This report describes some of the computer based techniques which can be used to aid in the visualisation of terrain. In the following discussion it is assumed that we have acquired data in a suitable form which allows us to define the surface on a computer. This usually starts as a series of points lying on the surface and stored as x,y,z triples. There are many ways of getting this data, the example here was digitised from contour maps using a large scale digitising tablet connected to a computer. The next step is normally to turn the digitised data into a series of polygonally bounded planes, called facets, so that they can be read, viewed and manipulated by 3D modelling and rendering software. The point data is this example is transformed into a regular polygonal mesh by an algorithm called Delaunay triangulation See an earlier discussion by myself for details of this algorithm and computer program which transforms randomly distributed spot heights into triangular or regular meshes and exports this surface description in a format suitable for most 3D modelling and/or rendering programs.Contouring
Contouring has been the traditional way of presenting 3D terrain data on 2D sheets of paper. It has advantages of having units (contour values) so precise height calculations can be made. Contour lines are usually drawn with line segments so they can easily be transferred to large scale hard copy devices such as plotters. The main problem with this visualisation technique is that it does not give a good 3D impression of the terrain, the best most people can determine is that some localised parts of the surface are higher or lower than others.Mesh representations
This is the most straightforward way of rendering the 3D data, it is a direct perspective viewing transformation of the computer database. (Almost any 3D modelling package can display, view, and print this representation, MicroStation was used here due to its advantages when handling very large geometric databases).
This rendering can also be in colour. The colour may be related to height but it could also be some other attribute such as ground cover, population, etc.
The example in Figure 1 is a 128x128 cell mesh and it consists of about 33,000 line segments. On many high end computer platforms this can be drawn in real time so that the user can "fly" about the landscape. This number of polygons can however cripple many desktop machines. For this reason good terrain modelling software allows the user to view the surface at a range of resolutions.Vertical contouring
Higher levels of realism can be achieved by simulating more closely how the terrain would appear in reality. There are a wide range of techniques for accomplishing this, each technique generally involves a trade off between realism and computation time.
Given the digital model it is possible to determine the path required to manufacture the surface using a computer controlled milling machine. In its simplest form this is a drill which can be controlled backward and forward and up and down over a piece of wood say, so as to cut away unwanted portions.
Figure 4 shows the result of milling our example landscape from wood. I used a OH-FANUC, model 2R-NC milling machine operated by the School of Engineering, Auckland University. The machine is controlled by the MasterCAM software For this example the wood is about 300mm square and the drill bit used in about 5mm radius. Note that it is not necessary to use a particularly fine bit because the wood is cut away at the edge of the bit not from the bottom. The bit size then only determines the narrowest valleys that are possible. This physical scale model of the landscape has big advantages for visualisation purposes. The model is to scale, although in this case there is a 2 times exaggeration in height. The viewer can instantaneously view the terrain from various positions and angles by simply turning the model about. Tactile exploration of the model is of course possible and can be informative as well as satisfying.
An extension of this technique would be to automatically draw features such as roads, boundaries, contours, etc onto the landscape. This could be done with a robot arm holding a pen or with a laser which would burn the line features onto the surface of the wood.