Mathematics for GIS Professionals
A point is an exact position or location on a plane surface. It is important to understand that a point is not a thing, but a place. We indicate the position of a point by placing a dot with a pencil. This dot may have a diameter of, say, 0.2mm, but a point has no size. No matter how far you zoomed in, it would still have no width. Since a point is a place, not a thing, it has no dimensions.
Points are usually named by using an upper-case single letter. In the figure above, the points P,Q and R are shown. In this web site, points are shown either as a black dot or with a somewhat larger orange halo. This indicates the point can be dragged with a mouse.
- If a set of points all lie in a straight line, they are called ‘collinear’
- If a set of points all lie on the same plane, they are called ‘coplanar’
A line (straight line) can be thought of as a connected set of infinitely many points. It extends infinitely far in two opposite directions. A line has infinite length, zero width, and zero height. Any two points on the line name it. The symbol ↔ written on top of two letters is used to denote that line.
A plane may be considered as an infinite set of points forming a connected flat surface extending infinitely far in all directions. A plane has infinite length, infinite width, and zero height (or thickness). It is usually represented in drawings by a four‐sided figure. A single capital letter is used to denote a plane.