Have you ever set in a meeting where 50% of the words uttered were acronyms? 

At a presentation on 3D Models, the following was stated, “the 3D Model is probably based on a TIN with BLs created from PTS collected from a TS or GPS and you have to be aware of the RATs.”

I looked at the guy next to me and tried to signal, don’t ask the question.  But he raised his hand and asked, “What is an RAT?”

The presenter said, “It is a small furry rodent with a long tail!”  There is always a wise guy!

It can be embarrassing to stop the conversation to ask what does this mean.  However, it is important to know what we are referring to when acronyms are used to hold a conversation.

Over the next few articles, I will be defining some traditional acronyms together with several issues involved with making accurate and complete 3D Models that can be used for automated machine control, contouring, profiles, cross-sections, design, earthwork, etc.  The resulting products and answers are only as good as the data that they are based upon.

The most common digital systems that make up 3D Models are list as follows:

Grid Model – where all the points are on an equally measured grid.  The X and Y measurements are equal between all of the points.  DEMs (digital elevation models) are grid models.

TIN – Triangulated irregular network is probably the most frequently used models for civil engineering, surveying and automated machine control.

Dynamic road model – this consists of a horizontal alignment, vertical alignment, templates, transitions, possibly super-elevations, etc.  Most of these systems can produce XYZ points that can be used as a TIN.

Comparisons of GRID to TIN 

The digital systems listed above are used for surface modeling.  One of the limitations of a surface model is that it cannot depict overhangs or vertical features.  That is all of the XYZ points are visible as you look down on the model area.  In fact I have heard a surface model referred to as two and one half D. 

I will try to diagram this limitation below.

Diagram A       

Diagram B

In Diagram A above, the ground is shown with a ledge and overhang.  The Xs mark the location topo shots have XYZ values.  However, in Diagram B, when the surface is modeled there is no overhang.  The point that is locating the base of the overhang is instead viewed from above and interpreted as a sump.  Also, the leading edge of the ledge is connected to the next nearest XYZ point.

In fact, the surface models will not properly handle a vertical wall.  So as a minimum it will be necessary to move the bottom or top of the vertical feature so that it is slightly leaning.  Most systems will work appropriately if the difference is at least 0.01’.  The diagram to the left shows the slight movement off from vertical. Remember the issues shown above, the bottom must be moved out or the top moved in.  The vertical feature could be a large retaining wall or something as small as a curb face.

It cannot be emphasized enough that regardless of the method used to build the 3D Model, analyzing all the areas of the DTM (digital terrain model) is important.  This will be time well spent and the results will be reliable.

Most of the site modeling systems employed today use surface modeling, whether it is a GRID, TIN, or dynamic road model.  It is important to have a good knowledge and grasp of how these systems work and which is best for the project.  The GRID system is more frequently used in GIS (geographic information system) more so than to create models for automated machine control.  The TIN and dynamic road model are the predominant systems used currently. 

The next articles will continue to describe and give procedures for building accurate 3D Models.  We will be drilling into issues for creating accurate models and the various methods of quality control.

Ron Ciccarone, LS