- •Contents at a Glance
- •Contents
- •Hardware Hacks
- •GPS Secrets
- •Hidden Secrets
- •Garmin Secret Screens
- •Hard Resets
- •Soft Resets
- •Warm Resets
- •Full GPS Resets
- •Diagnostic Screens
- •Autolocating
- •Magellan Secret Screens
- •Magellan Meridian Series
- •After a Hard or Soft Reset
- •Summary
- •Cables Demystified
- •The Data Cable
- •Power Cords
- •Combo Cables
- •Combining Cable Types
- •Multi-GPS Cables
- •Multi-Data Cables
- •Multi-Data/Power Cables
- •Multi-Data/Power/GPS Cables
- •Making Your Own Data Cables
- •Materials You Will Need
- •Don’t Want to Buy a Connector?
- •Making Power Cords
- •Power Cord Assembly
- •Testing
- •Precautions
- •GPS/iPAQ Connections
- •Cradle Modification
- •Testing the Connection
- •Making Combo Cables
- •Making Multi Cables
- •Summary
- •Power Hacks
- •GPS Power Needs
- •Alkaline Batteries
- •Lithium Batteries
- •Rechargeable (NiMH) Batteries
- •Battery Do’s and Don’ts
- •Power Hacks
- •Carrying Your Own 12-Volt Power Supply
- •Battery Packs
- •A Different Kind of Battery Pack
- •Alternative Power Supplies
- •Summary
- •Antenna Hacks
- •The GPS Antenna
- •Quad-Helix Orientation
- •Patch Antenna Orientation
- •Best Performance Summary
- •External Antennas
- •Antenna Placement
- •Other Things to Avoid
- •Reradiating Antennas
- •Personal Reradiating Antenna
- •Communal Reradiating Antenna
- •Reradiating Antenna Considerations
- •Setting Up a Reradiating Antenna in a Car
- •Testing the System
- •Making the System Permanent
- •Carrying a GPS Signal via Cable
- •How Much Signal Do You Need?
- •Cable Losses
- •Connector Losses
- •Using a Signal Repeater
- •Building Your Own Mega GPS Antenna
- •Materials
- •Building the Antenna
- •Summary
- •Screen Damage
- •Screen Protectors
- •More Screen Armoring
- •Commercial Protection for GPS and PDAs
- •Mounting GPS
- •Car Mounting
- •Mounting a GPS for Biking, Hiking, and Skiing
- •Making a Personalized Case
- •Summary
- •Software Hacks
- •Hacking the Firmware
- •Firmware
- •Updating Warnings
- •Updating the Firmware
- •Hacking GPS Firmware
- •Bypassing the Garmin eTrex Vista Startup Screen
- •Bypassing the Garmin eTrex Legend Startup Screen
- •Bypassing the Garmin eTrex Venture Startup Screen
- •MeMap Personalization
- •Manual Firmware Editing
- •Magellan GPS Firmware Modifications
- •Recovering from a Failed Firmware Load
- •Garmin
- •Magellan
- •Summary
- •Connection Types
- •Which Connection Is Best?
- •Troubleshooting Problems
- •PC Connection Trouble
- •General PDA Connection Trouble
- •General Bluetooth Connection Trouble
- •Software-Specific Issues
- •Erratic Mouse Pointer after Connecting a GPS
- •Windows XP Problem: Microsoft Ball Point
- •Microsoft MapPoint Troubleshooting
- •USB-to-Serial Converters
- •Summary
- •GPS Data Collection
- •Position, Velocity, Time
- •Waypoints
- •Working with the Data
- •EasyGPS
- •G7toWin
- •Creative Uses of GPS Data
- •Sharing Waypoints
- •Adding GPS Information to Digital Photos
- •Lightning Detector and Plotter
- •Wardriving
- •GPS in Programming
- •Summary
- •Examining the Data
- •NMEA
- •NMEA Sentences
- •NMEA Sentence Structure
- •A Closer Look at NMEA Sentences
- •Examining NMEA Sentences
- •NMEA Checksum
- •SiRF
- •Using NMEA Sentences
- •GPS NMEA LOG
- •GPS Diagnostic
- •RECSIM III
- •Using NMEA
- •GpsGate
- •Recording Actual NMEA Sentences with GpsGate
- •Recording Simulated NMEA Using GpsGate
- •Data Playback
- •Why Bother with NMEA?
