Backround Information
Geodrift.com was created to answer one question: What is the deviation of GPS position measurements?
The coordinates reported by a GPS receiver are always an estimate only. As with every measurement there are measuring errors and multiple factors that enlargen the errors. A GPS receiver reporting an 'accuracy' of 3 meter may nevertheless display a position that is 10 meters away from the real position!
The picture to the left was generated by placing a GPS receiver on the ground and recording the position reported over more than 10 minutes. Each dot represents a single measurement. In 10 minutes the measured positions varied and were spraded in a rectangle of about 6 by 4 meters.
Read our Duration Track Test page to make and visualize similar recordings with your own GPS.
The basic principle for calculating the position requires that one knows where the satellites are (exact position in orbit) AND that they have very exact clocks AND that the signals reach the receiver undisturbed. Unfortunately no orbit is perfect and even atom clocks start to deviate over time.
The signals send by the satellites come through the atmosphere with potential interferences and may sometimes be reflected (trees, houses) before reaching the receiver. The receiver makes its calculations with limited precision and roundinge errors may accumulate. Those are getting bigger with bad satellite constellations (they are to 'near', not big enough of an angle between them). All these factors are called DOP (Dilution Of Precision).
Looking at the list of factors it becomes clear that a GPS receiver cannot recognize all of them and can only estimate what the current DOP is. Even harder is a prediction on how accurate or precise the calculated position is. Unfortunately there is no standardized method to determin the deviation and so different manufacturers use different methods and deliver results which are not comparable to each other. The values are typically combined with a probability, some using a 50/50 method. The deviation displayed has a probability of 50%. If the value displayed is 3 meters that means that with a chance of 50% the deviation does not exceed 3 meters. However, the chance of it beeing bigger than 3 meters is also 50% - AND - the real deviation could be 10 meters and more.
As the manufacturers of GPS receivers generally do not publish how their deviation is calculated displayed values of various receivers (even from the same manufacturer) cannot easily be compared.
One goal of the project Geodrift is to collect enough data about measurements from various GPS receivers to make an independent statement about the quality and the reliability of GPS systems in regards to their measurements and deviations.
Project participants are called upon to use their GPS receivers to measure Geodrift stations and to enter them on our web pages.
Wikipedia Information: