Introduction to GPS technology

What is a GPS?

A Global Positioning System or GPS as it is commonly known is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the United States (US) Department of Defence. GPS was originally intended for military applications, but in the 1980s, the US Government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day.
How does a GPS work?
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. There are 24 satellites that make up the GPS space segment that are orbiting the earth at approximately 19,000 kilometres above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 11,000 kilometres per hour. GPS receivers take this information and use triangulation to calculate the user's exact location. The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map. A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance and distance to destination.
How accurate is a GPS?
Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. GPS receivers are accurate to within 15 meters on average. GPS receivers with WAAS (Wide Area Augmentation System) capability can improve accuracy to less than three meters on average. Differential GPS (DGPS) can correct GPS signals to within an average of three to five meters.
Recommended GPS settings
  • Datum = WGS84


  • Reading (positional) format = Degrees/Minutes/Seconds (DD'MM"SS.SS) or Decimal Degrees (DD.DDDDDD)


  • Projection = Geographic


  • Country = South Africa


  • Time Zone = Other


  • UTC offset = +02:00 (South African Time Zone)


  • Time format = 24 hour


  • Units = metric


  • North ref = true


    • Mapping coordinate information
    The innovation interactive mapping section of the BGIS website (http://bgis.sanbi.org) allows users to map their x, y point locality data. To upload coordinates, the data must be saved as .csv or .pnt file. The file can be up to 4 000 000 bytes in size.

    Unfortunately not all GPS receivers automatically save data in either .csv or .pnt formats. The most common format that GPS data is saved in is GPX. There are many sites that allow one to easily convert data from GPX to a .csv format. We suggest you use the following site to convert your data: http://www.gpsvisualizer.com/convert_input.



    To use this tool, simply browse for through your directory for your coordinate information. Be sure to select comma-separated value for your output, so that it can be easily read by Excel. Press the convert button. It will take a few minutes to process your data but eventually a results window will appear on your screen. From the results window, click on the link to download the converted .csv file to your desktop or simply cut and paste the data into Excel yourself.



    To upload data onto BGIS, coordinate information has to be in decimal degrees (DD) e.g. -33.931500° and 18.626694°. Don't forget to include the negative sign that denotes if the data is in the Southern Hemisphere or Western Hemisphere.
    There are many conversion tools available on the internet that allows users to convert data from degrees/minutes/seconds (DMS) to decimal degrees (DD). A nifty little calculator can be found on the following website: http://www.satsig.net/degrees-minutes-seconds-calculator.htm
    Steps for uploading coordinate data onto BGIS
    1. Click on the upload tool.
    2. Browse through your directory to where the file can be found.
    3. Select your file.
    4. Click on the "upload CSV/PNT" button.
    5. Select the fields that contains your x and y coordinates. Your x coordinates (longitude) denote whether the data is in the Eastern or Western Hemisphere, whilst the y coordinates (latitude) denote whether the data is in the Northern or Southern hemisphere.
    6. Give your layer a name and specify the symbol colour and type.
    7. Click on the submit tab.
    8. View your data
     
    		 
     
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