Areas and Volumes in n4ce
Whilst a digital terrain model (DTM) is required for volume calculations, areas can be calculated using the Query Area option. However, if you wish to segregate areas of a model using groups and/or calculate slope areas a DTM is essential. As described in the previous section, tools are available for creating, editing and viewing these Models.
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Areas can be calculated directly from triangle facets making up the DTM. What makes this process a little more interesting is the application of groups. A Model can be split up into different zones, establishing grassed area, tarmac, concrete or even house footprints.
The calculations will automatically identify the plan and slope areas of each group and present these in both Table and Report format, as shown below. Ungrouped areas are also identified likewise. |
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The most common and universally accepted method of computing volumes is by triangular prisms. Whilst there are others methods, notably using section areas and the trapezium rule (end areas) none is as accurate and robust as correctly applied prisms.
n4ce Lite uses the DTM triangles to provide the height difference between itself and another reference surface. We call these Prismoidal Volumes, which can be likened to calculating the volume of toberones.
When calculating the volume between two DTMs an intermediate or isopachyte surface is created. Height differences at triangle apexes in both DTMs are calculated along with where triangle sides from both models cross. This is called complex prismoidal volumes and takes account of the three major problems when calculating volumes between two DTM, namely:
i) common boundaries
ii) honouring break lines in both models
iii) sparse vs. dense DTMs. |
You will also note other options in the dialog above. The top five options use sample points from the current model to calculate a best-fit 3D reference surface. This could be useful for example, when a reference DTM is not available, like calculating the volume of a spoil heap. Points along the base of the heap could be used to calculate a best-fit plane and triangular prisms calculated up to or below this surface. |
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If you are selecting another DTM as the reference surface this must be present in the Project and you will be given a list of available DTMs, as shown opposite.
You will notice in this example that the DTM has group areas. These will be taken into account when quantities are determined, and reported accordingly, as shown below. |
Results of calculations can be plotted in a Table and/or appear in a Report that can be printed out or copied into a Word document. The results shown here have been split up into grouped areas.
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