All about GPS receivers, Coordinates, Maps and Smart Phones
This page explains all about your GPS; how it works, the features on it, satellites and coordinates and how to decide which one to buy. You’ll also find information about the maps the GPS can use as well as using a smart phone for caching.
The basics – the GPS satellite system explained
The basics – different types of GPS available
The features of a handheld GPS for geocaching – receiver, compass, waypoints & more
The features of a smart phone for geocaching – receiver and compass
All about coordinates and map datums
How the GPS receiver picks up satellites; accuracy and WAAS and EGNOS explained (simply!)
What maps can you use on a GPS receiver? Can I create my own?
Maps on a Garmin GPS
Maps on a Magellan GPS
What maps can you use on a smart phone?
Which handheld GPS is which? How do I choose one for Geocaching? Comparison table of Garmin and Magellan
Garmin and Magellan information. Wikis and forums – a wealth of information, hints and tips from other users.
The Basics – the GPS satellite system
All GPS receivers whether they are car sat navs, handheld GPS or your smart phone need to pick up signals from orbiting satellites to work out where they are. Professional GPS’s (the sort used by surveyers for example) can also pick up land based radio signals to get much greater accuracy. This is called DGPS – Differential GPS and is not available on the sort of GPS’s geocachers use.
There are three GPS satellite systems in operation maintained by the United States, Russia and the European Space Agency (ESA). That provided by the US military is the one in common worldwide use and the popular Garmin and Magellan GPS receivers use this system. The Russian GLONASS system provides the same signals though not the same satellite coverage. Most of the newer Garmin handhelds can receive GLONASS. The European Gallileo system is currently being put into service but is not ready. When it is it should be compatible with US system so available to any GPS device.
The American GPS system is a network of more than 30 satellites that orbit the earth at an altitude of 20,200 Km (12,600 miles) which means each one takes about 6 hours to pass overhead from horizon to horizon. Your GPS receiver has 12 satellite channels allowing it to receive signals from 1 to 12 of these satellites. To get a good ‘position fix’ it needs to receive signals from a minimum of 4 satellites. The more satellites your GPS can pick up, the better the accuracy of your position. See the section below on Features of a GPS for more details about the GPS receiver.
The Russian GLONASS system doesn’t have the same number of satellites as the US system and offers the same degree of accuracy. Being able to pick up signals from both satellites won’t improve your accuracy but will ensure you are receiving a signal from as many as possible which means you’ll get a position fix quicker.
The Basics – different types of GPS available
There are 4 broad types of GPS available. With the exception of the car sat nav they all provide basic navigation functions allowing you to enter and save waypoints (caches for example), navigate to waypoints and create and navigate routes. They all have a compass allowing you to see which direction to go. The radio receiver that picks up the satellite signal varies between models so some are more sensitive than others which means they pick up the satellite signal faster and work better in poor signal areas (under trees for example). The number of waypoints and routes available and how the compass works differs between the types as does the amount of internal and external memory in which you store waypoints and maps.
Please note the GPS devices shown are just examples and not a complete list. For that please refer to the comparison table at the bottom of this page.
Basic Handheld GPS
These provide basic navigational functions and have a compass. They do not support maps. Limited number of waypoints and routes. Can be connected directly to a PC but may need a special serial cable (eTrex H). With the exception of the eTrex 10 these GPS’s are not ‘paperless’, that is you cannot download all the cache information to them other than basic cache GC number and coordinates.
Examples: Garmin eTrex and eTrex H, Garmin GPS60, Garmin eTrex 10
Garmin eTrex HGarmin eTrex 10
Mapping and Paperless Handheld GPS
Same navigation functions as the basic models but support maps allowing you to see in varying detail where you are. You can display routes and waypoints on the maps. They have the ability to download cache information so you don’t need to print out cache pages. The information is the same as you see on the web page, description, hints, logs and waypoints*. The paperless GPS receivers nearly all have greatly increased memory (external microSD card) allowing more maps to be stored and a considerable increase in waypoints (caches). They mostly have an electronic compass and usually an altimeter. Some models are ‘touch screen’ which makes entering data much easier.
