GPS Waypoints while your travell

Taking Waypoints while your travel - Convenience vs accuracy.

Alan Tomlinson brought to our attention the question of accuracy while taking waypoints on the move. This prompted us to do a few experiments and perform a few calculations to determine the effect of movement on waypoint accuracy.

To summarise our findings:

if Na > Nc then Stop

if Na < Nc then don't worry - be happy !

or to put it more verbally:

- If your Need for accuracy is greater than your Need for convenience -

then - Stop before taking a waypoint !

- If your Need for accuracy is less then your Need for convenience -

then - Don't worry to stop but beware!

We started to investigate all the possible sources of error, in general, which can be made and can accumulate when GPS readings are taken while on the move in a vehicle and compiled the following potential error causes:

1	The GPS temporarily lost signal
2	GPS set to a different datum than assumed
3	Making an error when writing down the co-ordinates
4	Making an error when typing the coordinates
5	Making and error when converting degree formats (i.e. from degree, decimal minutes to decimal degrees)
6	Making and error while importing the waypoint data into our database.
7	A Mobile-phone signal distorted the GPS signal or influenced the GPS receiver at that time ???
8	A large PDOP (Positional Dilution of Precision) at the time of the data logging.

1. The last error cause, that of PDOP (positional dilution of precision) is the error made due to an "in-line" geometry of the satellite constellation) at the time a signal is received. When traveling in a vehicle, we normally position the GPS on the dashboard. This contributes to the DOP error as the rest of the vehicle effectively screens out the satellite signals and allows only a relatively narrow band of satellites to be received which contributes to a larger DOP figure.2. Signal acquisition and processing time during speed. When traveling at speeds of say 60km/h, we may be inaccurate with as much as 83 meters, if signal acquisition and processing time are 5 seconds.

We know that GPS manufacturers claim updates every 1 second, but have a look how long it takes for a GPS to update its real readings with poor signal quality (between mountains for example) and DOP figures above 2.0 We ran a quick (totally unscientific) test (just for a feel of it) and came to the conclusion that taking GPS waypoints "on the move" should not be considered good practice, if you're looking for accurate waypoints. However, For general traveling and logging your trip, be aware that your the convenience factor should not overshadow the purpose and use of the GPS Waypoints taken "on the move".