A short glossary of map reading
Twenty-four terms we keep coming back to, defined the way we'd explain them to someone reading a topographic sheet for the first time.
Aspect
The compass direction a slope faces, often read from the way contour lines bend around a hillside rather than measured directly.
Azimuth
A bearing measured clockwise from north, expressed in degrees from 0 to 360, used to describe a direction of travel.
Bench mark
A fixed, surveyed point of known elevation, historically set in stone or metal, used to tie later measurements to a common vertical reference.
Contour interval
The constant difference in elevation between one contour line and the next on a given sheet, printed once in the margin and applied throughout.
Contour line
A line connecting points of equal elevation. Where the lines sit close together the ground is steep; where they spread out, it's closer to flat.
Datum
The reference surface a map's coordinates are measured against. Two maps built on different datums can disagree about the same point by meters.
Declination
The angle between magnetic north, where a compass needle points, and true north, at a given place and date.
Generalization
The editorial process of simplifying a map's features as scale decreases, so a sheet stays legible instead of becoming an unreadable tangle of detail.
Graticule
The network of latitude and longitude lines drawn across a map to show geographic position independent of any local grid.
Grid north
The direction of north as defined by a map's printed grid lines, which is not always identical to true north because of how the projection was built.
Grid reference
A set of coordinates, usually numeric, that locates a point within a map's grid system precisely enough to find again.
Hachures
Short parallel lines once used to depict slope and shading, mostly replaced by contour lines on modern topographic sheets.
Index contour
A contour line printed heavier than the rest and labeled with its elevation, usually every fourth or fifth line, to help a reader count quickly.
Legend
The printed key explaining what each symbol, line style, and color on a map is meant to represent.
Magnetic north
The direction a compass needle points. It drifts slowly year to year and differs from true north by an amount that depends on where you're standing.
Projection
A mathematical method for representing the curved surface of the earth on a flat sheet, always at some cost of distortion somewhere on the map.
Relief
The shape and elevation variation of the land's surface, however a particular map chooses to depict it — contours, shading, or spot heights.
Representative fraction
A scale expressed as a ratio, such as 1:24,000, meaning one unit of measurement on the map equals 24,000 of the same unit on the ground.
Scale
The relationship between a distance measured on the map and the corresponding distance on the ground, expressed as a fraction, a ratio, or a bar.
Spot elevation
A single point on a map labeled with its exact elevation, printed independent of the surrounding contour lines.
Terra incognita
A historic label for regions left blank or marked unknown on early charts, sometimes filled in later with guesses that turned out to be wrong.
Topographic map
A map that represents the three-dimensional shape of terrain, usually through contour lines, rather than showing only roads and boundaries.
True north
The direction toward the geographic North Pole, distinct from both magnetic north and grid north on most printed sheets.
Waypoint
A marked reference point along a route, used to check progress against a map or to note something worth remembering later.