What Are GPS Coordinates?
From Coloradohikingtrails
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How Does A GPS Receiver Work?
A GPS receiver (GPSr) calculates it's position by by measuring the distance between itself and three or more GPS satellites. The technique is a variation on triangulation, the tried and true method of navigation used by navigators and surveyors for centuries. In the old days you took bearings (compass sightings) on existing known locations and triangulated these on a chart to compute a fix on your location. Once you have a compass bearing you can draw a line through the known location and you know you are somewhere on that line. Do the same thing to a second point and the two lines will intersect. This is your position. If you try a third point it should intersect at the same place the other two lines intersect.A GPS receiver uses a slightly different approach. It measures its distance from the satellites and uses this information to compute its location. To measures its distance to the satellite, it measures the length of time the signal takes to arrive at its location, and then based on knowing that the signal moves at the speed of light, it can compute the distance based on the travel time. However, unlike the known location of the olden days, these sites are moving. The solution to this problem is to have the satellite itself send enough information to calculate its current location relative to your receiver. Now, armed with the satellite location and the distance from the satellite, we can expect that the GPRs is somewhere on a sphere that is described by the radius (distance) and centered at the satellite location. By acquiring the same information from a second satellite, it can compute a second sphere that cuts the first one at a plane. If it acquires the same information from a third satellite, the new sphere would intersect the circle at only two points. If we know approximately where we are (which we do - a reasonable expectation of the GPSr's current location is provided by the "Almanac" data downloaded from the satellites), we can discard one of those points and we are left with our exact location in 3-dimensional space. If it acquires the information from a fourth satellite, it would show us to be at exactly (or almost exactly) the same point we just computed above. Most newer-model GPS receivers can download data from up to 12 satellites simultaneously, using these data to improve the accuracy of the location fix.
By continually downloading data from the satellites, the GPSr can continuously recalculate its location (i.e. "maintain a fix"). Once it has a number of fixes, it can derive much more information than just location data. For example, a GPSr can compute the travel direction (compass heading) by comparing current location to previous location. Similarly it can keep track of travel distance, compute speed, record travel time and other valuable information.
Therefore the most obvious use of a GPSr is to tell you where you are. But if you can tell the GPS where you want to go (by providing it with the coordinates of the destination), you can use it to navigate to the destination, or to tell others, in a standardized format, the location of your destination (or of any other spot on Earth). Along the way, you have the GPSr "remember" these coordinates as waypoints, in order to document memorable photography locations, fishing holes, trail intersections and other sites in order to easily find them in the future. These are the uses of GPS Coordinates.
What Are GPS Coordinates?
GPS Coordinates are a pair of numbers called Latitude and Longitude that make up two of the three parts of a Geographic Coordinate System. The third part is elevation, or altitude. With a Latitude coordinate and a Longitude coordinate, you can precisely locate any position on the Earth. By adding altitude, you can specify any location on, in or above the Earth. But since we're only planning on hiking on the Earth's surface, for purposes of this discussion altitude doesn't need to be taken into account.
So What Are Latitude And Longitude?
The Earth can be thought of as a sphere, not unlike an orange. Imagine slicing the earth in half along the equator, so it is divided into two halves, the half above the equator, also known as the Northern Hemisphere, and the half below the equator, or the Southern Hemisphere. The surface where we cut it (using the orange analogy, the flat surface through the middle of the orange where we cut it) would be a "plane" running through the equator. Latitude is the angle between any point on the earth's surface and the plane of the equator (which is the surface where we just cut it in half). Each pole is 90 degrees: the north pole 90° N, or +90°; the south pole is 90° S, or -90°. The 0° parallel of latitude is designated the equator, the same plane that divides the globe into the Northern and Southern Hemispheres. Therefore any point on the earth half way between the equator and the north pole would be at 45°N, and any point half way between the equator and the south pole would be at 45°S, or -45°. So a latitude will define a line (technically a plane) running around the earth parallel to the equator, but it won't give us a point on the earth.
In the example above, we sliced the earth horizontally, along the equator, dividing the earth into north and south. If we were to slice it from the north pole to the south pole (or think of it as from top to bottom), we would divide the earth into an eastern hemisphere and a western hemisphere. Longitude is the same as latitude, except it is the angle east or west, between any point on the earth's surface and the plane of an arbitrary north-south line between the two geographical poles. The equator defines the zero-latitude reference line. The line passing through the Royal Observatory in Greenwich (near London in the UK) is the zero-longitude reference line, called the Prime Meridian. Longitudes east of the prime meridian are defined in degrees "E", and west of the Prime Meridian as degrees "W", or -degrees.
So a Longitude will define a line (technically a plane) running around the earth parallel to the north-south plane we discussed above, but it won't give us a point on the earth. But by combining Latitude and Longitude, the horizontal position of any location on Earth can be specified. The point defined by Latitude and Longitude is the point where the Latitude plane and the Longitude plane intersect.
For example, Fort Collins, Colorado has a latitude of 40.58° North, and a longitude of 105.08° West, or -105.08. (You can view these coordinates with Google Maps at http://maps.google.com/maps?ll=40.58,-105.08&spn=0.3,0.3&q=40.58,-105.08) This means that Fort Collins is 40.58° north of the equator, and 105.08° west of Greenwich, England (the Prime Meridian). In other words, a vector drawn from the center of the earth to a point 40.58° north of the equator and 105.08° west of Greenwich will pass through Fort Collins.
Traditionally, degrees have been divided into minutes (1/60th of a degree, designated by ′ or "m") and seconds (1/60th of a minute, designated by ″ or "s"). There are several formats for degrees, all of them appearing in the same Latitude-Longitude order:
- DMS Degree:Minute:Second (40°34'48"-105d4m48s)
- DM Degree:Minute (40°34.8'-105°4.8m)
- DD Decimal Degree (40.5800°-105.0800d)
To convert from DM or DMS to DD, decimal degrees = whole number of degrees, plus minutes divided by 60, plus seconds divided by 3600. For those of us that are mathematically challenged, there are web sites available, like this one here: http://www.gpsvisualizer.com/calculators, that have free Latitude and Longitude converters. DMS is the most common format, and is standard on all charts and maps, as well as global positioning systems and geographic information systems. NOTE: This web site uses the Decimal Degree format, as this is the format required by Google Maps and Google Earth.
In today's digital age, we often think of coordinates in conjunction with GPS receivers or Google Earth. But Latitude and Longitude were used with maps long before these modern-day tools were invented, and many avid outdoorsmen (and women) shun the use of GPS receivers and prefer maps - because maps still work if dropped, and their batteries never go dead.
How Do I Use These GPS Coordinates (Latitude & Longitude)?
On this web site, these latitude and longitude coordinates are used to generate the Google "Trail Locator" Map on the site's Main Page, and to generate the location data for the map information on each of this site's hiking trail descriptions.
With a GPS Receiver, these coordinates can be used for many purposes. Although the primary purpose is navigation, many uses are just plain fun! For more details about how these coordinates can be used with a GPS receiver, see the FAQ article "What Can I Do With A GPS Receiver?".
