Some information about GPS receivers, coordinates and waypoints

	GPS Receivers - the basics!
	How do I know it has locked on to the satellites?
	Coordinates and Map Datums - all about latitude and longitude
	Can I use maps on my GPS?
	WAAS and EGNOS - what is it all about?
	Waypoints and GPS receivers - how to goto
	Can I use my GPS anywhere in the world?
	How to use the compass
	How to use the iPhone for geocaching

GPS Receivers - the basics!

A Global Positioning System (GPS) receiver picks up radio signals broadcast from a network of 24 satellites that orbit the Earth some 12,000km above the surface. Using this signal the GPS can calculate its location on the surface to within about 20 to 30 feet. GPS will work anywhere in the world. There are two main GPS satellite networks. The American Military provide the satellites that we currently use with our handheld and car sat nav receivers. The Russion GLONASS system is limited in coverage and only works with specialised receivers not that are not commercially available. The new European GALILEO system is currently not developed and only has two satellites in orbit under test.

Which type is which? Well, there are many models offering different features but they all have a similar receiver and all calculate where they are. Later models have a more sensitive receiver which means they can get a satellite 'lock' quicker and in locations such as under trees or in 'urban canyons' better than older models. More expensive GPS receivers can display maps of your location and offer quick and easy transfer of data to and from your PC.

Personal Digital Assistants (PDA) or electronic diaries can also offer similar functions if they have a GPS receiver (normally built in but can be connected via Bluetooth). They too may be able to display maps.
There are many forum posts about GPS hardware. Review the information there before posting a new question, you may find the question has already been asked.

Differential GPS or DGPS uses a ground based radio signal in addition to the normal satellite signal to provide a very accurate postion. The normal Garmin type GPS receivers do not support this directly but would need some interface equipment. Professional GPS receivers as used by surveyers for example are able to use DGPS directly and can achieve accuracy down to a few centimeters.

Main forum, GPS and Technology. Read the FAQ thread.

How do I know it has locked on to the satellites?

The newer chipsets (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 travelled a long distance away then it will have to download this information again and may take a few minutes to get a lock.

Look at this display from a Garmin Colorado. 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 blue bars below show the signal strength of each satellite, the numbers show which satellite is which. So in this display, satellite 30 is directly overhead, 29 and 5 are quite high in the sky whilst 14, 31 and 12 are about 45 degrees above the horizon. 2, 4, 9, are very low but still being received but 24 and 20 are too low and the light grey colour shows they are not being received. Satellite 39 is one of the WAAS/EGNOS satellites (see more on WAAS/EGNOS later). It may seem odd that 12 has a higher signal strength than 14 though they are about the same height above the horizon. Many things affect the signal, including the way the GPS is being held, obstructions such as trees or buildings and so on.

Finally, with all these satellites being received the GPS is showing an accuracy of 12 feet (see more on accuracy later in the WAAS/EGNOS section).

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). Lat and Long is measured from the Greenwich Meridian which is 0 degrees east and west and from the Equator which is also 0 degrees north and south.

There are several different formats to use and different countries produced their own version including 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
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 websites

Geocaching.com have a simple to use coordinate convertor

This site also supports the Ordance Survey Grid references

This is a more comprehensive site

Can I use maps on my GPS?

Yes but it depends on the GPS receiver. The more basic receivers (like the Garmin Etrex H) can't support any form of maps. The Garmin Etrex Summit, Venture, Legend, Vista and 60 series do support maps. Whilst the Oregon, Dakota and Colorado support more advanced maps. The Magellan GPS units also support maps.

What maps are available?
Both Garmin and Magellan produce their own topographical and street map software. These are not as detailed as the British Ordnance Survey Landranger 1:50000 maps that most people in the UK are familiar with. These 'topo' maps can be quite expensive. Garmin have produced OS 1:50000 Landranger maps (and some 1:25000 scale) of the UK and Ireland specifically for the Oregon, Dakota and Colorado GPS receivers (they do not work on other Garmin units). Again these OS maps can be quite expensive. Some GPS recievers come with pre-loaded maps in their internal memory or have sufficient internal memory space to load additional maps. Some also support SD memory cards to allow maps to added.

Other maps available
You may be able to load OS maps on any Garmin GPS that supports maps by creating image files of the maps. You can also load the OpenStreetMaps onto suitable Garmin GPS receivers. These are not as detailed as the OS maps but are very good and most importantly are free! A UK geocacher has produced the maps required. They should run on all the Garmin GPS receivers that support mapping.

Things to be aware of
To load maps onto a GPS you need to have sufficient memory capacity. The more basic units (like the Garmin Legend) only have about 24 Mbytes of internal memory and do not support removable memory cards. This limits how much of an area you can store. The more expensive units have increased memory and do support SD cards. This means you can load more maps into the GPS and with SD cards can load the maps onto those and switch between them if you wish. There are a confusing number of GPS receivers with options of pre-loaded maps and different memory sizes. You should check out the Garmin and Magellan websites for complete details. But here is an example.

	Oregon 200	Oregon 300	Oregon 400t
Internal Memory	24 Mbytes	850 Mbytes	850 Mbytes
SD card supported	Yes	Yes	Yes
Pre-loaded maps	No	No	Yes - European 'topo'

Link to the free OpenStreetMaps (thanks to TalkyToaster!)
To load various map types to a GPS/PDA/Phone
Garmin website
Magellan website
I won't put direct links to their mapping sites as these change. Please just search the sites for their mapping solutions.

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.

More detailed information on WAAS and EGNOS here

Waypoints and geocaches

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.

GPS Compass

All handheld GPS receivers have a compass built in. On the more basic units the compass works using the satellite signal. This means it will only show the direction when you are moving. When you stop the needle will 'freeze'. This can be misleading when close to the cache and you move very slowly, the needle may not respond. The more expensive units have a magnetic compass that will point whether you are moving or stationary. This is very useful when near the cache location as you can move slowly around and watch the compass arrow as it points to the cache. You need to keep the GPS unit held level though. The newer units like the Garmin Dakota and Oregon 450/5XX have a '3 axis' compass which means you don't have to hold it level which makes using it much easier.

The magnetic compass in a GPS needs to be calibrated periodically (especially when you change the batteries). Follow the instructions with the GPS on how to do this.

For geocaching it is best to set the compass to the 'Bearing Pointer' which means the arrow points directly to the waypoint you are navigating to.

Can I use my GPS anywhere in the world?

Yes you can, the network of GPS satellites orbits the whole globe so your GPS can pick up the signals wherever it is. However, if you travel more than say 500 miles your GPS can take several minutes to get a 'lock' on the satellites when you switch it on.

What about WAAS/EGNOS? This only works where there are the necessary ground stations to broadcast the signal to the WAAS/EGNOS satellites. This is fine in Europe and the USA but in some areas, although you may pick up the WAAS/EGNOS satellites it won't be a corrected signal and you'll not get an improved accuracy figure. It won't affect the normal signal.

Using an iPhone for placing caches

The geocaching application is fine for finding caches but when you place a cache you need to record the coordinates accurately. The iPhone 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.

You can use the geocaching application by navigating to a cache (any cache) and on the compass screen it will show you your current position which you can write down. You can also use the compass screen which gives you an 'instant' readout of location (in degrees minutes seconds which you need to convert).

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. Cost is under £2 ($3).

In all cases you need to wait for 10 minutes or so to ensure the iPhone GPS has a good signal. Only the MotionX application gives you a GPS signal strength and accuracy figure.

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Geocaching Resources for the UK and Ireland