Explain "Weak Traverse Angle"

in the presumtion you are using a decent edm, the weak angle theory no longer holds water. yes, the sine of an angle changes quickly near 0 and 180 degrees. it is my expectation that the comment came from a surveyor from the theodolite and steel tape days. weak angles were a concern at the time.

these days, an edm measures angles and distances very (almost equally) well, so there is not a "weak" angle.

my two bits, and no offense intended. there is a T1A and a steel tape in my basement that just wept a little over a weak angle

Weak angles can create weak strength of figure for your traverse loop. Sometimes this cannot be avoided. Strength of figure is very important for loops, not so much for CORS stations and their relation to a GPS rover collecting static vectors.

Just for drill, run your traverse numbers through again, only begin and end on different points. If the numbers are still good, you may be okay, otherwise, you may see what the old man is talking about. There is a routine in C&G that does this or you can do it by hand since you have a small traverse.

For the real world though, I'd probably be like you, but it is very critical to understand what he's trying to say and use that when traversing.

Good advice Kris and every "younger" surveyor should try what you suggest...

If you start and end your traverse calculation on each point, you will see a different misclosure for each run. The point where you have the greatest closure, is the point that most likely has the weakest angle. That is why you have a good misclosure at that point, because you are excluding the angle at that point.

After you run an angle adjustment, you will get the exact same amount of misclosure at each and every point (if you rerun that exercise with the new angles).

Running the test Kris suggests, will be a good check to see if that angle makes for greater misclosure. A good least squares adjustment software will isolate out those same angles you will find by running through the exercise of checking misclosures and will "weigh" the amount to adjust each angle.

I grew up through surveying with the same understanding of trying to minimize the amount of angles that are close to 0 or close to 180. I have started to believe that is not as big of a deal as I used to think. It is true that moving the angle slightly will change the sine or cosine value a lot greater amount, but I struggle with the idea that it is all just positions on a circular plate regardless of where your backsighting or forsighting.

Just some random thoughts here. The only advice I have is check the different closures as suggested. It will give you a better feel of what your real misclosure is. Since it is all the same traverse, you might vary from 1/20,000 to 1/150,000. The less it varies, the more likely your angles add up to close to the prescribed amount 180(n-2) for interior angles or 180(n+2) for exterior angles.

Explain "Strength of Figure"

p. 8, 1-8, Strength of Figure, Higher Surveying, Breed-Hosmer-Bone

daw

Weak angles can be factored into a LSA. Weight the angles that are weak.

FWIW I generally will have to have "weak" angles in an urban setting. Of course always try to work some way to cross check or something, but to stay on budget I would not worry about it as long as you have some closure and adjust things.

If you use a "Compass Adjustment" the error in angle closure will be distributed evenly, even it is perfectly obvious that there are weak angles (based on the length of the legs) in the traverse.

For instance if you have an angle with two legs 500 feet long and an angle with two legs 100 feet long, and you apply an equal amount of angular error to both angles,irrespective of the length of the legs, it will affect the position of the two points dis-proportionally.

This is one reason why LSA are the way to go.

Merlin's answer is pretty concise.

N

I would have to disagree. Yes the equipment is better than the old stuff, but still sorry procedure is sorry procedure. Why do you not use two nails that are 5 feet from an iron and only 6 inches from each other to tape in the iron? Poor procedure. The distance measured is too close to the same. The distances cannot be measured precisely enough to give you a solid location. Can you probably get it in correctly? It is possible, but it is still shoddy procedure. The same holds for weak angles with newer equipment. The inst may have a readout to 1", but look at the specs. The gun may record distances to the .005', but again it is an approximation. It just makes for weak math. The third side is so much smaller distance wise that the difference may fall within your acceptable error parameter but still be wrong. (Say you sighted wrong by 5" and your bubble was off by 0.015'). Just like you would not want to do swing ties to set an iron off of two nails near each other, you would not want to set your gun up and turn a shallow angle to move ahead. It is the same concept, just using bigger numbers.

At least that is my opinion.

Doubling Angles Requires 2 Direct & 2 Reversed

A good traverse also requires you to shoot your backsight distances.

You do not indicate the quality of your instrument?

