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On nautical charts, the top of the chart is always true north, rather than magnetic north, towards which a compass points. Most charts include a compass rose depicting the variation between magnetic and true north. However, the use of the Mercator projection has drawbacks. This projection shows the lines of longitude as parallel.
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. [1] Common chart datums are lowest astronomical tide (LAT) [1] and mean lower low water (MLLW).
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146 m (5.7 in) above chart datum [8] and 1.304 m (4 ft 3.3 in) below the average sea level. In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collated data about the sea level.
Mercator 1569 world map (Nova et Aucta Orbis Terrae Descriptio ad Usum Navigantium Emendate Accommodata) showing latitudes 66°S to 80°N. The Mercator projection ( / m ər ˈ k eɪ t ər / ) is a conformal cylindrical map projection first presented by Flemish geographer and mapmaker Gerardus Mercator in 1569.
Topographic maps conventionally show topography, or land contours, by means of contour lines. Contour lines are curves that connect contiguous points of the same altitude . In other words, every point on the marked line of 100 m elevation is 100 m above mean sea level.
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. [8] Common chart datums are lowest astronomical tide (LAT) [8] and mean lower low water (MLLW).
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Edmond Halley's New and Correct Chart Showing the Variations of the Compass (1701), the first chart to show lines of equal magnetic variation. According to Arthur Robinson, thematic maps were largely an Industrial Age innovation, with some Enlightenment-era roots; almost all of the modern graphical techniques were invented between 1700 and 1850.