18-Geom-B1 Digital Terrain Modelling · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2016 — 3 hours, closed book (an approved Casio or Sharp calculator permitted). TWELVE numbered questions constitute a complete paper; each carries the margin value shown and the schedule totals 100 marks. Most answers are required in essay format, so clarity and organization are graded. All twelve questions are solved below for completeness.
Reference texts: Li, Zhu & Gold, Digital Terrain Modeling — Principles and Methodology (CRC Press, 2005); Maune (ed.), Digital Elevation Model Technologies and Applications: The DEM Users Manual (2nd ed., ASPRS, 2007); Wilson & Gallant, Terrain Analysis — Principles and Applications (Wiley, 2000); Wolf, Dewitt & Wilkinson, Elements of Photogrammetry with Applications in GIS (4th ed., McGraw-Hill); Natural Resources Canada High-Resolution DEM (HRDEM) and CDEM product specifications. Canadian datums throughout (NAD83(CSRS), CGVD2013).
Note: this December 2016 paper is the same twelve-question essay set as the December 2014 and December 2015 04-Geom-B1 exams, in a slightly different order (here Q1 asks the primary use of the DTM/DEM/DSM, Q2 the five definitions, Q3 the sampling-interval drivers). The margin marks are internally consistent this year — every sub-part annotation matches its header weight and the schedule sums to 100.
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.
Given. Three closely related but distinct digital representations of height data over a landscape.
Find. The primary use of each of the DTM, DEM and DSM, and the surface each one models.
(a) Digital Terrain Model (DTM). A DTM represents the bare-earth surface together with the morphological structure that describes its shape — breaklines, ridge and drainage (form) lines, and spot heights at peaks, pits and saddles. Because those slope discontinuities are stored explicitly, its primary use is engineering terrain analysis: deriving reliable slope, aspect, contours and drainage, and supporting route and corridor design, earthwork/cut-and-fill quantities and site grading, where the fidelity of ridges and channels matters. It is the richest of the three products and the one an engineer designs on.
(b) Digital Elevation Model (DEM). A DEM is the simplest form — bare-earth elevation only, almost always a regular raster grid of height posts at a fixed spacing (one $Z$ per cell). With no explicit breaklines or feature coding, its primary use is general-purpose gridded terrain analysis: the workhorse height layer for hydrological modelling, viewshed/visibility, orthophoto rectification of the ground, slope/aspect rasters and regional/continental base mapping, where a uniform, computationally simple height field is wanted. Nationally, NRCan's CDEM/HRDEM products are of this gridded bare-earth type.
(c) Digital Surface Model (DSM). A DSM represents the top reflective surface of the landscape as sensed (the first return), so it includes above-ground objects — building roofs, tree canopy, bridges. Its primary use is any application that needs the outermost surface rather than the ground: line-of-sight and telecommunications/radar planning, obstruction and flight-path analysis, canopy-height and forest-biomass estimation (DSM minus DEM), building-height extraction, and true-orthophoto production in built-up areas. The essential contrast: DSM = ground plus everything on it; DEM = bare-earth heights on a grid; DTM = bare-earth surface enriched with terrain structure — a DEM/DTM is produced from a DSM by filtering out the non-ground points.
