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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,-2877732rectangle with 2D corners (33,-2865760,0) to (-8281,-2870880,0)
Create a new layer 'circlerectangle' and set it as 'layer to use' to separate the new circle rectangle of the topography
Geometry->Create->Circle->Object->Rectangle
33,-2865760,0
-38508281,-28777322870880,0
normal of its plane in 'Positive Z' direction
radius=4000
Topography (transparent layer) and circle rectangle in XY and XZ views
To trim the terrain we will extrude cylindrical lateral surfaces and use them for intersection
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A translation of 3000 m in z, extruding surfaces
Copy with extrusion
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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.
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(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
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