Im working in Civil 09. I need to adjust the surface I created in order for the contours to be correct. Right now I'm interpolating point elevations to adjust the contours. I find mysef adding lines and adding and deleting points. It's tedious. I tried changing the grid spacing to smaller one. It didn't seem to work. Any suggestions?
Hello caddirt, Welcome
First off: "All Contours are lies"
Secondly: as you discovered adding tin points and line is a very tedious way to make a surface. Pardon my abruptness - Don't do that, it will make future edits almost impossible.
You need to more accurately model your system to display the perceived contours, a few well place feature lines may be what you need, but not seeing what you are trying to grade its mostly a guess. Perhaps extracting elevations from some temporary planes made from FL or projection gradings....? Almost forgot: regular ans reasonable spacing of elevation points added to FL may help
Is this a parking lot? You may want to lookup "Islands in Asphalt"; 2011 AU by Eric Chapell (don't trust my spelling)
Could you be a little more precise? How can contours be incorrect - they are a derived product. Are you trying to smooth them? Maybe check your tin lines and ensure they are oriented correctly - sometimes you need to flip them.
Flip a face, thats sufficient for me. Sure its a product, a product of interpolation, and can be in a different place depending on which face it interpolates off. Its also by definition a guess.
Garbage In, Garbage Out also applies.
If your source data (topo survey) is not precise enough (lacks enough info to define breaks properly for the intended purpose), you'll be banging your head on your desk and/or keyboard all day long.
I am creating a sub grade topo based on the final grade design. My point group consists of the sub grade shots at every location shown. So if a final grade is 8 and it is in an area with 4" of top soil, I take 4" off of '8 and that is my sub grade shot. I also went along the given final grade contour lines every few feet and made sub grade shots were could The elevations change because there is asphalt ion one area, then concrete in another, then sod etc. So all of the elevations change mainly within a few feet of each other.
I'm not using field survey data, I got these shots by taking them from a designers drawing. There obviously weren't enough shots to use otherwise the data and then the surface would have been more precise.
It is easier for me to interpolate by superimposing the final grade layers over my surface. However, it is still tedious because in order for an elevation along a contour to be smooth and continous, I have to add more point elevations near it. It is working, I just thought there were some settings I could adjust.
@caddirt wrote:I am creating a sub grade topo based on the final grade design.
do you have a surface for final grade?
the definition still applies.
home tab> feature lines
Try extracting the contours from the surface (SurfaceExtractObjects), modifying the resulting polylines, then add them to the surface definition as contours (AddSurfaceContours).
Excellent suggestion - extract the contours, just delete the bits you aren't happy with, redraw these particular contours to add back into a surface.
A twist on this is that you can also add "contours" that aren't actually isolines, this works just like adding in breaklines, but if you are drawing 3D geometry that you want to create a surface out of, this can be handy.
Extracting contours and adding them again will make your surface look good, but won't make it right. Are you aiming for a pretty picture or a model you can use for earth volumes or other real uses? I wouldn't go by this route if you want to use the surface as a model in a real sence. It makes the surface 'right' at certain locations only, only where there happens to be a contour.
I suggest the feature line route. When you add the feature line to the surface set the 'supplementing distance' to a low number (and 'weeding' to even lower) so that you get regular triangulation points along the feature line. This will lead to a nice smooth surface that is also acurate.
Mark Green
Working on Civil 3D in Canada
You are correct. Modifying contours will not make a correct surface, as all contours are derived data, and as Joe-Bouza pointed out, they are "lies". However, the original post asked about "correct contours", not a correct surface. At first, I wanted to respond as you did, but then I realized that there may be situations, where correct contours are the aim. As a design engineer, I rarely (never) use contours to build a surface. Maybe the end result is not a design surface, but to model an existing surface for watershed analysis, thus the desire for "correct contours". I can only guess.
I'm not exactly sure of the intent either, but from reading the original post, I think the real problem is not enough data. When you are in that situation, the triangles become too large and then the contours wander significantly in a zig-zag fashion. The OP was addressing that by adding cogo points, I assume at interpollated elevations. My solution is to draw a feature line between the elevations you have, and use the 'supplementing distance' option to let C3D do its own interpollation: more accurate and much quicker. More TIN lines, smoother surface. Still a guess really, but probably a better guess than otherwise.
I agree there are certain situations where the contours method might be better. (Rarely).
Surely contours are not "lies" (deliberate attemps to deceive), but rather they are all just rough guesses and fudge.
Mark Green
Working on Civil 3D in Canada