Global Mapper Tutorial by Opteryx
Global Mapper homepage here.
1) Import your terrain data.
2) Then go to Tools → Configure, set Projection to UTM.
3) Now select the Digitizer Tool (looks like a pencil) RMB click and select Create Area Shapes → Create Rectangular / Square Area.
4) Now Click and Hold LMB somewhere in the middle of the area you want to crop and drag so that the square shape forms. Select OK.
5) Now you need to deselect and reselect the Digitizer tool so that you can select the individual vertices of the box.
6) Select say the top left vertex by dragging and holding the mouse over it, right-click on it and select Set Position of Selected Vertices. From here you can set your vertex coordinate in the Geographic Coordinates (longitude/latitude) thingy, from the Global Projection UTM you can easily calculate the other vertices coordinates by adding and subtracting, UTM is meters so It's very easy.
7) After all your vertices are correctly placed you select the entire box area that you want to crop, go to File→Export Raster and Elevation data → Export XYZ Grid.
8) Now go to Highlight Export Bounds and check Crop to Selected Area Feature(s). Then options, set Coordinate Separator to “Space”, set both Y and X axis res to 10, then press OK and save your XYZ file.
Now if you've done everything right and I've remembered it correctly it should import nicely in to Visitor 3!