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.