(Coincidently, I prepared this question before I saw the "3 point resection thoughts" thread.)
Who has an idea of how often, or under what circumstances, you actually use the angles-only resection technique, these days?
I'm talking about "classic resection". That is, set up at an unknown point, observe sets of angles -- but not distances -- to two or more control points, and calculate your unknown point position.
I know that, in theory, if I don't have GPS, my control is inaccessible, and i don't have reflectorless total station, then I'd need it. Are there times when it happens to you, or other situations I'm not thinking of?
My boss seems to think it is extremely rare and I can forget about it, but I know I've used it (OK, it was 30 years ago) and a few Copan users have asked about it.
Note, I'm not asking about the "modern resection" or freestation technique (i.e., angles-and-distances resection).
Cheers,
i would use it if i needed to, but it does seem like it would be in very uncommon circumstances. now where did i put that T4?
I have not done it in a while (25 years?), but I have done them in the past using intersection stations (i.e. radio towers, water tanks). So, to answer your question, none recently.
The solution to an angles only resection is much weaker than one in which angles and distances are measured. The latter is much less affected by station geometry.
I have not used it to determine my instrument position, but just last week I used it to locate two transmission line towers that were inaccessible. My control was positioned about 200' left and right of the centerline and 1000' from each of the towers. Using a 3" Topcon robot I turned angles to the ends of the tower arms and the mounting point for the center insulator (all three should line up). On both towers the three resulting points were lined up within .02' and the center point was within .05' of being the true midpoint. Not bad for arms that had a spread of 100'.
> ... observe sets of angles ... to two or more control points...
Of course, i should have said, regarding resection, "... to three or more" points 😐
I do it frequently with my compass work (bearings only), simple and quick in Starnet.
I used to do it a lot for topo control from my own control nets on military bases where GPS is blocked. (I would always include naturals on control nets).
A good tool to have in the box.
I think you're describing intersection (observing angles to an unknown point from 2 or more control points), which i believe is relatively common.
Last time I used it was about 1976 in the Tygart River Valley (WV).
Used the T-2 to observe three well spaced radio towers with third order NGS positions. Got good results and saved about 12 miles of traverse. I had really forgotten about it until you posted.
If the question is "should we include it in our product" the answer is Yes.
People may not use it for Real Work much but it will be there in case a curious young'un wants to learn.
First U need a minimum of 3 known points , 2 if have distances as well.
As I have had edm since 1980 I only use distance and angle resection.
RADU
It seems angles-only (classic) resection IS still used!
After asking for clarification from a Copan user, it seems that in Italy (and many other places, i'd think) there's still a need for angles-only resection:
In the years before GPS, many of the official survey control points were church spires and other such unoccupiable places, established via intersection. The old control network is not compatible with WGS84 and many old control points are of course still unoccupiable.
So surveyors are now often required to use both control networks, hence the need for classic resection (when reflectorless total stations are not available) from church spires, etc.