Background: The Properties of a Map
 
All maps must contain three basic elements to be useful:
  1. Location
  2. Distance calibration and
  3. Direction calibration
Locations should provide two pieces of information. These are the portion of the Earth's surface covered by the map and a means of locating any point within the map area. The first is generally done with an insert image showing the map position relative to some larger feature, Earth, continent, etc. What it is relative to depends on the size of the region covered by the map.
A map must show direction so that one can use it to orient oneself with a compass, measured relative to geographic north. Unfortunately, a compass points to magnetic north not true north. This point is defined by the Earth's magnetic field. Magnetic declination is the difference between the two norths and should be indicated on a map. It varies with position on Earth (both in direction, e.g., east or west, and magnitude). It also varies with time.
Distance indicates how large an area of the Earth's surface a map covers, done by using a scale. Three different types of scales are used. Verbal scale is described by words, e.g., one inch equals a mile. It is convenient but lacks precision. A graphical or bar scale uses a bar to denote a certain distance on the ground. It is more accurate. The bar can be divided into fractions of measurement units. The most accurate type of scale representation is a proportional scale. Indicated by a ratio or proportion, e.g., 1:10,000. This scale indicates that one unit on the map equals 10,000 units on the ground. This works very well for metric systems but is somewhat confusing with the English system of measurements. 1:63,360 is one inch to one mile. Sometimes this is simplified to 1:62,500, which is slightly less than an inch to a mile.
  

 

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