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 circle with radius=4000 (4 km assuming that probably the model represent meters) and 2D center at (-3850,-2877732,0)

Create a new layer 'circle' and set it as 'layer to use' to separate the new circle of the topography

Geometry->Create->Circle

-3850,-2877732,0

normal of its plane in 'Positive Z' direction

radius=4000

Topography (transparent layer) and circle in XY and XZ views