This article does not have anything to do with dogs.

Continuing with our review of issues when developing tin surfaces, one of the very common processes is to create a TIN from contours.  It really does not matter whether they are existing contours or proposed contours; the process is basically the same.

You will need to get the contour lines in a CAD format.  This can be done several different ways.  Digitizing was one of the original methods to get linework into a CAD format and many people are still using this process of tracing lines on a set of plans.  Another method is to request a CAD file or a PDF file (new packages also import this format) from your client or his engineer.

Once you have the contour lines in a CAD format, you must check each line to be sure it has an elevation.  I have seen files with contour lines that all have an elevation of zero, they all have an elevation of -99999, they have scattered elevations that have no relevance to the project at all, and once in a while they all have the correct elevations.  If the elevations are bogus, you must edit the elevation of each line and enter the correct elevation.

Most CAD contour lines are an entity called a polyline.  One of the reasons to use this type of line is it can be splined or smoothed curved, but not an arc.

Depending on the size of your project, it could contain hundreds of contour lines.  Let’s just hope that they all have the correct elevations.

Another feature of a contour line is that it will have the same elevation throughout the entire line, but I am sure all of you already knew that.  I won’t charge for that piece of information.

In FIG. 1 are a small group of contours that have elevations as we discussed above.

Let’s try to build a TIN using this small group of contours.  Most CAD design systems have some means of converting the polylines with elevations to points with breaklines.  You remember our earlier discussion of BREAKLINES.  If the points along the contours are connected with a breakline, this will help control the way the software builds the TIN.  See Fig. 2

Are there problems with this initial TIN?  Does a duck like water?  There are several problems with the way this TIN is built.  Let’s try to define each of the problems and see if we can produce a solution.

The first problem outlined in red is a phenomenon known as FLAT TRIANGLES.  What are flat triangles you say?  This is when the tin triangle formed from the contours touches a contour of the same elevation or as in this case the triangle is formed using the same triangle.  The three vertices of the triangle have the same elevation, thus we get the term FLAT TRIANGLE.  This causes the TIN in the Red Outline to be virtually flat, when we know there is some slope in this area.  So let’s look at a couple of steps to correct this problem.  See Fig. 3

First, some CAD design software has settings that tell the program to attempt to make some logical corrections to remove the flat triangles.

By just changing one setting we can see that a significant improvement has been made to the TIN.  The program has attempted to connect to the nose of the next contour rather than link across to the same contour.  The areas outlined in green are now linking appropriately.  The triangles in the green areas are no longer flat.  See Fig. 4

If your program does not have the settings available described above, you are still able to correct the improper triangle formation.

You will notice in Fig. 5 the darkened lines are breaklines added to control the surface.  It took quite a few breaklines to remove the flat triangles in the areas shown in green in Fig. 4.  This shows that you do not have to have the special settings to make the TIN link correctly.  However, it makes you task a whole lot easier.

Shown in Fig. 6, is a 3D view of the site.  You should be able to see both the corrected flat triangles, but in the near area are flat triangles that need to be corrected.

Review

1. Obtain CAD drawing with contour lines

2. Contours has have correct elevations

3. Convert the contours to points connected by breaklines along the contour line

4. Check the TIN to see if properly liked

5. Check for FLAT TRIANGLES

6. Fix flat triangles with software settings or use breaklines to correct

There are other issues with the TIN that need to be address.  These will be covered next month.

TO BE CONTINUED...