Visualising trips and travel characteristics from GPS data

Road & Transport Research, Jun 2003 by Stopher, Peter R, Bullock, Philip, Jiang, Qingjian

Both these situations require resolution if the track record is to be converted sensibly to a set of trips that can be shown to participants in the survey, and if reliable information is to be gleaned from the records for analysis and presentation to decision makers. The former situation is also more troublesome if a stop occurs within the portion of the record where the signal has been lost. This would occur, for example, with someone who drives from a suburban area into the CBD, stops to transact some business within the CBD, and then resumes travel and emerges in a different part of the CBD boundary than entry. In this case, it is necessary to determine if a stop might have been made, which would constitute the end of one trip and the beginning of the next trip.

ALGORITMS FOR DATA PROCESSING AND ANALYSIS

Because the collection of data by GPS devices has the potential to provide very large quantities of data, it is necessary to develop automated procedures to analyse the data, and convert it to readily visualised information for respondents, analysts, and decision makers. The need is for some rule-based algorithms that can do the following:

* Detect a trip end when the ignition has not been turned off;

* Detect a trip end that occurs when direction is reversed, but there is a very short or non-detectable stop;

* Determine whether a trip ended during a period of signal loss in the middle of a trip; and

* Determine whether there is a loss of signal at the beginning of a trip and repair the data record if there is.

As noted previously, we experimented and found that a rule in which a trip end was assumed to occur if there was 120 seconds or more of elapsed time between two successive in-motion data points worked well to detect trip ends where the ignition was not turned off. Such locations are found by removing from the data any data points where the movement between successive data points is less than the accuracy rating of the GPS device. In our case, the GPS devices in use are rated to have an accuracy of within or -20 metres. However, when stationary, the position rarely changes by more than a few metres, with a speed of 0.0 kph, or nearly so. Therefore, by removing those points where the speed is shown to be zero, and there is little change in position, we can detect when there is a stop lasting two minutes or longer, and define that as a probable trip end. Again, by inspection of known short stops, we found that setting a limit on the change of latitude or longitude to 0.000 051 [degrees], which represents a distance of movement of less than 7.4 metres, we could be fairly certain of detecting a stop. In summary, we flag a trip end if:

* The difference in successive latitude and longitude values is less than 0.000 051 degrees; and

* The heading is unchanged or is zero; and

* Speed is zero; and

* Elapsed time during which these conditions hold is equal to or greater than 120 seconds.

If there is a break in the record, meaning that the engine was turned off, of between 30 and 120 seconds, this is also defined as a potential trip end.