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Valued Contributor
Posts: 101
Registered: ‎03-05-2011

# Re: Grading Contours not true to slope values

02-10-2013 12:44 AM in reply to: SDcivil

3 guesses:

1) Something wrong with featureline

2) Something wrong with how you're measureing the offsets.

3) Something wrong with the surface

Good luck lol

- Brian
"Very funny, Scotty. Now beam down my clothes."
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# Re: Grading Contours not true to slope values

02-10-2013 05:23 AM in reply to: SDcivil

Another thought: is it possible the feature line was created from polyline geometry that would have benefited from a map clean operation? i.e pseudo nodes, segments doubling back on it self?

Thank you

Joseph D. Bouza, P.E. (one of 'THOSE' People) Civil 3D 2012 & 2013
HP Z210 Workstation
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Note: Its all Resistentialism, so keep calm and carry on

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# Re: Grading Contours not true to slope values

02-10-2013 06:00 AM in reply to: SDcivil
Contours are drawn according to the slope of the triangle face they cross.

A grading object between two wildy skewed feature lines will produce two very differently sloped triangles, so the resulting surface smoothness will be dependent on the number of triangles.

Your best bet is to smooth this surface to guarantee a uniform set of slopes.

Matthew Anderson, PE
Global Services
Autodesk, Inc.

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# Re: Grading Contours not true to slope values

02-10-2013 10:29 AM in reply to: Neilw

Neilw wrote:

Could you explain "Grading occurs perpendicular from the feature line and not from the contours" Kati?

In my mind contours should be nominally spaced in plan view regardless of how they are created.

When you use the grading tools in Civil 3D (aka projection grading) the criteria is applied perpendicular to the featureline. If you project out at 2:1, the 2:1 is measured perpendicular to the featureline. If the featureline has no slope, then the resultant surface has a slope of 2:1. If the featureline has a slope, you need to add the two slope vectors to get the resultant slope.

If the featureline is level (slope of 0) the resultant looks like this:

If the featureline has a slope, the 2:1 slope (aka 50%) is still measured perpendicular to the featureline but the resultant surface is a combination of the two.

To get the resultant, you add the two slopes together (remember how to add vectors?). The resultant slope is sqrt(.5^2+.1^2).

So in the case of the original poster that said his surface had 9.7' between 5' contours, this would result in a slope of 1.94:1 or 51.5%. If the slope from the featureline is 2:1 (50%), then in order to get 51.5%, you would have to solve this equation: x^2+.5^2=.515^2. Thus X would be 0.123, or 12.3%.

Valued Contributor
Posts: 101
Registered: ‎03-05-2011

# Re: Grading Contours not true to slope values

02-11-2013 12:14 AM in reply to: Joe-Bouza

Post an example of what you're having a problem with.

- Brian
"Very funny, Scotty. Now beam down my clothes."
Valued Contributor
Posts: 75
Registered: ‎01-03-2008

# Re: Grading Contours not true to slope values

02-13-2013 01:33 PM in reply to: SDcivil

SDcivil,

In case you haven't given up...

Is your area of concern around a curve or does it have any curvature at all, or does the object you're grading from have varying slope.     The slope should be measured perpendicular to the object you're grading from not the individual contours.

You're probably use to create all your contours by offsetting to what ever slope (distance) you wanted.  You will find, that most of the time, the grading that you created this way, is actually not correct.  By this I mean, It was correct to someone measuring perpendicular to each contour.  But that's not necessarily correct if your slope is measured from something that's not perfectly parallel with the contours.

Do this test...  Create something like a swale with a CL create the first contour for the side slope, then offset that contour 3.00 for a 3:1 side slope.  Now measure the distance between contours going "perpendicular" to the CL of swale.  See image if you don't understand what I mean.

Note:

If you're not going to use the grading objects to create "correct" grading.  Don't bother using Coridors either.  (I'm kidding....  Just understand that what you did in the past may never have been correct.)

Jonathan Stewart
P.O. Box 7811
Lancaster, PA 17604
Ph. 717-468-3111

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# Re: Grading Contours not true to slope values

I've bee biting my tongue for a week now:

"All Contours are lies"; all it take is one look to see : flip a face.

...and from Sinc.."The other problem you are hitting, though, is that contours are the least-reliable form of vertical data."

Thank you

Joseph D. Bouza, P.E. (one of 'THOSE' People) Civil 3D 2012 & 2013
HP Z210 Workstation
Intel Xeon CPU E31240 @ 3.30 Hz
12 GB Ram

Note: Its all Resistentialism, so keep calm and carry on

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# Re: Grading Contours not true to slope values

02-17-2013 11:07 AM in reply to: bcinnv

Maybe this will help.  Attached is a plot of a basic straight street at a constant grade (no real cleanup required).  The 10 foot contours are shown at 2:1 slope.  The grading function calculates the controur spacing at 19.6', but a proper 2:1 slope should have this dimension at 20'.  Instead the 2:1 slope ratio is calculated perpendicular to the top/toe, which is incorrect becuase it is skewed to the slope direction.

Valued Contributor
Posts: 75
Registered: ‎01-03-2008

# Re: Grading Contours not true to slope values

02-17-2013 11:15 AM in reply to: SDcivil

That actually is CORRECT.  I think that's what everyone is trying to tell you.

If you have a cross-section detail of your roadway, that is an exact representation of the required slope.

Jonathan Stewart
P.O. Box 7811
Lancaster, PA 17604
Ph. 717-468-3111