Understanding GPS Coordinates

What are datums ,waypoints, tracks, and routes?

Geocaching relies on using a GPS receiver. You need to know how to read and understand the information on the receiver in order to be successful in finding a geocache. This section will:

• Discuss coordinate systems
• Describe what datums are and why they’re important
• Explain waypoints, tracks, datums and routes.
• How to set and navigate to waypoints.
• Initialize a GPS receiver (checklist).

Coordinate System
A coordinate system is a way to locate places on a map. An imaginary grid laid over a map is a typical coordinate system. A number of coordinate systems are in place. For GPS receivers and geocaching you need to know of only two systems: Latitude and Longitude and UTM. When looking up a site on the internet or if someone gives you coordinates, it will be in one or both of these coordinate systems.

Imagine a Grid
For example, if you look at a map in the phone book you will see a grid with a vertical row of letters on one side of a map, and a horizontal row of numbers on the bottom of the map. Street locations are given in a combination of letters and numbers. For example, let’s say you want to find Pushkin St. You look up Pushkin St in the street index and it gives you a coordinate of C10. You would put your finger on the letter C and move it along the grid line until it lined up with the number 10 on the horizontal grid. This would put you in the approximate area of Pushkin Street. You could then look in this area until you found Pushkin St.

Latitude and Longitude

If you wanted to be able to find a cache more accurately, you would use another grid system, latitude and longitude. Latitude and longitude are based on math, but it’s not hard. Angles are measured in degrees and theyre used to measure circles and spheres. Spheres can be divided into 360 degrees; because the Earth is a sphere so it can also be measured in degrees. This is the basis of latitude and longitude, a grid that is wrapped around a sphere.

What is a datum?

No, it isn’t going out with some cute boy. A datum is a standard frame of reference for maps. All maps have a datum

• The default datum for GPS receivers is WGS 84 more formally known as the World Geodetic System 1984. WGS 84 was adopted as a world standard and is derived from a datum called the North American Datum of 1983 or NAD 83.
• You can set which datum that your GPS receiver uses.

Why is a datum important?
Suppose that the coordinates of a geocache use the WGS 84 datum but your GPS is set to use the NAD 27 datum. When you go looking for that cache the location could be off by as much as 200 meters! There’s a good chance that you will never find the cache because you’re looking in the wrong place. The moral of the story is to make usre that the daum on your GPS receiver matches the datum associated with the geocache coordinates. The good news is that geocache coordinates almost always tell you the datum used, so you won’t end up getting confused.

What is a waypoint?
A waypoint is GPS lingo for a location or point that you stor in your GPS receiver. A waypoint consists of the following information.

• Location:
  • The location of the waypoint in whatever coordinate system the GPS receiver is currently using. Some receivers also store the elevation of the location.
• Name:
  • The name of the waypoint. Youget to choose your own name, but the length varies between GPS receiver models.
• Date and time:
  • The date and time when the waypoint was created.
• Optional Icon or symbol:
  • An optional icon or symbol associated with the waypoint that appears on the GPS receiver’s map page. It could be a tent for a campground or a fish for a favorite fishing spot.
    
  All GPS receivers can store waypoints, but the maximum number that you can save varies from model to model. As a general rule, as the price of a GPS receiver goes up, so does the number of waypoints that it can store.

Why are waypoints important?
Waypoints are important for geocaching because you create a waypoint on your GPS receiver for each geocache you’re looking for. After a waypoint is set, your GPS receiver has several features that help guide you to that waypoint. This will be covered in more detail in the geocache hunting section.

What is a route?
A route is a course that you’re currently traveling or plan to take. In GPS terms, a route is the straight –line course between one or more waypoints. If multiple waypoints are ina route, the course beteen two waypoints is called a leg. A single route can be made up of a number of legs.
After you create your route, the GPS receiver tells you how log each of the legs will be and also the total distance of the route. When you activate the (tell the GPS receiver your’re ready to use it), the following information is displayed:

• The direction you need to travel in order to reach the next waypoint in the route
• How far away the next waypoint is
• How much time it’s going to take to get there

After you reach a waypoint in the route, the GPS receiver automatically starts calculating the information for the next leg. This continues until you reach your final destination.

What are tracks?
The GPS receiver saves the coordinates of the current position to memory. This series of tracks is a track log or track history. Similar to waypoints, but there are two differences.

1. Names and symbols
   Although tracks and waypoints are both location data points, track doesn’t have names or symbols associated with them and can’t be edited in the GPS receiver.
2. Autocreation
   Unlike waypoints—which you need to manually enter—tracks are automatically created whenever a GPS receiver is turned on and the Track feature is enabled. When you reach your final destination, your GPS receiver can optionally use the track log to help you navigate back to your starting point by using the track data to guide you in retracing your steps. Check your user manual for model specific instructions on how to do this.