You could use EntityPickMode, then convert the picked x/y/z coordinate to the terrain using TFormPoint:
Open in online IDE; TerrainHeight Example ; --------------------- Graphics3D 640,480 SetBuffer BackBuffer() terra_size=32 ; initial size of terrain, and no. of grids segments, along each side x_scale=10 ; x scale of terrain y_scale=50 ; y scale of terrain z_scale=10 ; z scale of terrain marker_x=terra_size/2 ; initial x position of marker marker_z=terra_size/2 ; initial z position of marker camera=CreateCamera() PositionEntity camera,(terra_size*x_scale)/2,50,0 ; position wherever; just try and get good view of terrain! RotateEntity camera,30,0,0 ; again, try and get good view of terrain light=CreateLight() RotateEntity light,90,0,0 ; Create terrain terra=CreateTerrain(terra_size) ScaleEntity terra,x_scale,y_scale,z_scale ; Texture terrain grass_tex=CreateTexture(64,64,1+8);LoadTexture("media/mossyground.bmp") Rect 0,0,64,64,0 CopyRect 0, 0,64,64,0,0,BackBuffer(),TextureBuffer(Grass_tex) EntityTexture terra,grass_tex ; Create marker marker=CreateSphere() ScaleEntity marker,1,1,1 EntityColor marker,255,0,0 EntityPickMode terra, 2 While Not KeyDown(1) ; Change marker position values depending on cursor key pressed If KeyHit(205)=True Then marker_x=marker_x+1 If KeyHit(203)=True Then marker_x=marker_x-1 If KeyHit(208)=True Then marker_z=marker_z-1 If KeyHit(200)=True Then marker_z=marker_z+1 ; Get terrain height at marker position marker_y#=TerrainHeight(terra,marker_x,marker_z) ; If A pressed then increase marker_y value and modify terrain If KeyDown(30)=True If marker_y#<1 Then marker_y#=marker_y#+0.005 ModifyTerrain terra,marker_x,marker_z,marker_y# EndIf ; If Z pressed then decrease marker_y value and modify terrain If KeyDown(44)=True If marker_y#>0 Then marker_y#=marker_y#-0.005 ModifyTerrain terra,marker_x,marker_z,marker_y# EndIf ; Position marker, taking into account x, y and z scales of terrain PositionEntity marker,marker_x*x_scale,marker_y#*y_scale,marker_z*z_scale RenderWorld Text 0,20,"Press A or Z to alter height of terrain at marker's position" Text 0,40,"Value: "+marker_y# Text 0,80,"TerrainHeight: " + TerrainHeight(terra,marker_x, marker_z)*y_scale Text 0,60,"Terrain Y: "+TerrainY(terra,marker_x*x_scale,marker_y#*y_scale,marker_z*z_scale) CameraPick camera, MouseX(), MouseY() TFormPoint PickedX(), PickedY(), PickedZ(), 0, terra marker_x = Int(TFormedX()) marker_z = Int(TFormedZ()) Flip Wend End