In recent years there has been much research into the use of colours in the production of maps, particularly those depicting relief. There are what may be called the well-established conventional aspects of usage, the familiar associations; examples indicating blue tinting for the sea; stepped green, yellow, brown for relief; various land-use conventions such as brown for arable, green for forest; in maps showing temperatures the association of red with warmth and blue with cold; and the like. There is the association of magnitude with density of colour, for example, areas of heavy precipitation are often shown in dark blue, areas of light rainfall in pale blue, yellow or white. A.H. Robinson has discussed the three main colour dimensions. The first is hue, that is, the actual colour, the spectral colours of the rainbow and all the vast number of variants (as is shown, for example, in the British Standards colour range). The second is value, that is, the sensation of darkness (high value) and lightness (low value). And there is chroma, the actual reaction of the eye so that some colours are brilliant and intense, others are dull, pastel, even 'washed out'; this, in fact, is the amount of hue in the colour.