resected points so I don't have to remember to exclude that control point from the terrain model.
I'm not a fan of the zero-HI approach.?ÿ I just don't see any reason not to use a realistic HI.
With all the usage and even necessity with this location seems to hold, why not make a secondary control point and monument it?
Just curious, I'm still working on learning about what I don't know.
Also, on topo surveys I always include a realistic HI (measured or estimated) on resected points so I don't have to remember to exclude that control point from the terrain model.
I'm not a fan of the zero-HI approach.?ÿ I just don't see any reason not to use a realistic HI.
I use only selected points in my DTM, and all points are sorted to layers by the Descriptor Keys.?ÿ Control points are not being used (also - no boundary points, no utility points, no building shots, etc.), and it's easy to do.
I have a different code for resected (and unmonumented) instrument positions. So they sort to their own layer.?ÿ
I'm collecting a lot of things these days by reflectorless. Basically anything that sticks up. Building walls, signs, hydrants, risers and kiosks, trees, etc. No elevation on such things is valid for DTM, of course. SO the layering strategy is a necessity.?ÿ?ÿ
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There is a big difference between resectioning and what some survey packages use such as a "two point free station". The two point free station is much better but still may not always provide the precision desired. At least once a single two point free station did not provide me with satisfaction. I redid the two point free station swapping the order of points, then meaned the two new locations. That meaned position provided me with mathematical satisfaction.
Paul in PA
I'm collecting a lot of things these days by reflectorless. Basically anything that sticks up. Building walls, signs, hydrants, risers and kiosks, trees, etc. No elevation on such things is valid for DTM, of course. SO the layering strategy is a necessity.
We do the same thing, but use point groups for sorting. Add a unique character or two to a code, and it is sorted out of the "Surface Points" group. Import CSV and linework, update point groups, add breaklines to surface, rebuild surface. Done.
I'm not a fan of the zero-HI approach.?ÿ I just don't see any reason not to use a realistic HI.
For the sake of precision, if you are letting the point float every time you monitor, why introduce additional error into the adjustment? Why not eliminate the centering and measure-up errors?
For most recent monitoring projects I have done, we would not have been able to detect maximum acceptable movement without using zero-height resections. No matter how many points we tie or sets measured, centering and measure-up errors are going to stick around for the duration of the setup.
And monitoring is often a separate task for a larger construction project. In that case my monitoring control might not even be in the same coordinate system as the rest of the project - often I orient it to whatever we are monitoring for ease of movement detection.
Sure, sometimes we'll set a nail at the ideal instrument placement. If we can use that point for later work, I'll have the crew set up over it and record HI in the field books, but always enter zero in the instrument height. If I decide to bring that point (and the other monitoring control points) into the larger control network system, I will process several monitoring sessions with HIs separately against the project control.
Properly done, a zero-height resection is better than a single-backsight solution and will yield better results.
I hadn't thought of that but it makes sense.?ÿ The downside is you have to grade the point(s) separately which seems to defeat the purpose of using the TS in the first place.
For the sake of precision, if you are letting the point float every time you monitor, why introduce additional error into the adjustment? Why not eliminate the centering and measure-up errors?
There are no such errors, as it's not the same point -- it gets a new point number each monitoring event, and its position is determined by that event's resection.?ÿ I look at the adjusted coordinates to make sure they're within the expected tolerance, but otherwise don't use the point for anything.
The image below is a Star*Net screenshot zoomed in to the control point.?ÿ The N-S spread is about 0.013', E-W about 0.010'.
@thebionicman A full amen to that. I have even tested it to make sure and it's very true. The shape of a resection with newer instruments no longer matters.?ÿ ?ÿ"With modern instruments it is possible to do extremely tight work from resections with an angle near 180 degrees... The absolutes of yesteryear are fading fast.. "
That is a good bit of the story. The newer instruments and data colkection bring improved reliability as well. I still run sets of angles but haven't seen one bust in at least 5 years. Strength of figure is less of a concern as hand held computers are inherently more precise than interpolated values. All of it comes together to make measurement options endless.
Now if can just improve the ability to recognize evidence and treat it properly...
I used 3-point resections every once in a while years ago but that was because I was studying to take the exam. I remember the math would take a long time to manually do it and it would take almost a whole notebook paper to show my work.?ÿ
The trick is creating the unknown point in between the known points without creating angles too close to 0 degrees or 180 degrees.
2-point resections work as well but you will have 2 solutions.
Here are the different methods
I believed I used the Tienstra Method when learning this. It's satisfying when you get the correct answer after all that math. Check your work with Auto-CAD or a data collector. You can even enter this as a program in the HP 35s.