You do not indicate the quality of your backsights and foresights? Plumb bob and peanut prism, hand held rod, rod on a bipod, shooting to a prism center or to a traget, tripods and forced centering?

You do not indicate how well balanced the traverse leg lengths were?

However, all of the above are of less concern with modern instruments than in the past.

Suggestions; any concern of weak angles should have been compensated in the field by 4D&4R observations. That would also include combinations of very long and very short sights or questionable setup conditions, spongy soil or light frost under the tripod. With a data collector the slight extra time in the field more than offsets any worry time in the office.

Almost any traverse gives several opportunities for redundant observations, such as a corner pin from 2 different traverse points or remote azimuth observations.

3.5" and 0.02' may have been the best your instrument and techniques are capable of or it may have been sloppy, but as you report it you will never know. As a matter of fact it could be just gross errors canceling out.

BTW, adjusting poor fieldwork still gives a poor result.

Paul in PA

I would agree that LSA is the way to go, but I would also add, do some tie shots from multiple points when you must use a weak angle. If you shoot the same irons from multiple positions, it will help you constrain the error correctly.

A well laid out traverse, multiple ties and proper procedure make LSA a lot easier.

Least Squares Requires Redundancy

The more the better, but a simple fully closed loop is really insufficient for a solid solution.

Paul in PA

> I would have to disagree. Yes the equipment is better than the old stuff, but still sorry procedure is sorry procedure. Why do you not use two nails that are 5 feet from an iron and only 6 inches from each other to tape in the iron? Poor procedure. The distance measured is too close to the same. The distances cannot be measured precisely enough to give you a solid location. Can you probably get it in correctly? It is possible, but it is still shoddy procedure. The same holds for weak angles with newer equipment. The inst may have a readout to 1", but look at the specs. The gun may record distances to the .005', but again it is an approximation. It just makes for weak math. The third side is so much smaller distance wise that the difference may fall within your acceptable error parameter but still be wrong. (Say you sighted wrong by 5" and your bubble was off by 0.015'). Just like you would not want to do swing ties to set an iron off of two nails near each other, you would not want to set your gun up and turn a shallow angle to move ahead. It is the same concept, just using bigger numbers.
>
> At least that is my opinion."""

isn't this a route survey, not swing ties? what made your thoughts divert to swing tie, anyway? with something like a swing tie, then yes, of course shape and strength of figure are important. this is a small example of trilateration, right?

to a limited degree, i agree.
in general, with proper equipment and proper procedures, the results should be adequate. if they are not adequate, make a nested loop to strengthen the figure, use more measurements, etc.

we don't set marks from offset nails. we set the mark partially, then check, then set,check,etc. until the mark is at grade.

i'm just not smelling what you're stepping in. sometimes, "shallow" angles can't be avoided. if they can't, there are alternative methods. we can agree on that, right?

Moe, it is regional talk. When setting nails to set irons from, where I grew up we called them swing ties also. As in "what is the swing tie to drop the iron in?"

I am not naive enough to say they cannot be avoided. But when you have to use them, you tie to stuff from both positions so that you have a way to see if you screwed up.

To just run a traverse with a shallow angle because it is easier is poor procedure. Just like taking a foresight of 1000' from a backsight of 100'. An easy way to screw your traverse because of bad procedures.

> To just run a traverse with a shallow angle because it is easier is poor procedure. Just like taking a foresight of 1000' from a backsight of 100'. An easy way to screw your traverse because of bad procedures.
"""

right, i see what you're saying.

is it icy in your area? mighty crunchy roads here this morning...

Least Squares Requires Redundancy

Agreed. What I was getting at was if you must do a weak angle, make it a point to get multiples from those points. If you do not have something you are trying to specifically locate, set some random points to tie in together. Then you have a way to strengthen the weak area.

Weak angle sort of explained

IF you used a compass, and pacing, and "traversed" around 100 acres, and "Closed" within 0.05', would that mean that EVERYTHING was within 0.05'? No.

So, having a tight closure, does NOT necesarily mean that it OVERALL is an accurated traverse.

N

Weak angle sort of explained

I remember getting a 1:50,000 closure doing a compass and stadia survey decades ago.:-)