6. Height Heights above sea level in Ireland are measured in metres above a base line taken at Malin Head. This headland is used as the base line location because a headland can be an open area of sea and thus gives a more accurate sea-level reading. This base level is called Ordnance Datum (OD). Height can be shown on maps in the following ways:
Colour codes are used to distinguish height throughout OS maps. These colours give an immediate view of the lowland and highland parts of the map. Elevations below 100 metres are dark green changing to light green between 100 and 200 metres. This then changes to light brown above 200 metres and stronger shades of brown for each 100 metres above this.
27 28 29 (b) Contour lines
These are lines which join all points of the same height above sea level (or OD). They are shown at intervals of ten metres apart on OS maps. As well as height, they also show the shape
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and slope of the land. Contour lines placed close together indicate steep land.
(c) Spot heights 18 18 17 17
Spot heights show the height/elevation of particular points. Usually these heights mark all mountain/hill tops. They are drawn as a black spot with a number beside it representing its height in metres.
Height (m) 16 O 220 16 27 28 29 Fig. 16 Different ways of showing height on an OS map