Geographic Coordinates & Building Maps

(from the Discover Life website)

• Where are you?

  An important part of natural history research is to accurately record the position of your observations. Here we help you learn how to use latitude and longitude coordinates. This coordinate system is widely used to reference any location on the face of the Earth. You should know that

  • Latitude and longitude are measured in degrees (¡), minutes (') and seconds (").
  • There are 60 seconds in a minute, and 60 minutes in a degree.
  • Latitude and longitude use a grid system of lines based on the cardinal directions: North, South, East & West. Because magnetic north moves over time, you need to calibrate compass readings slightly before using them with grid systems on maps.
  • Latitude measures position north or south of the Equator, an imaginary line running around the Earth that has a latitude of 0 degrees.
  • Latitute values range from 0 degrees to 90, either north or south.
  • The North Pole is latitude 90 degrees north; the South Pole, 90 degrees south.
  • Longitude measures position east and west of an imaginary line called the Prime Meridian that runs through the Royal Observatory in Greenwich, a metropolitan borough of London, England, located at 51¡ 28' 38" (or 51.477222¡) of latitude north of the Equator (see map). The Prime Meridian has longitude 0 degrees.
  • The International Date Line is the on the opposite side of the world to Greenwich. Because there are 360¡ in a circle, the International Date Line is both longitude 180¡ east and 180¡ west of Greenwich.
  • Many computer programs require that latitude and longitude be expressed as in decimal values of degress rather than minutes and seconds. Use the following formula to convert from degrees, minutes & seconds to degress in decimal format:
    degrees.((minutes+(seconds/60))/60)
    They also require that southern latitudes and western longitudes are expressed as negative values. Thus, the South Pole becomes latitude -90¡ rather than 90¡S or 90¡ South.
• How to get your latitude & longitude
  • You can use a Global Positioning System (GPS) to measure your latitute and longitude. GPS read signals from satellites to calculate their position. Depending on the model, these systems can be very accurate. Some systems can give latitude and longitude a resolutions under a meter.
  • If you don't have access to a GPS, then calculate latitude and longitude from maps. To get the approximate latitude and longitude of a place in the USA, go to TopoZone.com, which allows you to display topographic maps of your area. Use the arrows on the edges of the maps to navigate so that your site of interest is in the displayed map's center. At the bottom of the map, then select DD.DDDD, to display the latitude and longitude in decimal format, and finally "Print this map." The latitude and longitude of the center of the map will appear at the top of the next screen.

  John Pickering of Discover Life
  12 August, 2001