18-Geom-A7 Geospatial Information Systems · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2015 — 04-Geom-A7 Geospatial Information Systems. Closed-book; an approved Casio or Sharp calculator is permitted. Format: twelve short-answer questions of varying value totalling 100 marks; all questions constitute a complete exam and 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); J. P. Snyder, Map Projections — A Working Manual (USGS PP 1395); 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.
Because every flat projection distorts the curved Earth in some way, choosing a projection means deciding which property to preserve and which distortion to tolerate for the task at hand. The main factors are: (1) The property to preserve — conformal (shape/angle) for topographic mapping and navigation, equal-area for choropleth and density mapping and any area measurement, equidistant for correct distances along chosen lines, or a compromise projection for general reference; the analysis to be performed dictates this. (2) The size, shape and latitude of the area of interest — projections are chosen so their line(s) of true scale run through the region to minimise distortion; a small mid-latitude area with east–west extent suits a Lambert Conformal Conic, a north–south strip suits a Transverse Mercator, polar regions suit an azimuthal projection. (3) The purpose and audience — precise measurement and engineering versus general visualization impose different accuracy needs. (4) Existing standards and legal/compatibility requirements — the projection other agencies use (UTM, 3° MTM, the provincial standard grid) so data will register, and any legally mandated system for cadastral or survey data. (5) Datum consistency — the projection must be tied to the correct datum (NAD83(CSRS)) so it aligns with other layers. (6) Acceptable distortion / scale-factor budget — how much scale, area or angular error the application tolerates. In practice for Canadian GIS, UTM or MTM (conformal Transverse Mercator) on NAD83(CSRS) is the default unless area-true analysis or a very large extent argues otherwise.