PC connection is easy via a USB cable so you can transfer cache information from the web page quickly.
Examples: Garmin eTrex 2x series, 3x series, Magellan GC, Garmin GPSmap60, Garmin GPSMap 62x, Garmin Montana 6xx, Oregon 6xx, Garmin Oregon 5xx, Garmin Monterra, Magellan eXplorist 710 (also 610, 510, 310)
*Paperless features are only available to Premium Members.
eTrex Legend HCxMagellan GCGarmin GPSmap60Garmin GPSmap62
Garmin OregonMagellan eXplorist 710
Many phones have a built in GPS receiver and you can install software to let you use the GPS for geocaching. You may also be able to store/run maps as well. Generally they behave as a ‘handheld’ GPS receiver but there are limitations such as poor battery life and ruggedness.
Certain smart phones have specific apps available for geocaching. These are explained later on this page.
Examples: iPhone (IOS), HTC (Android)
Car Sat Nav
These are specifically designed for use in a vehicle. They can be used for geocaching but it is not always easy. For example a handheld GPS (or phone application) shows you a compass arrow to follow and a countdown of the distance to the waypoint. This may not be available on a sat nav. Battery life is another issue as you can only expect a few hours time outdoors.
Examples: TomTom and Garmin Nuvi
The features of a handheld GPS for geocaching
Receiver: The GPS unit has a radio receiver that picks up the satellite signals. The actual radio receiver is made by several manufacturers and the GPS makers will use whichever one they think best. Does it matter which is used? Not really as they all do the same job. The newer GPS models usually have a faster more sensitive receiver that can pick up weaker signals (for example under trees or in cities with tall buildings around) and get a position lock quicker. If you look in the detailed specification then any receiver quoted as ‘high sensitivity’ is fine. The number of satellites and approximate accuracy is shown on the satellite display screen. The example on the right is a Garmin Oregon 550.
Compass: All handheld GPS units have a compass. The basic models have a GPS compass that uses the satellite signal to indicate your direction. You have to be moving for the compass needle to point correctly. If you stop, the needle will ‘freeze’ and if you turn to the left or right while standing still so will the needle. More advanced units have an electronic/magnetic 2-axis compass which works like a normal compass and points correctly when you are moving or standing still. If you turn to the left or right the compass needle will continue to point in the correct direction. You have to hold the GPS horizontally for it to work correctly. More recent GPS units have a 3-axis electronic/magnetic compass which means it points correctly regardless of how you hold the GPS. The 3-axis compass is the better one to use. You should also set the compass to ‘bearing’ pointer which gives you an arrow showing which way to go to the waypoint you are navigating to.
Waypoints: A waypoint is a location that you tell your your GPS to ‘go to’. The waypoint has coordinates. Geocaches use waypoints to show where they are, a traditional cache will be at the waypoint coordinates, a multi or mystery may have lots of waypoints where you have to find information for example. You can enter these waypoints into your GPS manually but it is much easier to enter them from your PC directly from the cache page or with a pocket query*. When you have the waypoint in the GPS you select it and then tell the GPS to ‘go to’ it. The compass arrow points the way and you’ll get a distance countdown telling you how far you have to go. The non-paperless GPS units store all locations as waypoints including geocaches. The paperless GPS units can store both waypoints and geocaches. This way you can navigate to the cache or to a waypoint, for example the start of a footpath or a stage of the cache. Entering waypoints/geocache coordinates and names with a more basic GPS like the Garmin eTrex H and Geko is quite time consuming with the way the number/letter selection method works. With the mid range GPS units like the Garmin eTrex range and GPSmap60 or Magellan GC there is an easier to use on-screen number/letter selection. The more advanced units like the Garmin Oregon/Montana and Magellan eXplorist 510/610/710 have a touch screen which makes data entry much quicker and easier. In all cases you can transfer data to and from your GPS by connecting it to your PC.
