Chapter 8 — GPS Data 171

FIGURE 8-8: Time zone information

FIGURE 8-9: Daylight saving information

Some GPS receivers, either when you buy them or when they have had exhausted batteries in them for some period of time, will ask you to input the time and date manually — this helps the receiver get a lock on the satellites, and the time will be corrected when the almanac has been downloaded.

Waypoints

A waypoint is a spot on the surface of the Earth as defined by coordinates that are inputted into the GPS and stored, usually along with an icon, a descriptive name, and some text.

There are two types of waypoint:

Manual waypoints

On-the-spot waypoints

172 Part III — Data Hacking

Manual Waypointing

A manual waypoint is one that you enter into the GPS for a location that you are not currently at. For example, before you leave home, you might enter the waypoint of the parking lot at your destination or perhaps the geocache that you are looking for (see Figure 8-10).

FIGURE 8-10: Manual waypointing

To enter a manual waypoint, you need to know the coordinates of the location to which you are heading. For this you need a map of some kind or some other repository of coordinates (a map, geocache listing, points of interest, etc.).

Be very careful when manually entering waypoint information into your GPS because a small numerical error can translate into a massive error on the ground! In addition, make sure that you’re using the appropriate datum for the coordinate system being used (for degrees/minutes/ seconds, degrees/minutes, and decimal degrees, use WGS84). Using the incorrect datum can cause inaccuracies of several hundred meters.

Coordinates form the fundamental aspect of navigating with a GPS other than using dead reckoning (where you follow a path and a bearing for a certain distance over a certain amount of time), and getting comfortable with using them is the first step to really using GPS properly.

On-the-Spot Waypointing

With on-the-spot waypointing, you travel to a place and waypoint that particular location, storing the waypoint in your GPS. This stores the location that is displayed by the GPS into the memory of the device. You can then navigate back to this spot later. For example, you could waypoint the location of your car before going off in search of a geocache so that you are sure to find it on the way back).

Routes

Routes are path data that is stored in the GPS. A route is a series of waypoints contained in the GPS that form a path of travel, similar to the connect-the-dots activity found in children’s activity and puzzle books.