Scale and accuracy Map

a global view map of europe, western asia , africa.

many maps drawn scale expressed ratio, such 1:10,000, means 1 unit of measurement on map corresponds 10,000 of same unit on ground. scale statement can accurate when region mapped small enough curvature of earth neglected, such city map. mapping larger regions, curvature cannot ignored, requires projections map curved surface of earth plane. impossibility of flattening sphere plane without distortion means map cannot have constant scale. rather, on projections best can attained accurate scale along 1 or 2 paths on projection. because scale differs everywhere, can measured meaningfully point scale per location. maps strive keep point scale variation within narrow bounds. although scale statement nominal accurate enough purposes unless map covers large fraction of earth. @ scope of world map, scale single number practically meaningless throughout of map. instead, refers scale along equator.

large scale maps, (e.g. 1:10,000), cover relatively small regions in great detail , small scale maps, (e.g. 1:10,000,000), cover large regions such nations, continents , whole globe. large/small terminology arose practice of writing scales numerical fractions: 1/10,000 larger 1/10,000,000. there no exact dividing line between large , small 1/100,000 might considered medium scale. examples of large scale maps 1:25,000 maps produced hikers; on other hand maps intended motorists @ 1:250,000 or 1:1,000,000 small scale.

it important recognize accurate maps sacrifice amount of accuracy in scale deliver greater visual usefulness user. example, width of roads , small streams exaggerated when narrow shown on map @ true scale; is, on printed map narrower perceived naked eye. same applies computer maps smallest unit pixel. narrow stream must shown have width of pixel if @ map scale small fraction of pixel width.

cartogram: eu distorted show population distributions.

some maps, called cartograms, have scale deliberately distorted reflect information other land area or distance. example, map (at right) of europe has been distorted show population distribution, while rough shape of continent still discernible.

another example of distorted scale famous london underground map. basic geographical structure respected tube lines (and river thames) smoothed clarify relationships between stations. near center of map stations spaced out more near edges of map.

further inaccuracies may deliberate. example, cartographers may omit military installations or remove features solely in order enhance clarity of map. example, road map may not show railroads, smaller waterways or other prominent non-road objects, , if does, may show them less (e.g. dashed or dotted lines/outlines) main roads. known decluttering, practice makes subject matter user interested in easier read, without sacrificing overall accuracy. software-based maps allow user toggle decluttering between on, off , auto needed. in auto degree of decluttering adjusted user changes scale being displayed.