What is a Map Scale and other questions?
Many, but not all, maps are drawn to a scale, usually expressed as a ratio such as 1:25 000, this means that 1 of any unit of measurement on the map corresponds exactly, or approximately, to 25 000 of that same unit on the ground. The scale statement may be taken as exact when the region mapped is small enough for the curvature of the Earth to be neglected, for example in a town planner's city map.
Large scale maps, say 1:25 000, cover relatively small regions in great detail and small scale maps, say 1:30 000 000, cover large regions such as countries, continents and the whole globe. The large/small terminology arose from the practice of writing scales as numerical fractions: 1/25 000 is larger than 1/30 000 000. There is no exact dividing line between large and small but 1/100 000 might well be considered as a medium scale. Examples of large scale maps are the 1:25 000 maps produced for walkers and hikers; on the other hand maps intended for motorists at 1:250 000 or 1:1 000 000 are small scale.
A comparison between 1:50 000 scale OS mapping on the left and 1:25 000 on the right.
It is important to recognize that even the most accurate maps sacrifice a certain amount of accuracy in scale to deliver a greater visual usefulness to its user. For example the width of roads and small streams are exaggerated when they are too narrow to be shown on the map at true scale; that is, on a printed map they would be narrower than could be perceived by the naked eye.
Sometimes the scale of a map is distorted deliberately. For example the famous London Underground Map shown here.
The basic geographical structure is respected but the tube lines (and the River Thames) are smoothed to clarify the relationships between stations. Near the centre of the map, stations are spaced out more than near the edges of map.
Further inaccuracies may be deliberate. For example cartographers may simply omit or remove features solely in order to enhance the clarity of the map. For example, a road map may or may not show railway lines, smaller waterways or other prominent non-road objects, and even if it does, it may show them less clearly (e.g. dashed or dotted lines/outlines) than the roads. This is known as de-cluttering.
Projection on World Maps
Maps of the world or large areas are often either 'political' or 'physical'. The most important purpose of the political map is to show state or country borders; the purpose of the physical is to show features of geography such as mountains, soil type or land use.
Geological maps show not only the physical surface, but characteristics of the underlying rock, fault lines, and subsurface structures.
Maps that depict the surface of the Earth also use a projection, a way of translating the three-dimensional real surface of the globe to a two-dimensional picture. Perhaps the best known world map projection is the Mercator projection which was originally designed as a form of nautical chart. This projection increasingly inflates the sizes of regions according to their distance from the equator which results in a representation of Greenland being larger than Africa, whereas Africa is actually 14 times larger. On Peters's projection areas of equal size on the globe are also equally sized on the map.
Airplane pilots use aeronautical charts based on a Lambert conformal conic projection, in which a cone is laid over the section of the earth to be mapped. The cone intersects the sphere (the earth) at one or two parallels which are chosen as standard lines. This allows the pilots to plot a great-circle route approximation on a flat, two-dimensional chart.
This refers to the measurement of underwater depth and in a maps case usually those of the oceans, in simplistic terms it can be considered the reverse of relief. It is not generally known but you could lose Everest in some of the oceanic trenches.
An idea of scale
|1:60 000 000
||This is the World on a small map, not a lot of detail but shows countries.
|1:30 000 000
||The normal scale for medium sized World wall maps which show large cities and geographical features.
|1:20 000 000
||Usually the scale used for the largest of World wall maps.
|1:1 000 000
||Ideal scale for a reasonably sized planning map of a single country.
||These maps will often include most of a country on one sheet and can be used for such things as an overview of a touring holiday. They are also often used as wall maps.
||Ideal for driving medium distance with minor roads still being shown.
||Ideal for cycling and small road / lane driving.
||Can be used for walking but more generally cycling and car navigation on minor country lanes.
||Usually used for walking and hiking maps as the level of detail is very good.
||Highly detailed and primarily used for city street maps.