All about GPS receivers, Coordinates and Maps
How the GPS reciever works, the features, how to choose one, satellites, coordinate formats and more!
Basic
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Mapping Examples: Garmin eTrex 20, Garmin GPSmap60, Magellan GC
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Paperless *Only available to Premium Members Examples: Garmin Montana, Oregon, Garmin GPSMap 62s, Magellan eXplorist 710
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Smart Phone Examples: iPhone, HTC
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Car Sat Nav Examples: TomTom and Garmin Nuvi
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| Connecting to a PC: To transfer data to your GPS from your PC you need to connect it. The basic models (Garmin eTrex H and Geko) have a serial connection which needs a special cable - not usually supplied with the GPS. The other Garmin and Magellan units all use a simple USB connection. With a suitable program you can enter waypoint and geocache information on your PC and transfer it simply and quickly to the GPS. If you are a premium member with a Garmin Oregon/Dakota or the newer GPSmap62 series you can transfer the data directly to the GPS in one simple operation - upto 5000 caches at a time! | ![]() Serial cable used on the eTrex and eTrex H and Gecko |
![]() USB connection used on most other GPS receivers |
Coordinates and Map DatumsLatitude and longitude have been around a long time (since people first started to sail the oceans basically) and refers to the lines going from pole to pole (longitude) and round the equator going up to the north pole and down to the south pole (latitude). Latitude is measured from the Greenwich Meridian which is 0 degrees east and west and Longitude from the Equator which is 0 degrees north and south. There are several different formats to use and different countries produced their own version including the United Kingdom with the Ordnance Survey British Grid. To get some standardisation various countries got together to produce the map datum World Geodetic Survey of 1984 (WGS84) which is currently still valid. Have a look at this wiki article which explains why we have and use this standard.So based on WGS84 we use Lat and Long which can be written in several different ways. Why? Because we can basically and you can use whichever you want! To give you an example, this is the junction of the M4 and M5 motorways near Bristol in the various formats: British Grid ST 61785 83780 They all get you to the same point but for consistency its best to just use one. Geocaching use Degrees and Decimal minutes on the WGS84 map datum and this is what your GPS should be set to. See the screenshot below from an Oregon GPS. So if you are talking geocaching to a fellow geocacher wherever they are in the World they'll understand where you mean because we all use the same coordinate notation. If you want to use your GPS with the position format of British Grid (Ordnance Survey) then make sure you change the Map Datum to Ordnance Survey. Similarly if you change from Ordnance Survey back to Lat/Long HDDD° MM.MMM then check the map datum is WGS84. If you end up with the coordinates on one format and the map datum on the other you'll find your GPS is incorrect by about 300 feet (100 meters). This is the settings screen from an Oregon GPS. From the settings menu, open the Position Format menu then you can change the Position Format and Map Datums. Screenshots of both settings (Lat/Long Degrees and Decimal minutes and the British Grid Ordnance Survey) are shown. |
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Coordinate Conversion web sites |
Satellites and AccuracyThe newer receivers in the GPS (SiRFstarIII for example ) have more channels which allows them to process signals from multiple satellites and so get a faster 'lock'. When you switch it on it starts to download information from the satellites it can 'see'. It will remember their locations for the next time you switch it on and get a lock much faster. If you've not used it for some time or have traveled a long distance then it will have to download this information again which may take a few minutes.Look at this display from a Garmin Oregon. The satellites being received are shown in the circle in the middle of the display, the outer circle of the satellite display is the horizon, the inner circle is 45 degrees above the horizon (imagine it like a circle drawn round the sky half way between the horizon and directly overhead). The centre of the circle is directly overhead. Any satellites below the 45 degree circle and near the horizon circle may be difficult to receive if your horizon is obscured by trees, hills or buildings. The green bars below show the signal strength of each satellite, the numbers show which satellite is which. So in this display, satellite 3 is almost directly overhead, 19, 3 and 6 are quite high in the sky whilst 22 and 24 are about 45 degrees above the horizon. 8,18 and 16, are very low but still being received but 11, 7, 28, 15 and 21 are too low and the light gray colour shows they are not being received. It may seem odd that 22 and 8 have a similar signal strength but 8 is much lower in the sky. Many things affect the signal, including the way the GPS is being held, obstructions such as trees or buildings and so on. Satellite 37 is one of the WAAS/EGNOS satellites which corrects the normal signal to improve accuracy. You know the signal is being corrected by the letter D's that appear in the normal satellite bars (see more on WAAS/EGNOS later). Finally, with all these satellites being received the GPS is showing an accuracy of 9 feet. The figure 31 feet on the right of the display is the altitude above sea level. WAAS and EGNOS Wide Area Augmented System and European Geostationary Navigation Overlay Service. These provide a 'high resolution' signal that improves the accuracy of your position. Typically from around 20 to 30 feet to about 10 feet. You need to enable it from the GPS setup menu. It will not significantly affect the battery consumption. |
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Using a Smart phone for geocachingThe iPhone, Blackberry, Android and Windows 7 phones all have dedicated in-expensive applications you can buy to find geocaches and allow you to download cache information, navigate to caches with the built in GPS and compass and view maps all with the phone. Full details are on the Geocaching website.The geocaching applications are fine for finding caches but when you place a cache you need to record the coordinates accurately. The smart phone uses both the GPS and the mobile radio signal (triangulation) to determine its position. If you have a poor GPS signal you may not know how inaccurate it is. In the case of the iPhone you'll see the accuracy figure will never go below 56 feet. This is determined by the GPS chipset in the phone. When you open the geocaching application your current position is displayed on the main screen so you can use this to record the coordinates when placing a cache. Please be aware though that when first opened the application will NOT be accurate. Leave the phone with a clear view of the sky for a minimum of 5 minutes to ensure it gets a good satellite lock. When recording coordinates you should walk to the cache site from several directions and from about 30 to 50 feet (10 to 15 meters) away each time and then average the readings to get them as accurate as possible. A much better way is to use a dedicated GPS application like MotionX GPS which turns your iPhone into a handheld GPS receiver. It uses the GPS to record your location and warns you if the signal is weak and it is using the mobile signal. It will show you GPS accuracy and position, allows you to add waypoints and record tracks and enter waypoints to navigate too. The application is one of many but I've used this one and found it works very well (I've found caches with it!). The coordinates can be set to HDDD° MM.MMM. MotionX GPS is free. In all cases you need to wait for 5 minutes or so to ensure the iPhone GPS has a good signal. The MotionX application gives you a GPS signal strength and accuracy figure, the geocaching application only gives you an accuracy figure.
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1 Garmin Ordnance Survey GB Discoverer maps (GB and Ireland - ROI/NI), Garmin topo and street maps, free Open Street Maps |
Magellan GPS
Detailed review I wrote of the 710, 610, 510 focusing on using it for geocaching
Detailed review I wrote of the GC
1 Magellan 'City Series Europe' (includes UK). Detailed street level 2D or 2D view, turn by turn routing and Points of Interest database. 710 includes voice guidance. |
Garmin and Magellan information
| Garmin Montana Wiki | |
| Garmin Oregon Wiki | |
| Garmin Dakota Wiki | |
| Garmin eTrex Wiki (10/20/30) | |
| Garmin 62/78 Wiki |






















