4

I have a number of sound files with timestamps (HH:MM:SS), made when walking a transect. I need to know where I was when each file was saved. To do this I have a GPX track of the route I took - I can extract the time for each point along the GPX track however the times don't match exactly with the sound files - the seconds aren't always the same.

These are my sound file time stamps:

Time       File 
19:32:53   1
19:34:26   2 
19:40:43   3 
19:40:47   4
19:49:21   5
etc

These are the times of the GPX points:

Time       Latitude         Longitude
19:30:29   51.4463364054   -2.4129919242
19:37:44   51.4464039635   -2.4128513597
19:37:59   51.4464663249   -2.4127552193
19:38:01   51.4464837592   -2.4127533752
19:55:08   51.4465564303   -2.4127591588
etc

So in the above example I need to estimate where I was at e.g. 19:34:26.

I could achieve this by removing the seconds however I would lose a lot of accuracy and would leave me with redundant coordinates.

Is it possible to do this in GIS? Or might it be best to plot this as a graph? Either way, how would I do this?

I'm on Linux and ideally would like to use Calc/Excel, QGIS or R.

  • See ?approxfun in R, you can use functions flon(newtime) and flat(newtime) created by approxfun(Time, Longitude) and approxfun(Time, Latitude) – mdsumner Sep 14 '16 at 6:38
2

My approach here would be:

  • Convert position times to integer seconds (ie hours x 3600 + minutes x 60 + seconds)
  • Create a linear interpolation of the positions for every second (not sure how in Excel, but in python it is relatively straightforward, I'll assemble some bits of code and post a sample later)
  • pick interpolated positions at times which match your sound file times

That's it! Maybe QGIS-python functions to do this exist already?

Here's my whole process laid out in an ipython notebook - I hope it is followable, and translatable to the tools you're using:

https://github.com/adamsteer/miscellaneous-debris/blob/master/Tinkering%20with%20timestamps%20and%20interpolating%20positions.ipynb

Cheers

  • Thanks for this. It looks like it should be straight forward to do a linear interpolation in Excel buy my worry was whether this would work with coordinates? Coordinates won't necessarily be linear with respect to time - would this be a problem? If I were to try this I guess I'd do Lat and Lon separately? – Thomas Sep 14 '16 at 11:16

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