18-Geom-B1 Digital Terrain Modelling · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2017 — 3 hours, closed book (one approved Casio or Sharp calculator permitted). The schedule prints TWELVE questions and states that "10 questions constitute a complete paper": Part A (Q1–Q8) is compulsory, Part B requires ONE of Q9–Q10, and Part C requires ONE of Q11–Q12, for a 100-mark paper. All twelve questions are solved below for completeness (a candidate would answer only Q1–Q8 plus one from each of Parts B and C).
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, 2014); Isaaks & Srivastava, An Introduction to Applied Geostatistics (Oxford, 1989). Canadian datums throughout (NAD83(CSRS), CGVD2013).
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. Four acquisition techniques spanning field survey, airborne imaging, spaceborne imaging and active airborne ranging.
Find. The principal merits (with the governing limitation) of each for producing DEM data.
2.1 Ground surveys. Direct field measurement by total station, differential levelling or RTK-GNSS. Merits: the highest attainable accuracy (mm–cm), full operator control over point placement so breaklines and critical morphological features are captured deliberately, and reliable results beneath tree canopy where remote sensing fails. It is the method of choice for engineering sites, earthwork volumes and small high-precision areas, and it supplies the ground-control/check points that validate every other method. Limit: slow, labour-intensive and uneconomic over large areas.
2.2 Aerial photogrammetric methods. Stereo image matching or manual stereo-compilation from overlapping vertical aerial photographs. Merits: efficient area coverage at high planimetric and vertical accuracy, a rigorous and well-established geometry, the ability for an operator to compile breaklines directly in the stereo model, and imagery that doubles as an orthophoto source. Accuracy scales predictably with flying height (base-to-height ratio). Limit: needs clear weather and image texture, and matching fails in shadow, water and dense vegetation (it yields a DSM that must be filtered to bare earth).
2.3 Satellite image-based methods. Stereo optical sensors (SPOT, ASTER, high-resolution commercial) or radar interferometry / InSAR (SRTM, TanDEM-X). Merits: the widest and fastest coverage — regional to global — reaching remote or inaccessible terrain, frequent revisit for change detection, and (for radar) all-weather, day-or-night sensing that penetrates cloud. It is the economical route to continental and national base DEMs. Limit: coarser resolution and lower vertical accuracy than airborne methods, and InSAR returns a canopy-biased DSM.
2.4 Airborne lidar terrain altimetry. Direct 3-D laser ranging from an aircraft, georeferenced by integrated GNSS/IMU. Merits: very high point density and cm-level vertical accuracy, 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. It is the current method of choice for high-quality DTMs and is the workhorse behind Canada’s HRDEM programme. Limit: higher cost than imagery, very large data volumes, and poor returns over open water.
