I have learned the hard way that Civil 3D has a Curve Tolerance value that is to be used when applying sample lines and computing average end cross section volume calculations along an alignment with a curve. How I discovered this was by double checking my self and using a straight alignment verses an alignment with a curve. When using the alignment with a curve the average end cross section volumes differ. I did some reading in Autocad help and it mentions that some times a curve tolerance value is needed when using curves and this value is changed in the compute or edit materials window. Here comes the question. "What is a curve tolerance?" The edit materials window prompts a user to enter a Degree value. How do I calculate this curve tolerance value? I assume it is related to one of the values of a curve.
Curve Tolerance corrects for the fact that sections are nearer to one another on the inside of a curve and further apart on the outside of a curve. You can't just multiply the area by the length between sections and get accurate volumes the way you can on a straight segment.
I'm sorry, I can't explain the curve tolerance setting, but I would love to hear from someone who can.
Delta Engineering Systems
Autodesk Authorized Value-Added Reseller
I remember seeing an explanation years ago but I cannot find it online now. If I recall correctly the explanation goes something like this:
If the angle between two sample lines exceeds the tolerance then the offset location of the centroid for the sections is calculated. The average of these two offsets is used to correct the arc length along the alignment between sections.
Imagine for example that the program is calculating the volume between two cut sections which are mostly on the outside of a curve. The average offset of the centroids will also be on the outside of that curve. The program will adjust the radius and therefore the arc length between the two sections will increase. That adjusted length will be used in the End Area or Prismoidal formula, rather than the distance along the alignment.
Thank you for clarifying this subject. The next concern is how do I calculate the Curve Tolerance value? What is it based on? I have not seen anywhere in the help documentation describing a way to calculate for this correction.
Here is to hoping some one out there knows how to calculate for this correction.
The tolerance is just a value that you use to determine when the curve correction function is applied, instead of the normal end area calculation along the centerline. Measure the angle between two adjacent sample lines to determine whether or not the correction would be applied there.
The tolerance that you choose to use will be based on the radius of your curves, the spacing of sample lines and where the materials being calculated are on the sections. For long flat curves with volumes that are distributed evenly between the left and right you probably don't want to use it.
Thank you Steve,
In my situation I have a project that covers 60 acres. The shape of the project is bean shaped in such that I created a baseline alignment through the middle of the project. I used a 1000 foot radius curve in the center of the baseline so that my project is divided evenly both north and south. I ran sample lines at an interval of 10 feet along the baseline and the sample lines lengths are pined to two alignments running along the north border and south border. This makes my sample line length variable. On the south side of this site (lets say the southern 25% of the site) we are designing a lager amount of cut where as the the remaining 75% of the northern part of the site is a small amount of fill.
I understand where the error in calculation is occurring. Because my sample lines length is getting closer and closer to the center point of the curve (In this example: 700 feet long in the 1000 foot radius curve). Because this is the south side of the project where I have a large amount of cut. I see where the average end area method is grossly wrong when used along a curve. I understand out of the hundreds of projects I have done this is the first where this calculation would be so grossly exaggerate. As compared to say a road on a large curve and a sample line length of only less then 100 foot long with a sample interval of 50 to 100 foot in separation. I know now that the Curve Tolerance correction needs to be applied. I now need to know how to calculate this Curve Tolerance value.
What I am looking for is there a formula to calculate this Degree value?
Example: 1000 foot radius curve at 10 foot sample interval and a sample line length of 700 feet = the Curve Tolerance value in Degrees. This decimal value is what I use in the curve tolerance column for the compute materials dialog box.
There has to be a method for calculating this degree value. I shutter to think you have to guess at this value in hopes it comes close to your other QAQC volume checks. Such as a Surface to Surface volume calculation.
I know for future design I will use straight alignments for quantity purposes and let the design set base lines be for survey staking and construction purposes only.
Thank you for the help.
The distance along the alignment between two sample lines divided by the curve radius would give the angle in radians between them. Convert the result to degrees. If this angle is greater than the tolerance value then the curve correction algorithm is used. Please note however that the value you enter here is just a trigger limit - it is not used in the calculation.
I've requested that this topic be escalated to Autodesk Support, to bring someone else into the conversation who understands the topic better that I do.