18-Geom-A7 Geospatial Information Systems · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2018 — 04-Geom-A7, Geospatial Information Systems. Closed book (one approved Casio or Sharp calculator permitted); duration 3 hours. Fifteen (15) questions are provided and any ten (10) constitute a complete paper; each question is of equal value (10 marks), so a complete paper totals 100 marks. Most answers are essay-format; clarity and organization count. All fifteen questions are solved below for completeness.
Longley, Goodchild, Maguire & Rhind, Geographic Information Systems and Science (4th ed.); P. Bolstad, GIS Fundamentals (5th ed.); Worboys & Duckham, GIS: A Computing Perspective (2nd ed.); Burrough, McDonnell & Lloyd, Principles of Geographical Information Systems (3rd ed.); de Smith, Goodchild & Longley, Geospatial Analysis; OGC Simple Feature Access (ISO 19125); ISO 19157 Geographic information — Data quality; ISO 19115 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.
The discrete-object and continuous-field views are the two fundamental ways of conceptualizing geographic reality; the choice made at this stage largely determines the data model, storage and analysis used downstream.
Discrete-object view. The world is regarded as an otherwise-empty space populated by countable, bounded things that have identity and clear edges — buildings, wells, roads, parcels, lakes-as-objects. Each object has a geometry (point/line/polygon) and one or more attributes, and there are gaps between objects where "nothing" of that type exists. This view maps naturally onto the vector model and onto object/feature tables, and it suits phenomena that are inherently individual and discrete (a cadastre, a utility network). Operations are object-oriented: adjacency, connectivity, containment, attribute queries.
Continuous-field view. The world is regarded as one or more variables that are defined everywhere — a function z = f(x, y) that has a value at every location, with no gaps and no discrete objects. Elevation, temperature, air pressure, rainfall, soil pH and land-surface reflectance are fields. This view maps naturally onto the raster grid (a value per cell) or a TIN/contour representation, and analysis is by map algebra, interpolation, slope/aspect and other surface operations.
Contrast and overlap. The essential difference is coverage: objects are discrete and leave empty space, whereas a field is exhaustive and continuous. Consequently objects carry topology and rich per-object attributes, while fields carry one value per location and support arithmetic across space. Some phenomena can be seen either way — a lake is a discrete object (a polygon with a name and area) or a value in a "water/no-water" field; population is individual people (objects) or a density surface (field). The database designer chooses the conceptualization that best fits the phenomenon and the intended analysis, and that choice cascades into vector-vs-raster storage.
