The Earth is divided into a grid of circular segments which are perpendicular to one another, called latitude and longitude.
Latitude lines run horizontally, and are parallel to the equator. Degrees latitude are numbered from 0° to 90° north and south. Zero degrees (0°) is the equator, 90° north is the North Pole, and 90° south is the South Pole. Latitude is commonly the first number expressed in a lat/long coordinate and is often expressed in the form of degrees, minutes, and seconds, there are 60 minutes in a degree and 60 seconds in a minute for instance: 38°47’30″N. (38 degrees 47minutes 30 seconds, north). More on this below.
Longitude lines (also called meridians) run perpendicular to latitude lines. Their spacing is widest at the equator, and converges at the Poles. The prime meridian or Greenwich Meridian (0° longitude) runs through Greenwich, England. Half way around the Earth, the degrees meet (180° east and west) in the Pacific Ocean, just west of the Midway Islands, and just East of the Fiji Islands and New Zealand. Longitude is commonly the second number expressed in a lat/long coordinate, and is often expressed in the form of degrees, minutes, and seconds.
Degrees are often divided into minutes (‘) and seconds (“). Each degree has 60 minutes and each minute has 60 seconds. Seconds can be divided further in tenths, hundredths, etc. for greater and greater precision. An example of using lat/long to describe a specific point is that the National Geographic Society in Washington, DC is located at 38°54’19” N, 77°02’14” W (38 degrees, 54 minutes, 19 seconds north of the equator, and 77 degrees 2 minutes, 14 seconds west of the prime meridian).
How to Measure Distance on the Map
The scale in the map legend provides the means for measuring distance. On USGS (United States Geological Survey) maps, the scale is given as a fraction, such as 1:24,000 or 1:100,000, and as a bar scale, which is a “ruler” divided into miles and kilometers. On a map scaled at 1:24,000, one inch on the map equals 24,000 inches (or 2,000 feet) on the ground.
True North vs. Magnetic North
Most maps are oriented to true north, meaning they are oriented along true north and south meridians of longitude. The top of the map usually points to the North Pole. The Earth’s magnetic pole is not located at the true or geographic pole. The magnetic North Pole lies south of the true North Pole, causing an error in compass readings. The angle that the magnetic needle points away from true north is called declination. What you see below is a declination diagram and it tells the difference between true north and magnetic north. In the case of this map, magnetic north is 11.5 degrees to the west of true north at this map’s location. On most modern maps North is up which means East is to the right, West is to the left and South is down.
What is a Topographic Map?
Topographic maps use contour lines to show changes in terrain and elevation, and are often overlaid with a wide variety of information, such as roads and streets, trails, land-use boundaries, tree and vegetation cover, and camping and hiking information to show how the lay of the land interacts with other natural and human-made features. Topographic maps are used for outdoor activities, engineering, energy exploration, natural resource conservation, environmental management, public works design, and commercial and residential planning.
Topographic Map Basics
It is found in the top right hand corner of the map:
Topographic maps use contour lines to portray the shape and elevation of the land. Contour lines are the curved, usually brown lines that connect points of equal elevation and make it possible for a topographic map to represent three-dimensional shapes on a two-dimensional surface. The space between the contour lines represents a set distance, called the contour interval. If the contour interval is 80 feet, for example, the vertical distance between two adjacent contour lines is 80 feet. Contour lines closer together on the map represent steeper terrain and lines farther apart represent flatter terrain. The elevation, in feet or meters, is written on darker or thicker contour lines, known as index contour lines. The contour interval can usually be found near the scale, in the map legend.
When contour lines cross a river they will make a V shape with the narrow part of the V pointing up stream
By using symbols, lines and colors, topographic maps illustrate both natural and human-made features. In order to read a map, it is important to understand what these symbols, lines and colors represent.