Ebook Hacking GPS: Part 2

GPS Data chapter This chapter is all about data. As you walk, drive, sail, or otherwise move around with your GPS, you are gathering a great deal of data. In this chapter, you will learn what you can do with this data while in the field and when you get back to base.You’ll see how to create and edit your own waypoints and routes and how to upload these to your GPS.Then, you’ll learn how you can download, modify, and upload data that you captured while using the GPS.You’ll also look at various appli-cations into which you can export your data for managing it and storage. This chapter also describes how you can add GPS information to digital photographs, plot lightning strikes, and go wardriving. For any program-mers that might be reading this, we will also be looking at some websites and applications that might be of use to you if you’re interested in writing your own GPS applications. Finally, you’ll learn how you can create your own data for upload to your GPS. GPS Data Collection As you move around with your GPS, it is continuously gathering data and storing it so that you can access it later.The newer and more expensive the GPS, the more information it can hold. For example, the Garmin eTrex basic version can hold the following: in this chapter ˛GPS data collection ˛Working with data ˛Using EasyGPS ˛Using G7toWin ˛Creative uses of GPS data ˛Sharing waypoints n 500 waypoints n 1 route n 50 waypoints per route n 1,536 tracklog points At the top to the range, the Garmin Vista has greater capacity in all areas: n 1,000 waypoints n 20 routes n 125 waypoints per route n 10,000 tracklog points 166 Part III — Data Hacking Put simply, this means you can move around longer with a Garmin Vista than you can with a basic eTrex without overwriting existing data. Let’s take a look at the different types of information your GPS collects and stores. Position, Velocity, Time Position,Velocity,Time (PVT) data is at the core of GPS.These three data categories cover where you are, what time it is, and how fast you are moving in relation to your last known posi-tion.This information is what GPS is pretty much all about. Let’s take a look at the three parts of this data. Position Position data is information about where on the Earth’s surface the GPS actually is.You can think of this as a spot reading taken at a point in time. Position information consists of two parts: n Latitude: These are lines that form concentric circles around the globe.The equator is the longest line of latitude, and they shrink in size until they become a point at the north and south poles (see Figure 8-1).They are measured in degrees.The equator is 0°, the north pole is +90° and the south pole is -90°. n Longitude: Lines of longitude extend from the poles (see Figure 8-2).There are 360° in a full circle, but longitude is measured 0° to 180° east and 0° to 180° west, with 0° passing through Greenwich in London and 180° passing through the Pacific Ocean. N W Equator E S FIGURE 8-1: Lines of latitude Chapter 8 — GPS Data 167 N W E S FIGURE 8-2: Lines of longitude You can take a measurement of latitude and longitude (although the convention is to use longi-tude followed by latitude) and combine them to get a fixed point of the Earth’s surface.This is similar to the grid system used in games such as Battleship in which by specifying how many squares to move along and up, you get to the square in question. For example, 0° longitude, 0° latitude (written as 0°, 0°) is a point in the Atlantic Ocean off the coast of Africa, while 180° east (or west, for that matter) longitude, 0° latitude is still on the equator, but this time on the other side of the world completely, in the Pacific Ocean, off the coast of Fiji. Degrees are a good start to plotting coordinates, but they aren’t as precise as possible.What you now need to do is further divide the degrees into minutes. Each degree consists of 60 minutes: 12° 12’ N 04° 08’W In addition to minutes, you can add even greater precision by adding decimal parts of a minute: 12° 12.255’ N 04° 08.345’W Instead of using decimal minutes, you can also subdivide minutes into seconds (where one minute has 60 seconds). However, decimal minutes are the general coordinate format used with the WGS84 datum system that we are going to use here, and they are the default units shown on GPS receivers. No matter what coordinate format you choose, the actual location represented on the planet is the same. The numbers may be different, but the location is the same. For example, the follow-ing three sets of coordinates represent the same spot on the globe: N 38 deg 26 min 48.517 sec, W 76 deg 5 min 38.192 sec N 38 deg 26.80862 min, W 76 deg 5.63653 min -76.0939423, 38.4468104 168 Part III — Data Hacking This coordinate system gives us the ability to pinpoint locations. 51° 30.075 North, 0° 08.545 West is the location of Buckingham Palace in London, England. If you enter this into your GPS (see Figure 8-3), you will get information about how to get there. As you move, the GPS will plot your position in relation to where you want to go, so you always know exactly where you are. FIGURE 8-3: Coordinates entered into the GPS Velocity Velocity isn’t the same thing as speed. Speed is a measure of how fast you are going in any direction.Velocity is more specific than that.Velocity measures how fast you are going and in what direction you are going. In a GPS, this is normally computed as a track angle (the direc-tion of travel with respect to True North). A GPS solution for velocity enables it to calculate whether you are moving toward or away from a particular point, and from this information a number of calculations can be computed, such as the following: n Distance to waypoint n Information about whether you are on course or not n Real-time plotting on a map of your direction of travel n Estimated arrival time n Estimated journey time Different units display this information differently. Figure 8-4 is a screen capture from a Garmin eTrex Vista. Chapter 8 — GPS Data 169 FIGURE 8-4: Waypoint information on a Garmin eTrex How Does GPS Calculate Velocity? Many people wonder how the GPS can accurately determine velocity from the positional information it captures for a snapshot period of time. This is a good question. In fact, the way that GPS calculates your speed is very clever, yet at the same time quite simple. It remembers where you were the last time it locked your position and uses this information to calculate your speed. Most GPS receivers update your position information once a second. For example, if you moved 30 meters since the last update, it calculates your speed as 30 meters per second (see Figure 8-5). y 30 meters In 1 sec. x FIGURE 8-5: Speed calculated from distance moved Is Direction Calculated in the Same Way? Yes and no. Most GPS receivers use only the signal from the GPS to plot your direction of travel, which means that the only frame of reference it has with regard to your direction is where you were the last time it looked and where you are now. So in that respect, yes, it uses that information to calculate your direction (see Figure 8-6). ... - tailieumienphi.vn