EPE (the “Accuracy” shown on your GPS receiver) is an estimation, rather than a measurement.
There is a difference between Accuracy and precision
To many people, accuracy and precision mean the same thing: to someone involved in measurement, the two terms should have very different meanings.
Accuracy - the degree of conformity with a standard
Precision - the degree of refinement in the performance of an operation
Look in your GPS manual and it will say something like “Accuracy of <15 meters”. This is it's degree of conformity to a standard i.e. within 15 meters of bulls eye. The EPE that is on your GPS however should often be as low as 4 meters. This is the degree of refinement in the performance of an operation. i.e a the uniformity of the co-ord within the aforementioned range of accuracy.
|Accuracy||Precision||Accuracy with Precision|
What affects GPS accuracy?
Lots of things can affect accuracy of your GPS and even though the basic principles used by different manufacturers are based on the same specification, different manufacturers use different propriety algorithms to achieve the end result. Even 2 identical models will most likely not give identical results. They may be similar but are not necessarily the type of results one might expect.
System SPS (SIS) accuracy is based over a 24 hour period so at any given time 20 metre accuracy is quite possible. Unless you are sure that the coordinates you have are absolute (ie. correct) then 20 metres really doesn't indicate accuracy. The system accuracy specification does not take into account how the user influences the position solution.
There are many things that can affect results. Keep in mind that the same things affect the hider as well as the finder, so you can't be guaranteed that the hider didn't have a different set of things affecting their accuracy when they hid the cache.
- Satellite geometry is much more important than satellite numbers. If you get three satellites in a line you won't get very good accuracy at all. Try and aim for a good spread of satellites around you. Generally speaking, signals from satellites on the horizon will introduce greater error into the final calculation, so you can often gain a bit more accuracy by deliberately blocking it out (with your body or a tree, etc).
- Signal-In-Space (SIS) errors. Ionosphere, Tropsphere etc
- Satellite errors, clocks, ephemeris, etc
- Obstructions such as trees, buildings, your own body or even the way in which you are facing. With a handheld GPS always try and face the equator, there are no satellites over either of the poles.
- Multipath or reflections. The issues with multipath signals or reflections are extremely hard to detect. Foliage (in general), buildings or other obstructions may actually be causing a reflection rather than completely blocking the satellite signal.
- Receiver. Additional issues than simply alogrithm and antenna. Receiver noise etc
- Processor speeds (not a big drama these days)
- Old and New satellies. A period where there are a mix of old and new generation satellites. The hardware of the newer satellites contributes to improving things considerably but depends on the constellation and any given point in time. GPS signals are extremely robust and the IIR-M (one of the newest satellites) has a little bit more power as will the next generation.
- Movement. Repeat observations at a single (or stationary) location permit an improvement in precision due to the effect of averaging over time. A moving GPS receiver does not offer this possibility. If you are trying to gain accurate co-ordinates, stop and wait.