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n MineSight 3D, triangulated surfaces can be created using multiple methods including importing the surface from a Digital Terrain Model (DTM) or a 3D Triangle File. This article will describe how the subcell count affects the computed elevation when gridding surface geometry to a Gridded Surface File (GSF) item, and will provide some general guidelines for adjusting the subcell count for different applications. The subcell count describes how many vectors are used to calculate the elevation value of a grid cell from a triangulated surface. The more vectors you use, the more accurately the GSF cell elevation will represent the average value of the surface. However, larger subcell counts take longer to process. How do you balance processing time and accuracy?H O W T O S P E C I F Y T H E S U B C E L L C O U N T
Assuming you already have a GSF (file 13) and a triangulated surface, open the Model View Editor for the GSF. The subcell count is accessed through the Grid Surface tab. In Figure 1, the model item to be gridded is TOPOG and the subcell count is 1 x 1. In this case, the resulting GSF cell value will use one central vector per grid cell to define the elevation of the entire surface.
Fig. 2Settings and illustration for gridding a GSF with a 10 x 10 subcell count (100 vectors per grid).
If the subcell count is changed to 10 x 10, then each grid cell elevation will equal the average elevation value of 100 evenly spaced vectors (Figure 2).
Obviously a GSF cell value calculated from a subcell count of 10 x 10 (100 vectors) will be a much more accurate representation of the entire cell than a subcell count of 1 x 1 (1 vector). However, as the number of vectors used to calculate a cell value increases, so does the processing time. For grid cells created with a subcell count over 25 x 25 (625 vectors), processing time may be greatly increased with little increase in the accuracy of the cell value.
What is the best subcell count to use? The answer to these questions depends upon what you are trying to accomplish.
ßFig. 2 à
1 x 1 subcelling should be used for viewing, contouring, and with any interaction with other
1 x 1 gridding, for example in MSBasis procedures pdhgrd.dat and grdrat.dat. A higher subcell
count should be used for MSEP and topo% coding if you’re not using a solid.
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Fig. 3Illustration of
topographic surface viewed as Filled polygons.
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Fig. 4Illustration of topographic surface viewed as Surface/Slab.
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V I E W I N G T O P O G R A P H I C S U R F A C E S
For basic viewing of topographic surfaces, a 1 x 1 subcell count provides a good representation of your topographic data in a relatively short amount of time. Additionally, when the display style is changed from Filled polygons to Smooth contours, the result is a continuous surface as shown in Figure 3 and Figure 4. Both are created with a 1 x 1 subcell count and can be used for dramatic visualization purposes.
S U R F A C E S F O R A N A L Y S I S O R P I T D E S I G N
If you are using the GSF for analysis or designing a pit using MSOPIT, you may consider using a higher subcell count in order to increase the accuracy of the overall grid value. Although your values may not change significantly at each cell, the cumulative result of your entire study area may be very large.
Fig. 5Grid elements located in the middle of the 20 x 20 meter cell using one vector (subcell count of 1 x 1). The yellow star represents the center of the cell.
Figures 5 (previous page) and 6 illustrate the difference between using 1 x 1 vs 10 x 10 subcell counts. The blue lines represent the outline of each GSF grid cell draped on a triangulated (brown) surface. The yellow stars represent the point where one central vector would intersect the cell. In Figure 5, the GSF cell value is identical to the elevation of the triangulated surface elevation at the cell center. Alternatively, in Figure 6, the average cell value is not necessarily the value at the center (the position of the yellow star). Instead, increasing the subcell count results in the GSF cell elevation representing the average of 100 vectors within the cell. Both interpretations are valid depending upon what you want to do with the surface.
C O N C L U S I O N
In general, for most visualization purposes we suggest using a subcell count of 1 x 1. This will result in a good overall illustration of the surface elevation. However, where more detailed analyses are necessary, we suggest using between 10 x 10 (100 vectors per grid cell) with an upper limit of 25 x 25 (625 vectors per grid cell).
áFig. 6à
Fig. 6Grid elements placed in the average location of the 20 x 20 meter cell based on 100 vectors (subcell count of 10 x 10). The yellow star represents the center point of the cell.
