In the past, we’ve had a lot of questions on how to compute volumes in user-defined polygons. If you had a Ph.D. in HYPACK®, you could usually find a way to do it, but it wasn’t easy.

With a new routine in the latest TIN MODEL program, called TrimTin, users can now ‘cut’ surface models so they conform to the edges of a user-defined polygon (border). The TIN MODEL program cuts the triangles along the polygon border and throws out the portion of the model that is outside the border. Volumes can then be computed.

For our example, I have an XYZ file of gridded multibeam data and I have a sub-area (defined by the border file) to which I want to limit the volume computation..

To create my border file, I used the HYPACK® MAX Border Editor and entered the exact XY values for each corner of the border file. I entered the last point inside the border area, to tell the TIN MODEL to keep the info inside the polygon. [I could have entered a point outside the polygon and TIN MODEL would then keep the material outside the polygon.]

The first step is to go ahead and make the TIN MODEL, using the original XYZ data file. There’s nothing new here, so I’ll skip the details. The resulting 2D model is shown in the graphic to the right.

Now it’s time to cut the tin along our border. In the File Menu, you’ll find the latest addition in an item called ‘Trim Tin’. When you click it, you’ll be asked to supply a HYPACK® MAX Border (BRD) file.

The existing TIN is now trimmed to conform to the edges of the border file. Additional XYZ data points are created at every point where a triangle leg intersects I can now use the remaining information and run volume computations on it using any of the methods available in the TIN MODEL program. For example, the report from the Philadelphia Method is shown in the text box below.

You can see that it only provides quantities in the areas contained in the polygon and 0.0s for lines that are outside the desired area.

The same technique (TrimTin) can be used to isolate contours only in a user-desired area. In the graphic to the right, I generated contours inside the polygon area by first, making the model; second, trimming the tin to a border file; third, exporting DXF contours from the TIN MODEL on the trimmed model.

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