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. Five acquisition techniques spanning existing-map conversion, field survey and airborne/spaceborne remote sensing.
Find. The principal merits (with the main limitation) of each for producing DEM data.
(a) Map digitization. Deriving heights by digitizing existing contour lines and spot heights from topographic maps. Merits: very low cost and immediate availability — no field campaign — giving national coverage from legacy mapping; suitable for small/medium-scale, low-relief regional models. Limit: accuracy is capped by the source map (contour interval, generalization, paper distortion) and the surface is only as detailed as the contours, so it is a fallback where nothing better exists.
(b) Ground surveys. Direct field measurement by total station, differential levelling or RTK-GNSS. Merits: the highest accuracy (mm–cm), full operator control over where points fall so breaklines and critical features are captured deliberately, and reliable results under canopy where remote sensing struggles. Ideal for engineering sites, earthwork and small high-precision areas. Limit: slow, labour-intensive and uneconomic over large areas.
(c) Aerial photogrammetric methods. Stereo image matching / manual stereo-compilation from overlapping aerial photos. Merits: efficient area coverage at high planimetric and vertical accuracy, well-established rigorous geometry, and the imagery doubles as an orthophoto source; the operator can compile breaklines directly in stereo. Limit: needs clear weather and texture, and image matching fails in shadow, water and dense vegetation (yields a DSM needing filtering).
(d) Airborne Lidar. Direct 3-D ranging from an aircraft with GNSS/IMU georeferencing. Merits: very high point density and vertical accuracy (cm-level), multiple returns that see through gaps in vegetation to yield a genuine bare-earth DEM after filtering, active sensing that works day or night, and rapid large-area collection. The current method of choice for high-quality DTMs. Limit: higher cost than imagery and large data volumes; poor returns over water.
(e) Satellite image-based methods. Stereo optical (e.g. SPOT, ASTER, high-res sensors) or radar interferometry/InSAR (e.g. SRTM, TanDEM-X). Merits: the widest, fastest coverage — global or continental — including remote/inaccessible terrain, repeat acquisition for change detection, and radar penetrates cloud and works day/night. Best for regional and global DEMs. Limit: coarser resolution and lower vertical accuracy than airborne methods; InSAR returns a DSM biased by canopy/decorrelation.
