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18-Geom-A7 Geospatial Information Systems · May 2014

Question 13 of 23: DSM vs DEM, and DEM Sampling Schemes

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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 13: DSM vs DEM, and DEM Sampling Schemes (5 marks)

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.

A Digital Elevation Model (DEM) represents the bare-earth terrain surface — elevations of the ground with buildings and vegetation removed. A Digital Surface Model (DSM) represents the top reflective surface — the ground where it is exposed, but the tops of buildings, tree canopy and other above-ground objects elsewhere. Their difference, DSM − DEM, is the normalized surface (object height) giving building and canopy heights.

DEM — bare earth (ground)DSM — top surface (over building & canopy)
Figure — DEM follows the bare ground; DSM rides over the building roof and tree canopy. Their difference is object height.

Two sampling schemes for collecting a DEM: (1) Regular (systematic) grid sampling — elevations are captured at the nodes of a uniform grid at a fixed interval. It is simple, produces a raster DEM directly and is easy to process, but it can over-sample smooth terrain and under-sample rugged terrain and miss abrupt features. (2) Selective / progressive (feature-based) sampling — points are placed adaptively according to terrain character: densely where the surface is rough or important and sparsely where it is flat, with explicit capture of breaklines (ridges, streams, road edges) and spot heights. This yields an efficient, terrain-faithful model (well suited to a TIN) at the cost of a more complex, judgement-driven capture. Other schemes include contour digitizing and random/stratified point sampling, but grid and selective sampling are the two standard end-members.