What kind of raw traverse closure would you expect to see with 35-40 points over 12,000 LF using a one second instrument and good field practices?
For an average 300 ft traverse leg, 1 second is 0.0015 ft or 0.44 mm. Your centering errors and EDM error could easily dominate over the angular error.
Let's say you have a std err of 1 second (2D2R per angle), centering of 1 mm or 0.0033 ft, EDM std error of 0.0033 ft, and used the first leg of the traverse as the basis of bearing. The 95% confidence on the last point of a 40-leg open traverse would be about 0.4 ft. (I hope I did that right)
Your actual closure could fall anywhere in a broad range, since it is one sample from the probability distribution. So 95% of the time, the actual closure will be smaller than 0.4 ft.
Occasionally you will be unlucky and see a near perfect closure despite the fact that errors have canceled and other points in the traverse are not nearly that good. For 3% of cases, you will see an actual closure of 0.04 or less despite the fact that many of the points are worse than that.
> What kind of raw traverse closure would you expect to see with 35-40 points over 12,000 LF using a one second instrument and good field practices?
That is a question that is easily answered by a least squares survey adjustment program like Star*Net. Basically, you enter the traverse as if it closes on a point with a different station name than the beginning point of the traverse and give the same ending azimuth of the line that the closing angle turns onto as the first leg of the traverse had.
Then, you compute the relative error ellipse between the beginning point of the traverse and the ending point that is the same as the beginning point, but which has been assigned a different station name. The error ellipse will give a very good basis for estimating what the error of closure for 95% of all runs of the traverse with the same equipment and methods should yield. It will vary between 0 and some maximum value.
The other method, if you haven't actually run the traverse and are trying to plan things, is to run the Pre-Analysis function on a traverse with approximately the proposed configuration, using the standard errors of angles, distances, and centering of instrument and targets to weight observations.
Really, though, if what you want to know is whether the traverse contains any blundered angles and/or distances, if you have LSA software, there shouldn't be any need to even waste time computing a closure unless for some reason the specification to which your survey is made requires it. The blunders will pop out when the whole works is adjusted with correct weights on angles and distances and with correct centering errors.
What Kind Of Procedure Did You Use?
Your procedure predicts your closure. 2D & 2R even with a 1" gun, because the point is to actually get 1" angles.
I do not think I ever worked on traverse that size without planning in some redundancies, cross ties, azimuth observations and/or GPS. All of these improve a least squares closure.
Paul in PA
We are required by contract to do a compass rule adjustment on it between azimuth pairs. I would love to have an extra pair in the middle but that is not an option due to conditions.
I think I found my issues though. Has to do with Leica exporting azimuth data instead of right angles for traverse You can set it to angle right for sideshots when exporting but it does not seem to apply to traverse. That said they put a bad starting coordinate in when running the traverse so all of the azimuth readings are off.
Tom
That's a lot of points for that short of a distance. You get more into the problems with setups and the quality of your tribrachs than the quality of the gun. I've run traverses like that but maybe not as long and struggled to get much better than 1 in 40,000.
> That's a lot of points for that short of a distance. ..
1 every 300 feet. That would be about right for an urban environment. 1 per block corner. Contractually obligating an adjustment is fine, obligating it be done by compass rule smells like a 30 year old survey text book on some engineers shelf. Musty.
This is thick woods but we have a lot of monuments to locate and a lot to then set.
I agree about the old standards but it's what they are paying us to do so that is what we are doing. That said, how well does least squares work for long generally straight traverses with no cross ties, etc?
Tom
LS is theoretically better than compass rule, but if you don't have any redundancy there probably isn't a big difference in performance.
Where LS shines is the ability to easily accommodate any measurement combination and any redundancy you have and give you both the statistically best answer and estimates of the resulting precision, assuming you give it realistic values of all the standard errors.
> ... That said, how well does least squares work for long generally straight traverses with no cross ties, etc?
It wouldn't be much different, if any at all, than a compass rule. But using LS you could sprinkle a few GPS vectors (RTK or otherwise) into your traverse to provide additional strength. Vectors between every 5th or 10th or 20th traverse points would really enhance your solution.
I am not a statistion or a favor of an adjustment of a traverse. I do adjust raw data to analize the results though. My old boss did this and I guess that is where I picked up the habit. I have never used an adjusted control traverse for construction but I did adjust to analyze the data sets.
I can't count how many nasty, short legged, boundary traverses we have run but:
Only had 1 that was really blown. I used the old line perpendicular to the azimuth error to pick it out quickly. Found exactly 3° on a set up....
Anywho, a 1" gun utilizing doubled angles in poor area have always met state standards. We usually had more acute vertical angles than horizontal and closed very well vertically and good horizontally. One would expect a 1 part in 30,000 on a good one and 2/3rds that on a bad one.
I would like to see some comments on the actual ratio of precision others have encountered.
My 0.02 hundreths.
I would like to add that I have been a few minutes off the original back bearing several times. It is the nature of the beast for a traverse like you mentioned.
