When I orient using resection, it's rare that I can see 3 known points so I usually get set up with just 2.?ÿ I read and hear that I should be using 3 and I understand "the more the better" with regard to identifying and eliminating errors.
But mathematically, isn't using two fully constrained??ÿ A known baseline/distance (albeit not necessarily accurate on the ground), but two measured lines and the included angle.
If I get "good" residuals using two, should that be adequate for, say, getting oriented on a property for construction staking??ÿ How about retracing (not that I do that)??ÿ Of course I always check into another corner or two to see that coordination.
Thanks,
Dave
Yes, I suppose that 2 is mathematically sufficient. Especially if you are running 3d and there is a significant height difference. I require 3 nevertheless. My confidence level with 3 is dramatically higher than with 2.?ÿ?ÿThis is born of training that says to occupy one, backsight another, and always check into a 3rd. Then you are ready to go. There have been times when that practice has saved my behind. The ways of the blunder are many, and varied.
If you use 2 and have something else to check into, then you have 3, don't you? If you really do have only 2 available, then you need to set more control.?ÿ?ÿ
With 2 you always get ??an orientation?. There is no QC on mistake ?ÿ
3 has redundancy and it??s self checking. A B and C: orientation by A B and B C and A C.
It??s just safer. And by least squares, better?ÿ
Ooh. I won't use a resection for instrument set up with less than four. Residuals may look good, but check points don't seems to match with.previous coordinates. Just me, but occupy one of the existing points, backsight the other and set the points you need.
Back in my college days, early 90's, we did resections using only angular measurements, think Wild T2, and 3 points were a minimum with good geometry. If you had an EDM you could then use a minimum of 2 angular measurements and a minimum of 2 distance measurements, and as long as your geometry was good, the result was pretty good. I think the second method was called a trilateration.
If your two points are good, you can locate your setup relative to what your measurements compare to the inverse between the two points.
0.02
If your two points are good, you can locate your setup relative to what your measurements compare to the inverse between the two points.
The inverse between your measurements of two given points provides no check, no new information.
If you do a trilateration using the distances from those two points, then the angle provides one check measurement, but there is a locus of points all having the same angle, so it isn't the greatest check.
Angle and distance to a third point is a much better check.
2 points are ok IF you ave an idea of WHERE you are located relative to the 2 points. With 2 points you are equally located left or right of the 2 points. Both locations will give you save angular readings. If you are going in blind then 3 points will definitely tell you where you are exactly.
"relative to"?ÿis not a definite solution
When resecting, you must stay within your box of control points that you have used for the location of your setup if your solution is acceptable within the quality of what you are attempting to do.
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Good answers.?ÿ Thanks.?ÿ You guys are great.
You have been given some great information to help with an informed decision for performing resections. I will add a few things to the post.
1. Understand the math that whatever software data collector is using. There are different approaches from different manufactures. Also some manufacture use different approaches when doing say a 2 point vs 3 or more points. Some helmert transformation?ÿ some basic triangle calculations integrated with coordinate and bearings.
2. Understand strength of figure and acute vs obtuse triangle math. Apply this in the field. Go back to law of cosines and law of sines. set down with a piece of paper and solve those triangles then change those acute vs obtuse triangles just a little like a few seconds a couple hundreds of a foot. This can give you that mental picture.
3. Check check and check again. Redundancy is your friend.
4. I am not afraid to use 2 points but here is some things I learned the hard way to build in extra checks. Try and avoid being online or close online between 2 points that are known. Doing by hand calcs you could easily end up on the wrong side of that known bearing. When I do use two points after completing the routine in your data collector. Re set up or establish your self. IE the point your DC just calced is now known. So this time tel it your setting up on that known. Back site the 2nd point you observed in the resection and stake out the first observed one.?ÿ I do like to stake out something independent even if all I have is something like the pc of curb and gutter or cl of a man whole anything to make for sure your orientation is not way off like a blunder you created. For example when you set up on a point and bs a point but you manually made a blunder and reversed the point numbers. Without that 3rd point check and if the differences in heights are close you would never know until you get back to the office or the contractor said hey that BLDG is on the wrong side LOL..?ÿ Always build checks in, no matter what. In my early years I was taught to always establish a natural bs or find a tower anything. I have witnessed many bs tripods vanish from the bs when I would turn back to check. Once surveying at a zoo the kids picked it up and was caring it away. I seen it bobbing along. Good luck!
1. Understand the math that whatever software data collector is using. There are different approaches from different manufactures. Also some manufacture use different approaches when doing say a 2 point vs 3 or more points. Some helmert transformation?ÿ some basic triangle calculations integrated with coordinate and bearings.
Excellent point. It is a good idea to familiarize yourself with how the different resection routines work. Depending on how your control is laid out on site, the method used can make a difference in final results.
Trimble has a reference paper on how Access resections work.
Leica also has a good explanation of resections. An old document but likely still accurate.
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