It will help you follow this lesson if you use the same terrain BMP as I did.  So download it here, delete your existing terrain mesh (select the group and delete it), and use the supplied image1.bmp in GtkGenSurf to generate the terrain I have used.

Delete the couple of silly cones that we created earlier, to give us some clear space - we don't really need them, they were just for ililustration of technique.

In the previous lesson we created the terrain which was generally fine, but a chunk of it obscured the door.  To clear this chunk out of the way we will edit the terrain manually by dragging their vertices about.  Manual editing is something you should do last, because if you decide subsequently to regenerate the terrain with GtkGenSurf, you will lose your editing (unless you go to great lengths to retain your edits and restore them after the regeneration - best avoided).

Manual editing of vertices is error prone.  Very.  Backup or save your map before you start.  It is also highly likely that Radiant will create errors even if what you have done is correct.  We can usually sort these, but then again, that backup may come in very handy.  You have been warned :)

We've selected the door to help us identify the selected terrain triangle in the 2D view, and to see how far down we have to lower it.

In the 2D view, get the side view that makes the selection the clearest to work on.

Deselect the door (in the 3D view will be easiest).

Press V to engage vertex mode.  Little green dots appear at each vertex of the triangle.

This illustrates one of the problems encountered with this activity: trying to spot which green dot to use.  You can click on the required vertex in the 3D mode if you can see/guess where it is - or you click on the one you think it is in the 2D view.  The one we want is the top right dot.

You should zoom in on the dot to ensure it is on a grid intersection.  If it isn't, reduce the grid scale until it is.  If you attempt to move the dot when it didn't start on an intersection you will make an almighty mess of your triangles.

If it isn't clear how far you need to drag the vertex, you can briefly select the door or the floor so you can see the required baseline, but you will need to press V again.

Drag the vertex down to the required baseline.

In the 3D view this now appears that the terrain is out of the way.  Check it by going inside the building, selecting the door and hiding it.

As you can see, there is still some caulk showing, so we need to drag another vertex down too.  But this time it is slightly more involved, and in fact is much more the usual state of affairs, being that we'll need to drag multiple vertices at the same time.

Take your 3D view outside again.  Get the 2D overhead view.  In the 3D view (it is easiest unless you hide the sky to make 2D selection easy) select all of the triangles that share the vertex at the corner of the building.  There are 6 triangles to select.

This is what the 2D view should show:

Press V and select the required vertex - this gets quite hard in 2D but sometimes with the rolling hills it can be impossible in 3D so even the quite hard 2D view is easier :(

As can be seen, the required vertex is not grid aligned.  As we are at grid scale 4 and the dot is clearly 1/4 up from a line, we'll need to drop down 2 scales: press 2 to get grid scale 2.

Ok now we have the required scale, but it has become hard to see where we have to drag it.  The best thing is to drag it just enough to put it onto a bigger, better grid scale.  So drag it one notch down, then press 4 to go back to grid scale 4.

Drag it down a couple of notches, and it should look like this:

Press ESC to let go of all the brushes.  You can see now that the door is unobscured, so you can reveal the door again.

When you edit your terrain mesh this way, Radiant may well generate duplicate planes, which will cause your compilation to open the debug window.  So after each editing session and before you compile, you should click the Brush Cleanup button.  If it finds errors it deletes them and tells you how many invalid planes were removed.  it also selects the mesh group.  You should deselect the group and click the button again until the Brush Cleanup reports 0 invalid/duplicate planes removed.  Finally deselect the group and save your work.

 

There will be occasions where the vertex you wish to manipulate shares its Z co-ordinate with another vertex of one or more of the selected triangles (ie, that point of the triangle shares its height with other points on the selected triangles).  In the 2D view you will not always be able to see the other vertices at the same height as they may be obscuring each other depending on whether you are looking down the X or Y axis at them.  When you try to select and drag the vertex, you may find you are dragging the wrong vertex :( This is because even though you may have selected the right vertex in the 3D view, when you click on the blue dot in the 2D view, Radiant will realize that there is another vertex at that point which is closer to you, the viewer, and so handily selects that one for you.  You may notice this has happened in the 3D view - but if you don't, you're going to start dragging the wrong vertices.  Watch out for this.  If you find this happening, try using the other 2D side view.  Or selecting the other troublesome triangles and move their offending vertex up/down a little to get it out of the way.  Final option is to carefully drag a notch up/down in the 3D view - this is hazardous as it is easy to accidentally drag sideways.

Final tip is to remind you to get down to the lowest grid scale needed to get your vertex onto a grid intersection.  Then move it to a nearby larger scale grid intersection, change the grid scale up accordingly by pressing a bigger number, even 9 if possible, then zoom out in the 2D view and drag in larger increments.  Always try to leave any dragged vertices on nice big grid intersections, it will help you a lot.

Do not drag vertices sideways in the 2D view, only up and down!!!!