Contour Line Question

No matter how great the software, surfaces always need human intervention. If there were a magic button to press, we would all be pressing it. Repeatedly.

I know in the CivilCAD software i used to use you had the option of "swapping" triangles to give a better output for the contours, it just depends on which settings your triangulations are set on.

Would only be logical to change these if you know what you are getting is wrong.

Change the contour style to triangles and contours. Then you can add and delete TIN lines to where they should be. Then modify the contour style, there are a couple different options for smoothing. Of course the more you smooth, the less representative it is of the actual interpolation.

> My question is do you submit your contour lines straight from the software?

NEVER!

auto generated contours are very crude looking. it is always worth your time to clean things up.

I have never done a finished topo that I didn't edit a few triangle.

For example, if you have four topo points that form something similar to a rectangle, the software will make two triangles out of it. Sometimes the contours look better if the diagonal line was drawn the other way. I add a break line the way I want the triangle drawn and rerun the tin. If you look at the tin and the contours together, you can see the ones that need fixing.

James

> I have never done a finished topo that I didn't edit a few triangle.
> If you look at the tin and the contours together, you can see the ones that need fixing.

I agree James.

Do all the editing you can from the tin and the contour editing should be minimal. I also think that anyone who wants to really understand contours should draw a few by hand interpolation on paper.

Scott

Some Times I Just Add Points

After looking at the contours I sometimes say "I should have had a point here." I evaluate the points I have, what I recall from the field and how I want it to look and add one or more 3D points. No point numbers just a 3D node and maybe an elevation note.

I added just nodes on my first CAD major subdivision project back in 1990.

Other times I see that my breaklines are not doing it and offset a 3D breakline horizontally and vertically. Sometimes I do that on purpose. I may shoot only the centerline of a local road or only the EPs of a highway. For the centerline I offset left/right and down. Depending on the project my EP breaklines can be more complicated. I may offset away and down for a ditch and away and up for a bank. Centerlines require options, tanget lines may be created by drawing 3D polylines across the road from EP to EP then redrawing from midpoint to midpoint of those cross lines. Then the only offset would be up, or just change the polyline elevation up the correct crown height. Special consideration for approaches to curves that are superelevated and no crown in the curve. Some times it is more work in the office than it would have been taking actual shots in the field, but the rodman or the robot operator was not put "in harm's way".

All of the above is easy because of the use of a field book and extra notes on various points.

Nowadays I am more likely to take a random sampling of ground topo shots and compare them with available LIDAR shape files.

Paul in PA

In summary, computer-generated contour lines are better than nothing but not nearly good enough for most projects. They might be fine for a gently rolling empty lot for which general information is needed. They are worth very little for intricate designs of key features on a diverse surface.

What it boils down to is what does the client really need to know. Perhaps all they really need are the hard elevations on existing hard surfaces and structures. In that case the contour lines are of minor value. But, if they need to know precisely how to reshape the entire project area, a quicky, grid pattern of shots with computer-generated contour lines will not be adequate. For example, if controlling surface water movement is critical, then one needs to find the true contour lines, not guess at them based on random shots. Nothing destroys the appearance of a new construction more than the routine appearance of mini-gullies and sediment being deposited in embarrassing places, like in front of the grand entry.

On the other hand, rough planning for future general development may necessitate only a simple topo. The key here is that the client and anyone in the future must understand that this is a very rough topo that most definitely should be replaced with one of higher quality when hard decisions need to be made Developers frequently like these rough topos so they can play around with possibilities for key elements such as roughly visioning natural problems to other goals. Something like a sewer line needing to suddenly appear from below surface, stay above ground for some distance, then re-enter terra firma. Not too esthetically pleasing in most homeowners' eyes.

Swap triangles? Sure. Manually draw? No. Smooth? No. The points are the points and the interpolation is the interpolation. It's a technical product. Screwing it around by eye doesn't have a very technical feel to it. Besides that, I also deliver the surface models to my more sophisticated clients. I want the model and the contours to match. If there's a problem with the contours there's a problem with the model on which they're based. No bueno.

