Don’t Mesh This Up!

Posted on October 23, 2018 by Matthew Anderson


During the first week of October, I presented a webinar on the role of Terrain Surfaces and Surface Modifications.  The intent was to cover the role of the terrain surface in the calculation of flooding as well as to discuss the differences between XPSWMM and InfoWorks ICM. Specifically, how they approach terrain and modifications.

In Case you Missed it: Watch the Webinar “Don’t Mesh This Up!” Available On-Demand

Sourcing the correct ground model is a key element. The Digital Terrain Model (DTM) shown below from GIS Resources is a perfect example of the differences between a DTM and a Digital Surface Model (DSM). The DTM shows pretty much bare earth without surface detail. The DSM includes surface elements that would be valuable to incorporate in a floodplain model as actual elements. For example this would include trees and shrubs (using Manning’s n values), as well as buildings that might fail, flood, or need an assessment for risk.

Both applications can generate terrain from similar sources, but they have differences depending on the source of the work.

A great example would be a user building an urban terrain model within Autodesk Civil 3D. The user might have spent time with terrestrial collected data and built extensive break lines. Preserving these changes in XPSWMM is as simple as LandXML export / import into the application. And, with the upcoming 2018.2 release later this month, XPSWMM will import a 2-million-point surface while preserving the triangulation.

Are you flexible or just a square?

This age-old question isn’t something I will debate. But I will discuss the benefits and challenges of both methods.

Grid Resolution is a key element of XPSWMM’s approach, especially when hoping to understand the space between buildings or other urban models. Multiple domains are needed to focus grid resolution where you want it (or the converse – loosen resolution where it isn’t needed). A 2D grid is relatively simple to set up but may not provide super fine detail.

A flexible mesh provides the focus as required and defined in the production of flexible mesh computations. The mesh can conform not only to the terrain automatically, but also to features like rivers, buildings, levees, and streets – all of which can be used as components in creating the mesh. The best example I have is the intersection shown below.  Notice how the mesh aligns with features within the model.

With the flexible mesh you can see the left turn lane, curbs, and more – giving you a more accurate representation from your model.

While the webinar covered some basic terrain and mesh differences at a high level answering several common questions, some attendees asked for more detail on both XPSWMM and InfoWorks ICM.  In the best tradition of Marshall Field, I will endeavor to “give the engineer what he/she wants”.  Future webinars will delve deeper into the details between the two, so stay tuned!

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