Objective: create a volume around a trimmed part of a topography
The starting data is the file topo_nlc.dat, that is a text file with xyz of a grid (5800 points)
Files->Import->XZY points...
Select the 'Triangulate' option like the image
XYZ points import file dialog
The terrain seems planar, but really the z of each point is different.
Its bounding box has dimensions 11300x19700x7.5 (could be seen with Utilities->Distance and clicking two opposite corners),
Probably represent meters (a 2D terrain rectangle of 11.3 x 10.7 Km).
The lower left point has coordinates x=-9400 y=-2884100 z=1557.5 (could be seen with Utilities->Id or Utilities->List)
Note: the Y coordinate is big (has 7 digits before the decimal), far from the origin, typical of an UTM (Universal Traverse Mercator) projection. It is near the limit of significant digits of a float number (4 Bytes) and could be problematic for OpenGL drawings (graphical artifacts drawing and selecting in screen)
We want to have only the region inside the rectangle with 2D corners (33,-2865760,0) to (-8281,-2870880,0)
Create a new layer 'rectangle' and set it as 'layer to use' to separate the new rectangle of the topography
Geometry->Create->Object->Rectangle
33,-2865760,0
-8281,-2870880,0
Topography (transparent layer) and rectangle in XY and XZ views
To trim the terrain we will extrude lateral surfaces and use them for intersection
Utilities->Copy...
A translation of 3000 m in z, extruding surfaces
Copy with extrusion
And calculate numerically the intersection of surfaces with:
Geometry->Edit->Intersection->Surfaces
Try to select the minimum surfaces possible, because will find the intersection of the selected surfaces all against all, and it could be very expensive in time.
(select in XY view first with a box a little bigger than the rectangle, and again with a box a little smaller to remove surfaces from the selection)
Selection of surfaces to be intersected
Note: Don't worry if the advance bar show more than 100%, this is because the final number of surfaces to be intersected grown because when a cutted surface is splitted the new surfaces are added to the collection to find more intersections.
If everything end successfully all surfaces will be divided. Some intersection could not be done because of tolerances (too small cuts, etc), and some manual extra operation could be required.
Now we can delete the unwanted part, e.g. to have only a volume of terrain under the rectangle for some geotechnical simulation.
Delete the unneeded surfaces.
Geometry->Delete->Surfaces
In the contextual menu click LowerEntities (to delete also its dependent lines and points)
and select the surfaces
Then the model must have only the surfaces that enclose a volume ,create it with
Geometry->Create->Volume->By contour
and select all surfaces
Final volume and detail of cutted surfaces