GPS, Map and Google Earth

To better understanding GPS unit operation and capabilities, it may be helpful to review the basic terms and concepts briefly explained below:

• 2D Operating: ModeA two-dimensional GPS position fix that includes only horizontal coordinates (no GPS elevation). It requires a minimum of three visible satellites.
• 3D Operating Mode: A three-dimensional GPS position fix that includes horizontal coordinates, plus elevation. It requires a minimum of four visible satellites.
• Almanac Data : satellite constellation information (including location and health of satellites) that is transmitted to your receiver from every GPS satellite.
  Almanac data must be acquired before GPS navigation can begin.
• Altimeter:An instrument for determining elevation, especially an aneroid barometer used in aircraft that senses pressure changes accompanying changes in altitude. The Garmin® eTrex® Vista and Summit models contain a basic GPS with a built-in barometric altimeter.
• Bearing : the compass direction from your position to a destination, bearing also call an azimuth
• Crosstrack Error (XTE): the distance you are off a desired course in either direction.
• Datum: A math model which depicts a part of the surface of the earth. Latitude and longitude lines on a paper map are referenced to a specific map datum. The map datum selected on a GPS receiver needs to match the datum listed on the corresponding paper map in order for position readings to match.
• Desired Track (DTK): the compass course between the "from" and "to" waypoints.
• Differential GPS (DGPS): An extention of the GPS system that uses land-based radio beacons to transmit position corrections to GPS receivers.
• Dilution Of Precision (DOP) A measure of the GPS receiver/satellite geometry. A low DOP value indicates better relative geometry and higher corresponding accuracy. The DOP indicators are GDOP (geometric DOP), PDOP (position DOP), HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (time clock offset).
• Estimated Position Error (EPE): A measurement of horizontal position error in feet or meters based upon a variety of factors including DOP and satellite signal quality.
• Estimated Time of Arrival (ETA): The time of day of your arrival at a destination.
• Estimated Time Enroute (ETE): The time left to your destination based on your present speed over ground.
• Geocaching: A high-tech version of hide-and-seek. Geocachers seek out hidden treasures utilizing GPS coordinates posted on the Internet by those hiding the cache.
• GoTo: A route consisting of one leg, with your present position being the start of the route and a single defined waypoint as the destination.
• Grid: A coordinate system that projects the earth on flat surface, using square zones for position measurements. UTM/UPS and Maidenhead formats are grid systems.
• Ground Speed: The velocity you are travelling relative to a ground position.
• Latitude: A north/south measurement of position perpendicular to the earth's polar axis.
• Longitude: An east/west measurement of position in relation to the Prime Meridian, which is an imaginary cycle that passes through the north and south poles.
• Navigation: the process of travelling from one place to another and knowing where you are in relation to your desired course.
• Point of Interest: A specific point location that someone may find useful or interesting
• Position: An exact, unique location based on geographic coordinate system.
• SiRFstarIII: the latest high sensitivity GPS microcontroller chip manufactured by SiRF Technology. The chip supports 20 parallel channels, meaning that the chip can listen to, or search for, the signals from 20 different GPS satellites simultaneously.
• Speed Over Ground (SOG): The actual speed the GPS unit is moving over the ground. This may differ from airspeed or nautical speed due to such things as head winds or sea conditions. For example, a plane that is going 120 knots into a 10-knot head wind will have a SOG of 110 knots.
• Track (TRK): The direction of movement relative to a ground position.
• Track Log: a features in GPS unit that allow the user to create a log (record) of the travels, which can save for later use.
• Universal Traverse Mercator (UTM): A grid coordinate system that project global sections onto a flat surface to measure position in specific zones.
• Velocity Made Good (VMG): The speed you are travelling in the direction of destination.
• Waypoint (WPT): A geographical location which created by marking current location.
• WAAS: Wide Area Augmentation System, a service to improve the overall integrity od the GPS signal for user in North America.
• WGS-84 World Geodetic System, 1984: The primary map datum used by GPS. Secondary datums are computed as differences from the WGS 84 standard.

Note: most of the definitions above used for Garmin GPS units.