Wednesday, January 23, 2013

GPX in relation to auto-pause


GPX is a XML formatted data to record the GPS locations that was logged by an application. It could contain data other than GPS information plus date and time. For example it could contain Heart Rate data.

I would not want to puzzle readers with the XML format. It is something which I myself don't want to dwell into. Let's just take it as a format that records data in a particular way so as to allow for data manipulation.

Now, sports trackers stores the logged data in their own ways. It might not even store the data in the XML format. However, most will allow the data to be exported as GPX (a text file that has .gpx as extension). My interest here is to analyse what it does in relation to auto-pause.

A typical data log is as below (note this is not the actual XML as FB cannot display the original code  properly)

[trkpt lat="1.2921916666666666" lon="103.78459"]
      [ele]76[/ele]
      [time]2012-08-29T14:54:43.72[/time]
[/trkpt]

The first line stores the lon and lat information as attributes.

The next line stores the "elevation" as data. The date and time is stored as data in the next line. Both lines become the data of the first line.

The last line indicates the end of the data for the first line. The whole thing consists of one set of data or in lay man's term " a record".

Initially, I was mistaken that the datetime information is the real datetime. The sports tracker simply ignores certain data that falls below certain speed limit. I was totally wrong.

The sports tracker reads the GPS data at regular intervals. It will record the location and the date time information at regular intervals. The auto-pause was done between two readings. If the speed falls below the speed limit setting. It simply ignores the entry. Thus, you get a datetime that is not real time. If you move at a speed below the speed limit then the location will not be recorded. The location and time will simply freeze till you move beyond the speed limit.

Thus, instead of recording datetime, it actually records the time only. Datetime format is used simply because certain sports take more than one day to complete. It also removes the trouble to distinguish between real time recording (no auto-pause) and the auto-pause feature.

Now I am in a dilemma. Originally, I though I could use the logged data as evidence in case of accidents. With the auto-pause, I cannot use it as evidence as it is not real time. But if I turn off the auto-pause, every stop (e.g at traffic light junction) adds extra time to the sports I do thus reducing the speed record. I just can't have the best of both world.

Luckily, the two sports-tracker I use allows me to adjust the overall distance and time so as to calculate the overall speed. With this, I could do a real time logging then adjust the overall time taken to get the desired speed. This means that I need another data logger that has auto-pause feature turned on. Well, it is still better than having a pseudo real time data logging right?

Since I subscribe to two sports tracker site. I might as well use two device to track real and pseudo time record and adjust the overall time and distance on the real time recording so as to get the correct reading for the real time logger. But then I would be labeled as paranoid sports person. Ah... You win some, you lose some. It all depends on how you look at it.



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