BY ANDERS PERSSON
When you exercise, the histories of your proceedings are stored in the Forerunner. The track log will hold a record of your whereabouts, including position, speed, heading, elevation, time of day and, if you have he Forerunner 301, heart rate.
The storage principle deployed in these units is what Garmin calls the “automatic mode”. This means that there’s no fixed rule, stating that a data point will be stored with a certain time or distance interval. Instead, the Forerunner projects a line through the last two points in the track log, extrapolating this line into your future (literally!). As long as you keep on moving along this line, not deviating from it more than a certain, internally defined tolerance, no new track log point is stored. When you do turn away from the line, a point is stored, and the process is repeated with the two now most recent points.
Although not really that easy to envision graphically, a similar approach is used for the other variables, like elevation and speed.
The result of this strategy is that as long as you maintain steady state, i.e. same speed, direction etc., no new points are used from the track log memory. But if you’re moving in an erratic manner, many points are stored to mimic your movements.
How this memory is managed in the Forerunner 201 is well explained in the users manual, on page 6. Basically, it breaks down to this:
When you reset the timer, you free up the current log, compress the (up to) 3000 points into 250 points (maximum), store that in the most recent stored event and erase the least recent of the 10 stored events.
The Forerunner 301 uses another approach, which isn’t at all described in the users manual.
When you start a new event, 5,000 points are made free, if necessary. Thus, if the log of past events is shorter than 20,000 points, no old event is erased. However, if there is less than 5,000 points free, the track log of enough events are purged, to make 5,000 points available to the new activity. This implies that if the most recent event was very large, but the oldest events were short, several of the oldest events may have to be sacrificed to release enough memory for the new activity. On the other hand, if the oldest event is a large one, releasing that may allow you to fulfil several new, short events, before you loose another old track log.
In the Forerunner 301, the track log will also store heart rate information. This means that track log data points may be consumed, even if the Forerunner 301 has lost track of where it is, either because you are under foliage or if you deliberately have turned the GPS off, to use it indoors, for example. Thus, if you upload the track log to the Mapsource program, which is concerned about geographical track log data points only, it may seem that the track log memory is smaller than what I’ve just stated. But that’s an illusion, created by the need to store heart rate data as well.
For events, which aren’t deleted in the Forerunner, but where the track log has been erased, you can still see all event data, including the positions of your lap times were taken, except the trail you took to go between the different lap points. This lap memory can hold 5,000 such times, according to the manual. This amount is the same in both units (201 and 301).
Received November 9, 2005. Posted November 10, 2005