18-Geom-A7 Geospatial Information Systems · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 04-Geom-A7 Geospatial Information Systems. Closed-book; no calculator permitted. Format: twenty-three short-answer questions of equal value (5 marks each); a candidate answers any twenty, but all twenty-three are solved in full below. Datum and coordinate conventions follow the Canadian spatial reference framework — NAD83(CSRS) horizontally and CGVD2013 vertically.
Reference texts: P. A. Longley, M. F. Goodchild, D. J. Maguire & D. W. Rhind, Geographic Information Systems and Science (4th ed., Wiley, 2015); P. Bolstad, GIS Fundamentals: A First Text on Geographic Information Systems (6th ed., XanEdu, 2019); P. A. Burrough, R. A. McDonnell & C. D. Lloyd, Principles of Geographical Information Systems (3rd ed., Oxford, 2015); M. Worboys & M. Duckham, GIS: A Computing Perspective (2nd ed., CRC, 2004); ISO 19115 Geographic information — Metadata.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
TIN stands for Triangulated Irregular Network — a vector representation of a continuous surface (usually terrain) as a mesh of contiguous, non-overlapping triangles whose vertices are irregularly spaced sample points of known elevation. Each triangular facet is treated as a planar surface, so slope, aspect and interpolated elevations can be computed directly within any triangle.
A TIN is produced as follows: (1) collect an irregular set of mass points — significant elevation samples (from survey, photogrammetry or filtered LiDAR), placed densely where terrain is complex and sparsely where it is smooth; (2) add breaklines (ridges, channels, road edges, shorelines) and boundary/exclusion features that the surface must honour; (3) triangulate the points, almost always by Delaunay triangulation, which maximizes the minimum interior angle of the triangles (avoiding thin slivers) and guarantees that no sample point lies inside the circumcircle of any triangle; (4) enforce breaklines as fixed edges (a constrained Delaunay triangulation) so triangles do not cross linear terrain features; and (5) attach the elevation to each vertex so the mesh models the surface. The result adapts its resolution to terrain complexity, uses fewer points than an equivalent grid, and preserves sharp features through breaklines.
