Terrain Model
Terrain models can be generated from a set of contour lines and a closed shape that represents the boundary of a site. The contour lines can be vector lines or splines, open or closed, laid out in a manner that makes topographical sense. For example, they cannot intersect. Open contour lines should be crossing the site boundaries. They may have a height value (Z is not 0) or they may all lie flat on the ground, in which case we need to instruct the Terrain Model tool to generate heights for them before they are used for the generation of a terrain model.
To derive a terrain model, with the Terrain Model tool active and while pressing the shift key, click on the contour lines, in the order heights need to be assigned to them. If they already have heights, the order is insignificant. After shift-picking the last contour, release shift and click anywhere in the project window, then, with the Terrain Model tool still active, click on the site. The terrain model is generated immediately. The type of terrain model or it's numeric values may be altered while the new terrain model is highlighted in orange directly after clicking the site boundary. When the result is satisfactory, the operation can be completed by clicking a tool icon to go on to the next tool operation.
There are three types of terrain models, which can be selected from the Terrain Model Tool Options palette.
Terrain Model options.
Meshed: The site surface is meshed and then the heights of its points are adjusted according to the heights of the contour lines. The latter is done through interpolation, which may be more or less accurate, depending on the option selected from the Interpolation pop-up menu. The density of the mesh depends on the value entered for Mesh Size. Turning on Triangulate produces a triangulated mesh, which is a variation of the plain mesh.
Stepped: The contour lines are inserted to the site and then extruded according to their heights.
Triangulated: The contours are again inserted in the site, but are not extruded. After they are positioned to the appropriate heights, the non-planar surfaces between them are triangulated.
Site (Starting) Height: The value in this field specifies the lowest elevation of the terrain model, which becomes the basis for the other elevations.
Contour Heights: There are two options in this group: Use Existing and Set New. The former is used when the contour lines already have heights. The latter when heights need to be assigned, which is done using the value in the Interval field.
Smooth At Interval: This option applies only when the contours are vector lines, rather than splines, which are already smooth. When on, the vector lines are rounded. This option only impacts the stepped and triangulated terrain types. It has no effect on the meshed models.
Precheck For Intersecting Contours: As already mentioned, the contours being horizontal slices of landforms, generally make no sense to intersect each other. Intersecting contours are not accepted by the stepped and triangulated types, which will fail when the condition is encountered. This option instructs the program to check the contours before any attempt is made to generate a 3D model.