*Pocket queries and paperless caching require Premium Membership.
More information about paperless caching and transferring caches and waypoints.
Altimeter: Many GPS’s have an altimeter to display your height above sea level. The more basic ones will use the GPS satellites to work this out whilst the more advance have a barometric altimeter. This type does need calibrating as it uses air pressure which varies day to day (and with altitude). An altimeter is not an essential feature for geocaching.
Compass arrow and ‘countdown’ distance shows where to go and how far
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. You can copy cache information from your browser to the GPS. If you are a premium member with a Garmin or Magellan paperless GPS then you can transfer the data directly to the GPS in one simple operation – up to 1000 caches at a time using a Pocket Query.
The Features of a Smart Phone for Geocaching
The iPhone and Android phones all have dedicated applications you can use for geocaching. These allow you to find caches, download cache information and log your finds online. The apps are available from the respestive online stores.
Smart phone GPS receivers are now quite sensitive and suitable for geocaching. Adding waypoints and navigating to them may be less easy than with a dedicated handheld GPS. Don’t forget your ‘smart’ phone is also your MP3 and video player, web browser, email application and much more. The compass and GPS are just additional applications. A handheld GPS on the other hand is designed for really only one job, navigating.
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 to determine its position (triangulation). If you have a poor GPS signal you may not know how inaccurate it is. This is determined by the GPS chipset in the phone. This article in the Help Center explains how to use the apps to find your location and display the coordinates. 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.
Coordinates and Map Datums
Latitude 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). Longitude is measured from the Greenwich Meridian which is 0 degrees east and west and Latitude 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 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
Decimal degrees 51.55160, -2.55255 (the – indicates West of Greenwich)
Degrees minutes and seconds N51°33’05.7″, W2°33’09.2″
Degrees and decimal minutes N51°33.096′, W2°33.153′
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.
Coordinate Conversion web sites
Geocaching.com have a simple to use coordinate converter
This site also supports the Ordnance Survey Grid references
This is a more comprehensive site
Satellites and Accuracy
The newer GPS models have more receiver 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 and 5 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 15 to 20 feet (5 to 7meters) to about 10 feet (3 meters). You may need to enable it from the GPS setup menu. It will not significantly affect the battery consumption.
What maps can you use on a GPS?
Both Garmin and Magellan produce maps for their GPS receivers. There are ‘Topographical’ and ‘Street’ map versions available. The topographical maps (also referred to as ‘topo’) show features of the countryside such as rivers, woodland, footpaths etc. Street maps are just that with few topo features showing. On the more expensive GPS receivers these maps usually support turn by turn routing for navigation by car, bike or on foot. Maps may be included with the GPS model or available to purchase separately.
All Garmin and Magellan GPS’s that support maps, come with a Base Map which in the case of Garmin is very basic and not much use. The Magellan Base Map is much more detailed.
Ordnance Survey maps
Both Garmin and Magellan produce Ordnance Survey (OS) maps for the UK (and Northern Ireland and the Republic of Ireland on Garmin). These are based on the 1:50000 scale Landranger maps and 1:25000 Explorer maps (includes national parks). These OS maps are very detailed but are very expensive.
Open Street Maps (OSM) – Garmin only, these do not work on a Magellan GPS
OSM maps of many parts of the World are available from various sources and these are quite easy to install on the GPS. A UK geocacher, TalkyToaster has created a series of maps for the British Isles and Ireland that are based on OSM and these have the look and feel of the OS maps. These are very detailed and a very cost effective alternative to the Garmin OS GB Discoverer maps. There are several versions available and are very reasonable priced. You can either download them or buy a pre-loaded microSD card. Full details are on TalkyToasters website.
Note: TalkyToasters OSM maps are also available to run on the PC and in Garmin’s Basecamp software.
