Below we compare the difference between mesh and breaklines, and how the two can work together.
What is the difference between Mesh and Breaklines?
In general, meshes are created by decimation of the point cloud, followed by nearest-neighbor algorithms. The result is a Triangular Irregular Network (TIN) that retains the shape of the ground in a general sense. Nearest-neighbor may or may not work in when there are features like ditches or berms. Breaklines, which generally follow features like drainage ditches or berms, help to guide TIN creation to retain the shape of features regardless of the point distribution. Typically, this means you get better ground definition using fewer points.
When is Mesh the optimal output?
Breaklines are typically used for defining Digital Terrain Model (DTM) topography since terrain models are considered 2.5D and not truly 3D. This means that looking down on a plot of land from above, no place on the surfaces can contain multiple elevation values, which makes modeling caves or tunnels difficult within a single DTM. At a single XY position there could be a Z for the ground, another Z for the top of the cave, and another Z for the floor of the cave. This can be overcome by creating multiple DTM’s to define each elevation layer, but this is a cumbersome and inefficient way to create a model. When concavity is required, or if a full 3D model is desirable, mesh is the obvious choice since it can be true 3D and is not hampered by this limitation.
When are breaklines most appropriate?
Breaklines create shaped facets that define terrain much better than simple points. In a surface model where features like existing drainage ditches are a critical component of the analysis or design, these facets create a much more accurate model. Nearest-neighbor may connect the left and right tops of a ditch if the points at the bottom are sparse, simply because the two top points may be closer together. This point connection could create a false dam in the ditch. If the bottom and tops of the ditch are continuously connected with breaklines, this will not happen. The idea that the feature is defined by the connections between points also means that it requires fewer points to define that shape. In a TIN created by meshing, it may require hundreds of points to define the bottom of a slope, where a breakline can define that feature with as few as two points. Another added benefit of that point reduction is that contours created from breaklines have a much more “organic” look to them. They are naturally smoother and don’t look digital.
Can you use Mesh & Breaklines together?
They are almost always used together to some degree. Even Breakline-Builder from Datasight creates mass points in areas where no grade breaks exist. Point data is very useful for defining features like sloped terrain, shaped roadway crowns, large depressions, sloped concrete slabs, parking lots, and anywhere else with regularly shaped terrain that doesn’t have breaks in it. But if there are sharp features that need to be captured, breaklines are the only way to ensure that those features are captured correctly.
