I've got to disagree with your position on resections. If done well with good geometry they can be a way of eliminating errors. I resect to at least 3 points of offsite control - of the same type you seem to be using as check ins. Check into a forth.?ÿ
I agree with Norman. Yes, if you have a minimum number of points to resect from then there can be problems. But, with redundancy I feel it is a great method that can provide instantaneous checks right in the field.?ÿ
I should have been clearer: I should have said never rely on a minimum resection. I've lost count of the number of times I've seen people do two distances or three angles. When it is pointed out that they would do better with more targets the invariable answer is "Well, if I do it my way I know the answer is always right - if I do it your way then there will be a discrepancy and I don't know what to do"
That reminds me when I was getting ready to do a 130 mile second order level line in the late 80's. My boss wanted to use a certain crew, so he told me to show them how to do it. I explained to the party chief (old timer) that we would set new marks every mile or so, and there were about 30 existing marks along the route, we would run between them, and then double run any sections that didn't close between the marks, or that were too long between exisitng marks. He said he wanted to just use one mark, if we had multiple marks constrained how would we know which ones were correct? Head-slap. Needless to say I used a different crew
@benchmarkses Don't forget to measure them the same way. From which way you turn the instrument and sight in the targets. All those little things make a difference.
I have a settlement monitoring project (vertical only) that I've been working on for about a year and half. Monitoring events are monthly during the wet season, quarterly the rest of the time. It's very small-scale, a drainage pump station, and I can see all of the monitoring points as well as the three off-site control points from one setup. On the first day I set a cup tack in a pavement headerboard to mark that location, but I resect to the three control points to establish the instrument position. However, just for fun I set up over the tack every time, measuring the HI with a tape, as I was curious how much movement I'd see over time. For a very casual control point it's been remarkably stable horizontally: maximum minus minimum is 0.012'N and 0.010'E. The vertical shows more movement, about 0.035', rising and falling with the season.
Can you explain what you mean by never rely on a resection? What would you use instead?
See my further comment about redundancy in the observations.
If possible it's always better to have "fixed" positions to set up on as this eliminates one source of error by having all the observed sights similar from one set of observations to another (leaving aside vehicles parking very close to the line of sight and introducing either a vertical or horizontal refraction error). The stability of these points can be checked by either having a number of other fixed points available so that you can also resect the position or traverse to it at intervals. You might normally resect but then traverse if the resection residuals were poor.
Critically the other points need to be away from the area of interest and also away from whatever might be causing the movement in the area of interest. Thus if you think an underground pipe is leaking and causing subsidence it is pointless in putting "fixed references" on manholes to that pipe even if they are 100 metres from the site.
Remember, monitoring is all about CHANGE. Actual position is generally less important than change of position, so the essential element is that everything which is done must be as close to a repetition as possible. Don't recalibrate the instrument half way through the job (unless you have dropped it!)
OK I just wanted to thank you all for the replies - I haven't had much time to post in the last couple of weeks.
I'm none the wiser about a better method, I've been using control points as far as reasonably possible (up to 150m) from the monitoring site, and usually use 4 retro targets. I have purchased some monitoring prisms (with ATR) to use for control points which is working nicely at the moment (XPad has an auto resection to these prisms which is super easy). The only issue with these is where to site them without danger of them being knocked off or removed. So I'm hoping that at least the resection will be solid and reliable now. My original issue was whether I need to change instrument in order to get better accuracy, so I'm trying the prisms in order to avoid having to do that.
The instrument is a great instrument. One thing you might do, is have the shop check your instrument. We had one of ours start to have a difference of about 2mm between different modes, and the shop was able to get it straightened out.
That said, the factory specs are +/- 1.5 mm for the EDM, so...run your check and adjust, if the T value is high, check with your local shop.
@dmyhill thanks 🙂