contour lines index contour triangulation pillar spot height concave slope convex slope even slope
stepped slope gradient
Relief on OS Maps
Interpreting height Height on Irish maps is calculated from a point on Malin Head, Co. Donegal, known as Ordnance Datum (OD). Measured in metres above sea level, height is shown four ways on OS maps (Fig. 12.10).
Colour layering z z z z
Contour lines zz
z z zz
Variations in colour are used to show changes in height. The colour changes every 100 m.
These are imaginary lines drawn on OS maps that join places of equal height above sea level.
Contour lines are drawn at 10 m intervals.
The 50 m, 100 m, 200 m and 250 m heights are sometimes indicated by a stronger line called an index contour. Index contours always have heights shown on them.
Triangulation pillars zz
Fig. 12.10(a) Contour layering table from the OS legend.
Sometimes referred to as triangulation stations, these are concrete pillars placed at the summit of hills and mountains to mark their exact height above sea level.
zz
On OS maps, triangulation pillars are identified by a black triangle with a number placed beside it.
Spot heights zz
zz
Spot heights are used to indicate the height above sea level at various points on the map.
They are identified by a black dot with a number beside it.
Fig. 12.10(b) Interpreting height on an OS map.
47
Contour lines: These grey lines join places of equal altitude – height above sea level.
Triangulation pillar: The altitude/ height of the land at this location is 526 m above sea level.
67 68 69 70 71 72 47
46
46
Spot height: The altitude/height of the land at this location is 459 m above sea level.
Colour layering: Green represents lowland areas up to 200 m. Brown represents higher areas, with the shading becoming darker as height increases.
