Calculate a grid and volume from one surface to a datum.
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TerrainGridSingle |
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This function allows you to generate a grid model of one specified surface. Each 'sheet' is processed in turn, starting at the top right-hand corner. Lines of grid points are dropped vertically at the specified interval, and levels are interpolated at each point as they are displayed. All other surfaces are ignored in this process.
The resultant DTM grid, and hence derived contours, will reflect the single chosen surface only.
A grid interval must be defined. This interval controls a trade off between speed and precision. The site is broken up into squares called sheets. These sheets can be viewed by selecting Preview inside the dialog box. Each sheet comprises of 40 x 40 grid points at the specified spacing. A maximum of 200 sheets is permissible. If this number is exceeded, larger grid spacing must be specified. A smaller grid interval produces more sheets, and thus a more concentrated grid, which yields a higher accuracy in the volumes and smoother contours, but takes longer to process.
A 'Search hole' distance controls the way in which the data is interpreted. As each grid line is scanned, elevations are initially interpolated from the line data at the spots where the grid line crosses them. If the distance between these interpolations is greater than the specified 'Search hole' distance, no elevations will be generated at those locations and contours and volumes will not be obtained in this region. This setting can therefore be used to avoid contouring or quantifying through dams or large buildings, across the arms of horseshoe shaped sites, etc.
A specific elevation datum is set in order to enable a constant datum elevation to be used for sequential volume measurements. The datum elevation that is set may not be higher than the default provided.
Two volumes from the model are printed:
The Perimeter Base Volume is obtained by creating a uniform base around the perimeter of the gridded surface. This base is assumed to run from the first grid point on a grid line, directly to the last grid point on the line. Thus, for each line of grid points, a base will be created between the first and last grid points in that line. Any grid point elevations below the base will be considered as cut, and any above will be considered as fill. This volume is useful for dumps and stockpiles where the base level below the surface is not known, or where it can be assumed that there is a plane surface from one side to the other.
The Elevation Datum Volume is the volume between the gridded surface and a datum elevation below it. This volume is most frequently used in comparative measurements spaced over a period of time, and special attention must be given to the reliable definition of a perimeter in these circumstances.
In using these volumes, particular attention should be paid to the points raised in the general discussion on DTM Volumes.
Procedure
Select the Single surface option and click Next.
Select the surface and click Next to continue.
Fill in the relevant data and click Next to continue.
Fill in the relevant data and click Next to continue.
