Question 3 of 15: The buffer function and proximity analysis

Question 3: The buffer function and proximity analysis (10 marks — a 5, b 5)

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) The buffer function. Buffering is a proximity operation that creates a new polygon (a "zone of specified distance") enclosing every location within a given distance d of the input feature(s). It applies to any vector geometry: a point buffers to a circle of radius d, a line buffers to a corridor of half-width d on each side, and a polygon buffers to an enlarged (or, with negative d, shrunken) version of itself. The distance may be constant or driven by an attribute (variable-width buffer), buffers of many features may be dissolved so overlapping zones merge into one, and multiple-ring buffers produce nested distance bands. The output is itself a polygon layer that can be used in further overlay.

(b) Buffer in proximity analysis. Proximity analysis answers the question "what lies within (or beyond) a given distance of a feature of interest?" The buffer supplies the distance zone; an overlay (intersect / select-by-location) then tests which other features fall inside or outside it. For example, to find dwellings within a 500 m noise zone of a highway, buffer the highway by 500 m and intersect the result with the buildings layer; to enforce a 30 m riparian setback, buffer the streams by 30 m and erase that zone from developable land. Buffering thus turns a distance criterion into an explicit polygon that spatial selection can operate on — the standard way "within/near/beyond distance d" rules are implemented, and a building block of suitability and site-selection models (see Q12).