A large scale map can show a relatively high level of detail, whereas a small scale map contains more information in relation to its size. The scale of a map is expressed as the ratio between the size of the map and the size of the corresponding area on the ground. This can be expressed as a simple ratio, for example 1:100, where one unit on a map is equal to 100 of the same units on the ground. Alternatively, the ratio may be expressed using different units, for example 1cm: 1km, in which one centimeter on the map represents one kilometer on the ground. Many maps also show scale in the form of a scale bar which can be used to calculate any distance on the map. The scale dictates the level of detail shown by the map. Because maps are smaller than the areas they represent, symbols are used to indicate the features on the ground. Many symbols do not resemble the things the things they represent and so they are often explained in a key.
Lines of latitude and longitude form a grid from which the position of any point on the Earth’s surface can be measured. An obvious reference point is the equator and latitude is the angle of a place north or south of it as measured from the center of the Earth. Lines of latitude form horizontal rings around the Earth parallel with the equator. They become shorter as they get closer to the poles. Lines of latitude divide up the Earth through its poles, like the segments of an orange. Each line is a great circle going right round the Earth. Lines of longitude have no obvious reference point such as the equator, so English navigators used the longitude or their home port, Greenwich, on the Thames near London. Longitude is still measured east or west of the Greenwich meridian. Early navigators used the stars to find out their position. Latitude was calculated from the heights of the stars in the sky or the position of the Sun as it rose, set or reached its zenith. Longitude is also measured against the stars but it is necessary to know the exact time because the stars move east to west across the sky. It was only after the perfection of the marine chronometer by John Harrison in the mid 18th century that accurate longitude measurements, and hence accurate map making, become possible.