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18-Geom-A7 Geospatial Information Systems · May 2014

Question 20 of 23: Entity-Relationship (ER) Models

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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 20: Entity-Relationship (ER) Models (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.

An Entity-Relationship (ER) model is a conceptual data-modelling technique that describes the information content of a database in terms of entities (the real-world things to be represented), their attributes (the properties of each entity) and the relationships between entities, including the cardinality of those relationships (one-to-one, one-to-many, many-to-many). It is typically drawn as an ER diagram with entities as boxes, attributes attached to them, and relationships as connecting links.

PARCELowned byOWNERN1attrs: PID, zoning, area, geometryattrs: name, address
Figure — Simple ER diagram: PARCEL and OWNER entities linked by a many-to-one "owned by" relationship, each carrying its own attributes.

In a GIS the ER model is used at the database-design stage to plan the geodatabase before any data is captured. The geographic features become entities (feature classes) — parcels, roads, buildings, monuments — each with spatial and thematic attributes, and the relationships capture how they connect: a parcel is owned by an owner (many-to-one), a road connects intersections, a building sits on a parcel. This conceptual model is then translated into a logical schema (tables, keys, relationship classes) and finally a physical geodatabase implementation. Modelling the entities, attributes and relationships explicitly up front ensures the resulting database is consistent, avoids redundancy, enforces referential integrity, and correctly represents the real-world associations that spatial and attribute queries will later rely on.