TalkyToaster’s Open Street Maps.
Maps on a Garmin GPS
See details of the available maps including comparison of the maps with a series of screen shots (opens a new page)
Maps on a Magellan device
See details of the available maps including screen shots (opens a new page)
Create your own maps
Please note that the Ordnance Survey digital maps with the PC programs Memory Map, Anquet and Fugawi cannot be transferred to a Garmin or Magellan GPS though you can use the maps on the PC to transfer and load routes and waypoints from the map to the GPS. There are software applications available that allow you to ‘capture’ these maps from a PC map program (or scanned maps) and copy them to the GPS. This can be quite a time consuming process.
How are maps stored on the GPS?
Maps are stored in the GPS memory. Most GPS’s come with more than sufficient internal memory and all the newer ones support microSD cards for additional storage. Some GPS’s come with pre loaded maps. See the comparison table later for full details of this. Pre-loaded maps use an additional area of internal memory and doesn’t affect the normal memory available for you to use. A 2Gb microSD card is sufficient for TalkyToasters Open Street Maps.
Which are the better maps?
The topo and street maps from Garmin and Magellan and the free Open Street Maps are vector based which means you can zoom in very closely without losing any detail. Both the Garmin and Magellan Ordnance Survey maps are raster based which means they go a bit ‘fuzzy’ if you zoom in too closely. For this reason it’s best to have both the OS (or OSM OS style maps) on your GPS. You can easily switch between them. Magellan in fact auto-zoom the maps from the OS to the Base Map.
What maps can you use on a Smart phone?
All smart phones have map applications available, such as Google maps. The geocaching applications you buy also use maps. These are however run ‘online’ so you will need a data connection to use them. You can also purchase from Memory Map the application to run Ordnance Survey (Landranger series) maps you have with the Memory Map application on your PC locally on the phone without a data connection. If you don’t have Memory Map on the PC you can buy the maps for the phones. See the Memory Map website for details. For the iPhone the Motion-X GPS application allows you to download Open Street Maps to use offline.
Which handheld GPS is which? How do I choose one for Geocaching?
There are a lot of different handheld GPS receivers available with many different options. In the table below I’ve tried to give the main features as discussed further up this page to help you choose which GPS is the best for you based on the features you want and the price. A great way to find out about different types is to go to a Geocaching event and talk to other geocachers and have a look at what they use and what their preferences are. The Geocaching Association of Great Britain have a Calendar of events coming up.
The ‘paperless’ GPS units hold both waypoints and geocaches separately. This greatly increases the number of caches you can store and also keeps them separate to make it easier to go geocaching. You should also read the page in this site about paperless caching to understand more about how it works and how the cache information is sent to the GPS units. To get the full benefit of paperless caching you need to be a premium member of geocaching.com. There are also a huge number of forum posts from people trying to compare units and find the best.
Paperless caching page
Forum posts about GPS’s
GAGB Event Calendar
The GPS units listed below are all current models. There are other models but the list would be just too long! The ones listed here seem to be the most popular that geocachers use. There are also very old discontinued models but I’ve not shown those either. Some more recently discontinued models (like the Oregon 4xx) are still available from some places and are still very popular. These are in the table in italics. The guide price is just that, a guide. You should shop around for best prices. Be careful of deals that offer maps with the unit. Check which maps are included in the deal.
All the Garmin mapping GPS’s come with a ‘Basemap’ which is not a lot of use having only very basic road coverage which isn’t very accurate.
The features listed here are the key ones. The units have many more features so you should check Garmin’s product page for complete details. Other features (not shown) include dual battery,altimeter, route navigation etc.
All GPS’s listed have a straightforward USB connection to a PC except the eTrex H which needs a special serial cable which is not supplied with the unit.
Disclaimer: All the information provided here is for information only to help you make a choice and I take no responsibility for any omission or errors. Please visit the Garmin & Magellan websites for full specifications of each unit.