For this article, I am exploring the very basic essentials of a Triangulated Irregular Network (TIN).

The basic elements of a TIN are relatively simple.  It can be boiled to two objects, a 3D point and a breakline.  I realize some systems are including other objects to define TINs such as contours, but when the smoke clears it used them to create points and breaklines.

A 3D point can be defined as a traditional CAD point that has X, Y, and Z values.  Of course a good system will also allow a point ID (point number) and descriptor (i.e. TB=top of bank, CL=centerline, PP=power pole, etc.).  I used XYZ to define the point, because they could be NEZ or SWZ, etc.  I remember using SW coordinates prior to using computers and keeping the signs straight could drive you crazy.  That is why most coordinate systems are setup in the NE quadrant.

A large quantity of 3D points can be entered into the CAD system and the TIN generator can very quickly process thousands of points to form the 3D network.  Most systems today can very quickly produce a TIN from a huge number of points.  The speed is becoming important because of some of the new technology such as 3D scanners, which can produce millions of 3D points (yes I said millions).

Shown in Figure 1 is a group of 3D points with descriptors, and the TIN has been generated showing the triangles formed.  I highly recommend that if your systems allows, turn on the visibility of the triangles, it is very important to see how they are formed.  If you notice the descriptors, there are a few GRN (ground shots), some TB (top of bank shots) and some CL (centerline shots).  Follow the line of CL shots and the triangles that are formed, you will notice that there are triangles formed connecting GRN shots from either side of the CL.  I don’t think this is correct, but before we jump to conclusions, lets contour the TIN to see if this helps.

In Figure 2, the contours have been generated to better define the surface that has been formed by the TIN.    A red line has been drawn to show the approximate location of the centerline.  As pointed out in the diagram, the triangle edge seems to be crossing and in conflict with the centerline.  The contours are showing either mounds or sumps along the centerline.

In Figure 3, a 3D view of the model formed by the TIN helps us get a better idea of the surface.  As you can see ridges have been formed that cross the centerline that has been shown by the point codes.  However, without the point descriptions, we could not be sure that something was wrong with the model that has been generated.  Soap Box, collected points from the field should have a description and there should be a SKETCH.

More points could be added along the centerline control the way the triangles are being formed, but then the same issue occurs with the top of bank points on either side of the centerline.

So now enters the break line, some systems call it a fault line, others a feature line.  The function of a break line is to limit and affect how the triangles are built and connected.  It keeps the triangles from linking to points that are not associated.  The CAD system is fast and very smart, but it cannot read your mind.  It should have known there was a streambed there, yeah right.  The model is only as smart as you make it.  Just dumping a data collector or an ASCII file full of points into the systems and letting it rip does not work.

You must do quality control on every 3D model that you develop.  A thorough pass over the entire site will keep you out of E&O negotiations.  I realize budgets bulge at the end of the project, the old rule was you complete 80% of the project for 50% of the budget.  Then the last 20% to finish puts you 25% over the budget and we usually blame it on QC and admin.

Now let’s see how the breakline can help us control the triangles to form a better 3D model.  In Figure 4 we have connected the TB and CL points to create breakline.  We should see a significant difference the way the triangles are now formed.  Also, every CAD system is a little different not just any line will act as a breakline.  Each system has certain rules to follow, that is a very important reason to get formal training on the CAD modeling system that you will use.  A rule of thumb, 1 hour of formal training equals 10 to 20 hours of muddling through yourself.

Now in Figure 5 we see that the triangles and the contours are being generated correctly because of the use of the breaklines between associated points.

Lastly in Figure 6 we see the 3D model of the area that has the TIN created properly and the use of breaklines to control how the triangles are formed.

Let’s review the process and steps taken for this exercise.

A group of points with XYZ values were placed in the modeling system and an initial TIN was created.  Through exploring the triangles and reviewing the point codes it became apparent that the 3D Model was not correct.  Breaklines need to be added to affect and control how the triangles were formed.  Following the rules for the system breaklines were added and a correct model was formed.

So we have looked at the Basics of a TIN, the next article continue in small steps to cover various aspects of building an accurate model.