Download A Glossary of Terms for Map Interpretation

A Glossary of Terms for Map Interpretation
(compiled by Hugh Millward 2006)
altitude (elevation)
The vertical height of an object above a datum level, usually mean sea level.
arête (knife-edge ridge)
A steep-sided narrow ridge separating two cirques (corries).
azimuth
A bearing clockwise from north. May be true (orthodrome), grid, or magnetic.
back bearing
A bearing exactly opposite to the original bearing (180 degrees different).
bearing
The angular distance between a reference direction and another direction.
May be measured E or W from N or S (by quadrant), or as an azimuth.
bench-mark (vertical control point)
A survey marker, indicating the location of a precisely-known elevation.
butte
A small, flat-topped, steep-sided hill (a small mesa).
cardinal points
North, East, South, and West.
cirque (corrie)
A semi-circular hollow, with steep side walls, formed by alpine erosion.
col (saddle, pass)
A pass between two peaks or a gap in a ridge.
compass rose
The 32 (or 64) points of the mariner’s compass.
concave slope
A slope which is steeper at the top.
contour
A line drawn on a map connecting points of the same height above a datum (usually mean sea
level).
contouring
Following a longer route that maintains a constant height rather than climbing and descending
on a direct route. For example: traveling around high ground rather than climbing over it.
contour interval (vertical interval)
The difference in height between adjacent contours.
conventional sign
A symbol used on maps to represent ground features or information.
convex slope
A slope which is steeper at the bottom.
datum
A fixed elevation used as a starting point for a survey (usually mean sea level).
dissected plateau
A former plateau into which numerous valleys have been carved by erosion.
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draw (re-entrant)
The minor valley between two spurs, often lacking a stream.
eastings
Distance eastwards from the origin of a grid system, indicated by the first half of a map grid
reference.
escarpment (scarp, cuesta)
A ridge which has a steep slope on one side and a gentle slope on the other.
fan
A gently-sloping, semi-circular area formed by alluvial deposits.
fix
A position obtained by the use of bearings. See also ‘resection’ and ‘intersection’.
false crests
When traveling up a convex slope you often see the crown of the convex slope in front of you
and not the summit of the hill. These crowns are known as false crests.
global positioning system (GPS)
A navigation system using earth-orbiting satellites to fix ground positions and heights.
gradient
The degree of slope, measured by the vertical change in height between two points on the
slope against the horizontal distance between them (i.e., rise over run). Usually given as a
ratio or percentage.
graticule
The lines of latitude and longitude drawn on a map.
grid north
The direction to which the vertical grid lines on the map point.
grid declination
The angular distance between grid and true north.
grid reference
A method, using a grid, of giving a unique reference to any point on the ground.
hachures
Short lines drawn downslope on a map to indicate relief.
hanging valley
A tributary valley which enters the main valley well above the valley floor. Often found in
glaciated regions.
hogback ridge
A long, narrow ridge in which both sides are steeply sloped (in contrast to an escarpment).
horn (pyramidical peak)
A sharp, pointed mountain summit, formed by back-erosion from three or more cirques.
index contour
A contour printed with a thicker line to assist in identifying the contour values and the shape
of the ground.
intersection
A technique to fix the map position of a feature by occupying in turn several locations and
plotting bearings from each location to the feature.
knoll
A small, rounded hill or mound.
ladder grid
The grid values shown at intervals on the face of the map.
landform
Any ground feature with shape, such as valleys and mountains.
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latitude
The angular distance in degrees north or south of the equator to a point on the earth’s surface.
longitude
The angular distance in degrees east or west of the prime meridian at Greenwich to a point on
the earth’s surface.
loxodrome (rhumb line)
Line of constant bearing. Any straight line on a Mercator projection.
magnetic north
The direction to which the magnetized compass needle points. This direction varies over a
period of time.
magnetic variation (or magnetic declination)
The angular difference between magnetic and true north. This difference will vary over a
period of time.
map reading
The ability to look at a map and visualize in your mind’s eye the features and terrain if
represents.
marsh
Wetland without trees
mesa
A steep-sided, flat-topped hill, or small isolated plateau.
northings
Distance northwards from the origin of a grid system, indicated by the second half of a map
grid reference.
orient or orientate
To align a map in the correct relationship to the points of the compass or ground features. See
also ‘set’.
orthodrome
True azimuth.
pass
A low gap in a line of hills or mountains. See also ‘col’ or ‘saddle’.
plateau
A distinct and mainly level area of elevated land.
Polaris
The name of the star that always indicates the direction of north. Also known as the Pole Star
or North Star.
protractor (Silva-type) compass
A magnetic compass with a rotating compass housing, which can also act as a protractor and
ruler.
quadrant
One quarter of the compass rose (e.g. 0 to 90=NE quadrant).
relief
Variation in altitude in an area; difference between highest and lowest levels.
resection
A technique to fix an occupied position by the intersection of two or more back-bearings from
known locations.
ridge (crest)
A long, narrow crest of a hill or mountain.
ridgeline (water-divide)
The line of high ground separating two watersheds or stream basins.
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roamer (or romer)
A device to assist in accurately reading or plotting grid references.
saddle (col, pass)
A pass between two peaks, or a gap in a ridge. Also called a col.
scale
The ratio between a map distance and the corresponding distance on the ground.
set a map
To align the map so that the direction of the features on the map coincides with the direction
of the features on the ground, from the observer’s position. See also ‘orientate’.
spot height
A surveyed height shown on the map but not marked on the ground.
spur (nose)
A lateral ridge extending into lower ground from a range of hills or mountains.
streamline
The line of a stream or river within a valley, or the line where an intermittent or infrequent
stream would run.
swamp
Wetland with trees.
tarn
A small mountain lake in a rock basin (often in a cirque).
terrace (bench)
A low, bench-like "step" running along a valley side or coastline.
terrain
The natural and man-made features that characterize the ground.
triangulation pillar or trig point (=Horizontal control point)
A concrete survey marker often found near the highest point in an area.
tributary
A stream, river, or glacier that flows into another larger one.
true north
The direction to the geographical North Pole.
truncated spur
A spur which has been partly cut away at the end by glacial action leaving a very steep slope
or cliff.
u-shape valley
A glacially-formed trough with a flat floor and concave sides.
UTM Grid
A rectangular grid placed over maps produced using the transverse Mercator projection. The
projection is re-centered for each zone of width 6˚ longitude (and, in the military UTM grid,
for each zone of 8˚ latitude).
vertical exaggeration
Ratio between the vertical and horizontal scales on a cross-sectional profile
waypoint
Any specific position used to navigate. Generally used as an alternative to checkpoint.
Sources: Ordnance Survey Land Navigation CD-ROM (1999), W. Moore’s Dictionary of
Geography, 5th ed. (1974), S. Mayhew’s Minidictionary of Geography (1994), L.M. Sebert’s
Map Reading (1984), author.