For the surveyors out there who want to go a bit deeper into the Survey Database, today we’ll talk about traverse analysis and adjustments. Why do we need to adjust our traverses? Because they’re not perfect! Human error (that guy running the rod isn’t holding it perfectly plumb, or has the shakes, or just doesn’t put the tip of the pole in the same spot every time, or “heat monkeys” wreaking havoc with the guy in the scope) or mechanical error (the optical plummet isn’t exactly right, the instrument got dropped one too many times, the rod got ran over by the crew truck, etc) will introduce error into any traverse loop, and over great distances, that error can add up to the point that your data is outside acceptable parameters. So how do you adjust a traverse in Civil 3D? You’ll find out after the jump…
First of all, we need to know what defines a true traverse. A traverse is typically a network of points that moves from one occupied point (station) to another. The most important part of a traverse is the last shot – the closing angle. In the following screen cap, you’ll see our traverse that we’re working on today.
In this traverse, we started out with our total station sitting on point number 2. If we look in the survey network, we’ll see that our instrument operator entered an azimuth of 70.4558 as a direction from point 2 to point 1 (that’s the magenta line with the arrow.) The crew then shot a distance to point number 1, and stored it in the data collector. They then used point 1 as a backsight point and turned an angle to point 3. This is a fairly typical workflow when beginning a survey on assumed coordinates. From point 3, they shot to point 4, then turned from point 4 and shot back into point 1. Now, from this point, many people will just store a new point at point 1 and inverse between that new point and point 1 to check their closure and call it good. However, they haven’t measured the most important part of the traverse – the angle turned to point number 2 (the starting point) from point 4. This creates a truly closed loop that can now be analyzed and adjusted. Remeber – without that closing angle, you have no traverse.
If the traverse wasn’t defined in the data collector, you can create a traverse from a survey figure, a really nice extension to the survey functionality provided by Rob Todd of Autodesk. You can find more out about the survey extension and what it does here. Maybe a tutorial on creating traverses from survey figures will be forthcoming 😉
If we look in the survey tab of toolspace, we can see that we do, in fact, have a traverse defined in this particular network, as shown below.
One thing to stop and consider before we move on to the actual analysis and adjustment is our Equipment Database. This is where you set up your equipment and the settings contained within that equipment so that those factors and settings can be applied to your traverse. This is really the only reason you’d ever need an Equipment Database. However, please note – some (most?) data collectors allow you to set the information shown below in the data collector itself, and the adjustments are being made to the points as they are collected. If you apply the correction factors on top of already-applied correction factors, you will have double-corrected data, which isn’t correct at all.
Now, we are ready to adjust the traverse – we do this by selecting the traverse, right clicking, and selecting Traverse Analysis… The following dialog box will be displayed:
We have to make a few choices here – to do the analysis, to balance the angles, what adjustment methods we will use, what our closure limit is, what the acceptable angle errors per set are, and whether or not to update the survey database.
Two of the most important choices are the different adjustment methods that can be used. We have 4 options for horizontal – Compass Rule, Transit Rule, Crandall Rule, and Least Squares adjustment. For vertical, we can choose Length Weighted Distribution, Equal Distribution, or Least Squares. Note that Least Squares will only be available if you have Least Squares set as your horizontal method as well. Once you decide what method you want to use, you will enter your closure limits. When you are ready, click OK.
A few text files will now appear, with the results of your analysis. You can store these for record purposes or to see how the traverse was adjusted. You can see my analysis report here:
Elevation error : 0.0158
Error North : 0.0006
Error East : -0.0110
Absolute error : 0.0110
Error Direction : N 86-50-13 W
Perimeter : 1618.8029
Precision : 1 in 147395.2963
Number of sides : 4
Area : 149907.7 sq. ft. , 3.4414 Acres
Had I not already adjusted this traverse once, I could show you the other results… 😉
Now, I realize that most surveyors are pretty set in their ways – you may have a program or method that you use for adjusting traverses that works fine for you – that’s perfectly OK. Some of the old programs are tried and true, and many surveyors trust the numbers from them. I’m here showing how it can be done – it’s up to you to decide whether or not you want to use this method.