A Problem with Rock Overhangs
by Lennie Torgerson
InRoads will not correctly triangulate vertical surfaces, or surfaces that have overhangs. This creates a
problem when trying to triangulate surfaces of rugged rock faces.
If all of the points that make up the surface are rotated by swapping the y- and z- coordinate values, the
surface will tip over on its side. Out-croppings and overhangs will become hills and valleys in the rotated
surface and then InRoads can triangulate correctly.
Accurate triangle volumes or grid volumes may be calculated from comparing rotated surfaces.
If you wish to perform average end-area volumes, you can use a straight baseline alignment in the
rotated coordinate system.
The Concept Behind Pushing the Cliff Over
When we simply swap Y- and Z- coordinate values on the points, the surfaces that are triangulated, are
equivalent to the original and are rotated, but are inverted because the direction of the resulting Z-axis is
downward. You can see this in Figure 1. As the Y-Z plane rotates 90° about the X-axis, the Y-axis
positive direction points downward. The surface inversion can be easily corrected by changing the sign
on the Y-values.
If an original surface includes shots on both a roadbed with roadside ditches and a rock face with outcroppings, the shots may need to be split into two separate surfaces in order for each to triangulate
correctly. The roadway surface could be stopped at a top back of ditch points and then a separate rock
face surface may be started at that same line and continued up a rock face. When you tip over rock
overhangs, you don’t want a ditch or a roadbed to rotate up and create a new overhang.
(X, Y, Z) – An overhang will not be triangulated correctly. Figure 1 shows two shots on a pyramid-shaped
out-cropping. Point B is closer to the instrument because the Y-value is less; Point A is further away.
A(0,4,6)
Z
Y
B(5,1,4)
X
Figure 1.
(X, Z’, -Y’) – Coordinate system is rotated by swapping the Y- and Z- values for all points. If the Y-value
is not made negative, an equivalent inverted surface will be created. To keep the surface from inverting,
change the sign of all Y-values. Because Point B’s Y-value was changed from 1 to -1, it is higher than
Point A and the out-cropping has become a hill.
The Y-values can also be translated back into positive space, by adding a large number to all of the Yvalues. For example, the drawing in Figure 2 could be translated 10 units higher and then the two points
listed in the picture could be (0,6,6) and (5,4,9).
Y’
Z’
A(0,6,-4)
B(5,4,-1)
X
Z
X
Figure 2.
Y
Process using Bentley MicroStation and InRoads:
The process involves using four different pieces of software:
Use InRoads to display graphical elements in a DGN and use MicroStation to export the coordinates.
Use Excel to swap Y- and Z- values. Use Excel, PFE, or notepad to insert negative in new Z- values.
Use MicroStation to import graphical elements with the rotated coordinates. Use InRoads to import the
points into a surface and triangulate the surface.
1. Display the features that were shot in the field in
a MicroStation design file, in a clean model.
Features list should include breaklines and terrain
points. Features stored in a surface can be
displayed using Surface > View Surface >
Features. Cogo points stored in a geometry file
can be displayed using Geometry > View
Geometry > Horizontal Annotation (select the
point range and uncheck all the annotation).
2. Make a selection set of all displayed elements. In
MicroStation, use Export Coordinates from the
Tools > Annotation > XYZ Text tool palette.
Click [Single] and accept the select set. This
creates a text file.
3. Launch Excel, click File>Open, set Files of Type to Text Files (*.prn; *.txt; *.csv), navigate to the
text file you just created and select it. Select a Delimited data type in the Import Wizard, with
Space delimiters. The file will open with three columns of data: X in column A, Y in column B, Z
in column C. Use Excel commands to cut column B and insert it after column C. Set the column
number formats to the same level of precision that your MicroStation coordinate readout is set to.
Save the text file.
4. You can use Excel commands to make the new Z- values (in column C) negative. I have created
the following formula in cell D1 and then used the fill handle to copy it to the bottom of the
column: =-1*C1. Once the first 3 columns are displaying X, Z, and –Y, I create a new worksheet
named XZY, copy columns A, B, and C, and use “Paste Special – Values and Number Formats”
to paste only the values into the new worksheet. With XZY the active worksheet, I choose
File>Save As and give a new filename of XZY. After responding [OK] to a couple of warnings,
only the XZY worksheet is saved to XZY.txt.
Or,
A text editor, like PFE, could be used to insert the negative sign after you’ve used Excel to do a
simple column swap. That process is very quick if you record a macro and reuse it as needed.
To do this, open the text file in PFE. Position the cursor in front of the first value that requires a
negative sign. Turn on the macro recording (Shift+F7), type a negative sign (-), then use the
arrow keys on the keyboard (Down) and then (Left) to position the cursor on the next line. Stop
recording (Ctrl+F7). Play back the recording until you hit the end of the file by pressing F7 and
holding it down until you get a Command Failed message. Save the file.
5. Back in MicroStation, in another
clean model, use Import Coordinates
from the Tools > Annotation > XYZ
Text tool palette. Navigate to the
XZY.txt file. Set your active
attributes to a color you will be able
to see and set the weight up to about
10 if you are importing Point
Elements – they will be easier to see
that way. Click the [Process] button;
then fit the view to see all your
rotated points.
6. In InRoads create a new surface. Chose File > Import > Surface… and set the From Graphics
tab up similarly to what is shown below on the Import Surface dialog and click [Apply].
7. Triangulate the surface and save it. Display triangles, contours, render, whatever…
8. When computing volumes from the surface, any other surface must go through the same rotation,
prior to using triangle volumes for comparison.