- •Ensuring That Your GPS Works
- •Avoiding Data Corruption
- •Summary
- •More Data Tricks
- •Screenshots
- •G7toWin
- •G7toCE
- •Turning Your PC into a High-Precision Atomic Clock
- •Setting Up the Software
- •Setting Up the Hardware
- •Hooking Up Hardware to Software
- •Bringing a GPS Signal Indoors
- •Other Uses for GPS Data
- •Azimuth and Elevation Graphs
- •Surveying
- •Navigation
- •Signal Quality/SNR Window
- •NMEA Command Monitor
- •Experiment for Yourself
- •Summary
- •Playtime
- •Hacking Geocaching
- •GPS Accuracy
- •The Birth of Geocaching
- •Geocaching Made Simple
- •What Is Geocaching?
- •Geocaching from Beginning to End
- •The Final 20 Yards
- •Geocaching Hacks
- •Go Paper-free
- •Plan Before You Leave
- •Sort Out Cabling
- •Power for the Trip
- •Better Antennas
- •Protecting the GPS
- •Summary
- •GPS Games
- •The Dawn of GPS Games
- •Points of Confluence
- •Benchmarking/Trigpointing
- •GPS Drawing
- •Hide-and-Seek
- •Foxhunt
- •Other Games
- •Summary
- •GPS Primer
- •The GPS Network
- •How GPS Works
- •GPS Signal Errors
- •Summary
- •Glossary
- •Index
GPS Data
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 applications 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 programmers 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:
500 waypoints
1 route
50 waypoints per route
1,536 tracklog points
At the top to the range, the Garmin Vista has greater capacity in all areas:
1,000 waypoints
20 routes
125 waypoints per route
10,000 tracklog points
chapter
in this chapter
˛GPS data collection
˛Working with data
˛Using EasyGPS
˛Using G7toWin
˛Creative uses of GPS data
˛Sharing waypoints
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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 position. 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:
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°.
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
Equator
W E
S
FIGURE 8-1: Lines of latitude
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|
N |
|
W |
Greenwich |
E |
|
Meridian |
|
|
S |
|
FIGURE 8-2: Lines of longitude
You can take a measurement of latitude and longitude (although the convention is to use longitude 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 following 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
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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 direction 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:
Distance to waypoint
Information about whether you are on course or not
Real-time plotting on a map of your direction of travel
Estimated arrival time
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).
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y |
N |
x
FIGURE 8-6: Direction of travel calculated by comparing the last location to the current location
However, some newer, more expensive receivers contain an electronic compass that works just like any other compass, detecting the Earth’s magnetic field. This information can be combined with the data from the GPS satellites to provide you with even greater accuracy in plotting your direction of travel (see Figure 8-7).
FIGURE 8-7: Electronic compass in action
The lack of a compass is why some GPS receivers, depending on type, can’t actually tell you which way to go to get to a certain point unless you are moving. Without a compass, it can’t determine the receiver’s direction from only the GPS satellite signal.
This can be a real problem at times, and the pointer on some receivers will spin wildly when the unit is stationary. An electronic compass eliminates this behavior (see Figure 8-8).
Remember, however, that built-in digital compasses are subject to the same problems as standard compasses (such as being affected by close proximity to metal and other magnets).
Time
Thanks to the four atomic clocks on board each GPS satellite, you can be pretty confident of getting a good time signal from the satellites. Local time zone information can be inputted into the GPS (see Figure 8-8) along with daylight saving information (see Figure 8-9).