![]() The distinguishing geometric characteristic of a Delaunay triangulation is that a circle surrounding each triangle side does not contain any other vertex. One approach is called Delaunay Triangulation which, in one of its constrained forms, is useful for representing terrain surfaces. (There are algorithms for triangulating irregular arrays that produce unique solutions. ![]() Similarly, the smaller the contour interval you intend to use, the more spot elevations you need. In general, the density of spot elevations should be greater where terrain elevations vary greatly, and sparser where the terrain varies subtly. Related to this element of subjectivity is the fact that the fidelity of a contour map depends in large part on the distribution of spot elevations on which it is based. As you will see, deciding which spot elevations are "near neighbors" and which are not is subjective in some cases. You may notice that there is more than one correct way to draw the TIN. I omitted breaklines from this example just to make a point. Assignment 2 / Project 1 : Cross Contouring While drawing some rocks and branches i was struck by how the pits in the rock and the eyes in the wood reminded me of contour lines and started experimenting with drawing contour lines into some of my work. Photogrammetrists refer to spot elevations collected along linear features as breaklines (Maune, 2007). In practice, spot elevations would always be measured at several points along the stream, and along ridges as well. Cross-hatching refers to a softer, woven application often used to build up tonal areas of shadow. Cross contour can be applied in one direction or in any mix of directions as long as the mark follows the surface of the form. Wonder why I suggest that you not let triangle sides that make up the TIN cross the stream? Well, if you did, the stream would appear to run along the side of a hill, instead of down a valley as it should. Cross contour brings elliptical curves within the boundaries of form to map the surface. With a little imagination and practice, you can visualize the underlying surface from the TIN even without drawing contours. (Spot elevations produced photogrammetrically are called mass points.) A useful characteristic of TINs is that each triangular facet has a single slope degree and direction. ![]() The vertices of the triangles are spot elevations that may have been measured in the field by leveling, or in a photogrammetrist's workshop with a stereoplotter, or by other means. A TIN is a vector representation of a continuous surface that consists entirely of triangular facets. Cross Contour: In simple terms, cross contour lines reflect the movement of your eye in and around the object you are seeing. The result is a triangulated irregular network (TIN). ![]()
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