That makes sense in the larger scheme of things. From what you're saying there are natural regular features that dictate an overall pattern of drainage, and maybe interspersed with sudden, abrupt, irregular features, whether man-made or avulsive, that wouldn't lend well to a regular grid pattern, and possibly or not, require a density of shots requiring substantial time for interpolation or editing, when viewed in the overall scope.
There's times when you can't adequately describe the numerous types of actions that occur on a project site.

I almost always smooth the contours. They're approximations anyway, so smoothing doesn't significantly reduce their technical value, and the resulting curvy appearance helps to visually distinguish them from utility line representations, which can be pretty dense. I generally also provide the TIN, so if the design team wants to look at things more closely in the vicinity of a critical design feature, they can check to see if the model is dense enough for their needs.

Yo Flipface!

I know of no surface I have ever created where I did not flip a face, flipped it for real. Well, in LDD 2 and 3 the command was flip face and it was not in real time... Did they fix that before they destroyed LDD? Carlson has had the real time Swap Triangular Edge or whatever they call it.

Picture a missing break line. What about a smooth transition between two slopes where there are no breaklines. Software is dumb and unless you perform your survey in the perfect way the computer works, your surface will have some oddities.

My cousin works with the lidar data in PA. Her husband is a geologist. As they were converting the lidar to contours, there were two options: leave them messy or make them smooth. Engineers love smooth contours and regular shapes. Geologists love the messy ones because apparently they show what happened a bit better.

A true contour line is where the water line will actually be if there were any way for the area to flood. That's the simplest way for me to think of it anyway. As the water slowly moves uphill it will conform to the true contour. The trick is to be able to create that true contour. The contour will generally be somewhat curved until it hits a sharp, flat obstruction, at which point it may become straight along the obstruction, then return to curvilinear when following a natural surface. At some elevation it may overtop the obstruction thus radically moving the appearance of one contour line relative to the next. There may also be small, oddly-shaped little contour lines in low areas or depressions. A good example of this is a terraced field. These little island-type contour lines will occur in the terrace channels.

Again, it is crucial to know what the client is intending to do with the topo data. There is not a one size fits all type of topo. Or, at least, there should not be. The worker in the field gathering the data needs to be fully aware of the client's precise need for a specific project. It is not his fault if his supervisors do not communicate well with him. He will probably assume that the minimum effort is appropriate so as to maximize the firm's profits.

I know my opinion of smoothing is atypical. I've seen over smoothing cause contour lines to cross or overlap boundaries so I avoid them.

Yo Flipface!

Thank you for your inputs.
Problem with contour editing is that once you start editing a portion you end up analyzing everything. Most of the problem areas are waterways - ie ditches, creeks, gutter, embankments. Surveyor might miss a toe but shot a top or the position of a toe point is not near the top point and this would mess up the triangles.
I would really love to get a scanner for these areas. But someone told me that the "problem" with a scanner is that there may be too many points.:-/

I take grid shots and break lines, often lots of extra shots in bumpy areas, and sometimes also follow the contour lines in the field. The computer generated contour lines still need some cleaning up, though.

Fractal Geometrics (Dimensioning)

A few years ago I read several papers concerning the application of fractal dimensioning when creating a digital terrain model. Although it appears to have some merit, most of the trial sets were large natural hydraulic areas with a huge amount of relief.

I don't think it would be a successful application on the work we generally perform on a site topo. However scanned surfaces with a tremendous number of data points might yield some interesting contours if the proper fractal 'dimensioning' could be determined.

Until then, I'm a strict triangle face editor.

You need to take shots at all grade changes and add break lines in field to ensure that you are generating a worthy model. So not necessary to be even as close as 5m if natural surface is a constant grade.

There are times where the computer generated contours cross so you must edit these generated contours else you are delivering incorrect data because natural surface contours simply do not cross.

The problem is caused by the contour packages generating contours with a particular smoothing factor, so need to alter smoothing factor percentage in area where contours cross. ( Liscad uses 50% factor)

RADU

These are the types of lines that I need to edit. I think it has more to do with the algorithm of the software. I mean there is no way that surveyor on the field would know whether to add/remove points to get rid of these errorneous lines.