All about GPS receivers, Coordinates and Maps

The Basics - the GPS system

The American GPS system is a network of more than 30 satellites that orbit the earth at an altitude of 12,000 miles which means each one takes about 6 hours to pass overhead from horizon to horizon. Your GPS receiver picks up signals from these satellites. It needs to 'lock' onto at least 3 to get a position. The more satellites your GPS can lock onto, the better the accuracy of your position. See the section below on Features of a GPS for more details about the GPS receiver.
¹The Garmin eTrex 20 supports Glonass.

The Basics - different types of GPS available

Basic
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).

Mapping
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. The compass may be electronic. May also have an altimeter. Increased internal and external memory (SD card) allowing more information and maps to be stored. PC connection is easy via a USB cable so you can transfer cache information from the web page quickly.

Examples: Garmin eTrex 20, Garmin GPSmap60, Magellan GC

Paperless
The same as the 'Mapping' GPS but with the ability to download Geocaching 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 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.

*Only available to Premium Members

Examples: Garmin Montana, Oregon, Garmin GPSMap 62s, Magellan eXplorist 710

Smart Phone
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.
Certain smart phones have specific applications available for geocaching. These are explained later on this page.

Examples: iPhone, HTC

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 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) and get a position lock quicker. If you look in the detailed specification then any receiver quoted as 'high sensitivity' is fine.

There are three different types of aerial used with GPS units. A 'patch' aerial, a 'quad helix' aerial and a 'ceramic' aerial. GPS units with a patch aerial need to be held flat to work at their best. Those with a quad helix aerial need to be held vertically for optimum performance. The Oregon and Dakota models have a ceramic aerial which is mounted inside the case at the top (not visible). The Oregon and Dakota can be held either flat in your hand or vertically. How do you know which type of aerial your GPS has? The quad helix GPS units have an aerial sticking up from the top of the case as in this example of a Garmin GPSMap62s. Those with a patch aerial have no visible aerial unless you have an eTrex Legend when you can just see it under the plastic case at the top (a square block about 2cm x 2cm).

Compass: All 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 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. Newer 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 using one of various programmes available. 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. 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 Legend, Venture, Summit and GPSmap60 series there is an easier to use on-screen number/letter selection. The advanced units like the Oregon 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.

Coordinates and Map Datums

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
Decimal degrees 51.55160, -2.55255
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

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Satellites and Accuracy

More detailed information on WAAS and EGNOS here

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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 and some paths. Street maps are just that with few topo features and are for those GPS that support road navigation. Both Garmin and Magellan produce Ordnance Survey (OS) maps for the UK and Ireland. These are based on the 1:50000 scale Landranger maps and 1:25000 Explorer maps (includes national parks). These OS maps can be quite expensive.

All Garmin and Magellan GPS's 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. See the second of the images below.

There are also free maps available based on Open Street maps produced by a UK Geocacher. These only work on the Garmin GPS and are equal in quality to the Garmin topo maps.They are very easy to download and install on your GPS.

Example of a wooded area on the Ordnance Survey map and Open Street Maps. The level of detail on the Open Street Map is very similar to the Garmin and Magellan topo maps.

Example of the town center of Caldicot in South Wales on the Magellan City Series and Open Street maps. The Magellan map detail is very similar to the Garmin version.


This shows the Magellan base map that comes with the Magellan GC compared to the Open Street Maps on a Garmin Oregon. The Magellan map is on the left. Again, town center of Caldicot in South Wales and Junction 2 of the M48 by the Old Severn bridge.

How are maps stored on the GPS?
Maps are stored in the GPS memory. Most GPS's come with 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.

Please note that the Ordnance Survey digital maps with the PC programs Memory Map, Anquet and Fugawi cannot be transferred to a Garmin 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 program and copy them to the GPS.

Which are the better maps?
The topo and 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 the countryside the OS maps are best and for towns and cities the Open Street Maps are better. You can have both on your GPS and easily switch between them.

Which SD cards do GPS's use?

• eTrex Venture Cx
• eTrex Legend Cx
• eTrex Legend HCx
• eTrex Vista Cx
• eTrex Vista HCx
• GPSmap60Cx
• GPSmap60CSx
• GPSmap76Cx
• GPSmap76Cx
• GPSmap62
• Oregon
• Dakota
• Colorado
  

Using a Smart phone for geocaching

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.

Geocaching.com page for Smart phone applications

Which GPS is which? How do I choose one for Geocaching?

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 discontinued models but I've not shown those either. 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 Magellan base map is much better.!

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.

Garmin GPS

¹ Garmin Ordnance Survey GB Discoverer maps (GB and Ireland - ROI/NI), Garmin topo and street maps, free Open Street Maps
² Garmin topo and street maps and free Open Street Maps
³ Maps are country specific. If you buy the GPS in the UK you’ll get the European Topographical maps preloaded in internal memory. These maps are loaded into an additional internal memory and does not affect the normal amount of internal memory shown for that unit (or the SD card if available) so you could add more maps if needed.
⁴ You can transfer some cache information such as the hint
⁵ Camera can 'geotag' photos with the coordinates of where the photo was taken. View pictures on screen. Navigate to geotagged photos.
⁶ Can send/receive geocaches, waypoints, routes and tracks with other Garmin wireless compatible units