08-26-2020, 11:28 PM
(This post was last modified: 08-27-2020, 09:29 AM by turboscholz.
Edit Reason: better explained the data which is plotted in the image
)
Well... physicist, of course 
Sorry for the long text below...
Let me describe my use case: Last month I was reading a very interesting article about the quality of bike lanes in the "Fahrrad Zukunft" magazine. There, they describe how dedicated bikes in the Netherlands are used to measure the surface quality of bike lanes with quite expensive sensors. I than thought that many of those sensors are also available in a standard mobile phone today (with a bit less accuracy of course). And so I got in touch with phyphox: I created an experiment called "Radweganalyse" which measures the acceleration and the GPS coordinates at the same time. Than, I mounted my phone horizontally aligned on my bicycle handlebar, started the phyphox experiment and began to bike along a bike lane in my city. After cycling some minutes I stopped the experiment and exported the data.
The exported data from the acceleration sensor and the GPS sensor have different frequencies. (~200Hz acceleration, 1Hz GPS). With the help of some tools I could manage to resample the GPS data (for example via linear interpolation) to find the interpolated coordinates of every single acceleration measurement record. In this way I could create a resampled Location.csv file which also contains 200 data points per second. This made it finally possible to create a gpx file (with another tool) in which I could use the z-acceleration to be plotted as a function of the GPS coordinates. See the following image as an example result:
This was really promising!!! (The data of the hight diagram is actually the z-acceleration in m/s².) Together with a friend I created I script which could even find the positions in the measured data with the highest z-acceleration values, i.e. positions of the bike track where the surface quality is worst. For example when I drove above a manhole cover ("Gullydeckel") I measured quite high z-acceleration values.
Okay, and now I come to the actual issue: When visiting the found positions with high-z values again in real life it turned out that the position on the map is quite often not the real position and that all positions are systematically shifted! For example, the real position of the manhole cover was ~10 meters away from the position in the gpx map. (See the black dots in the image above, these are the measured coordinates with high z acceleration values, but when visiting most of them, the actual street "bump" was always some meters away from that position). Although the data of the GPS and the accelerometer where measured at the same time!
This means that either
My idea would now be to measure the coordinates also with that external GPS device and align that data with the accelerometer data of phybhox. And that's exactly why I would need the data of the phyphox export to contain absolute values of the timestamps. With the current time format I cannot merge both measurements (absolute time <=> relative time).
I hope I made this point clear somehow and I'm interested what you think about the issue.
Sorry for the long text below...Let me describe my use case: Last month I was reading a very interesting article about the quality of bike lanes in the "Fahrrad Zukunft" magazine. There, they describe how dedicated bikes in the Netherlands are used to measure the surface quality of bike lanes with quite expensive sensors. I than thought that many of those sensors are also available in a standard mobile phone today (with a bit less accuracy of course). And so I got in touch with phyphox: I created an experiment called "Radweganalyse" which measures the acceleration and the GPS coordinates at the same time. Than, I mounted my phone horizontally aligned on my bicycle handlebar, started the phyphox experiment and began to bike along a bike lane in my city. After cycling some minutes I stopped the experiment and exported the data.
The exported data from the acceleration sensor and the GPS sensor have different frequencies. (~200Hz acceleration, 1Hz GPS). With the help of some tools I could manage to resample the GPS data (for example via linear interpolation) to find the interpolated coordinates of every single acceleration measurement record. In this way I could create a resampled Location.csv file which also contains 200 data points per second. This made it finally possible to create a gpx file (with another tool) in which I could use the z-acceleration to be plotted as a function of the GPS coordinates. See the following image as an example result:
This was really promising!!! (The data of the hight diagram is actually the z-acceleration in m/s².) Together with a friend I created I script which could even find the positions in the measured data with the highest z-acceleration values, i.e. positions of the bike track where the surface quality is worst. For example when I drove above a manhole cover ("Gullydeckel") I measured quite high z-acceleration values.
Okay, and now I come to the actual issue: When visiting the found positions with high-z values again in real life it turned out that the position on the map is quite often not the real position and that all positions are systematically shifted! For example, the real position of the manhole cover was ~10 meters away from the position in the gpx map. (See the black dots in the image above, these are the measured coordinates with high z acceleration values, but when visiting most of them, the actual street "bump" was always some meters away from that position). Although the data of the GPS and the accelerometer where measured at the same time!
This means that either
- the GPS measurement of my mobile phone is not that good at all or that
- the times of the GPS and the accelerometer sensors in the data export of phyphox are not well aligned or
- something else which I did not think of up to now happened.
My idea would now be to measure the coordinates also with that external GPS device and align that data with the accelerometer data of phybhox. And that's exactly why I would need the data of the phyphox export to contain absolute values of the timestamps. With the current time format I cannot merge both measurements (absolute time <=> relative time).
I hope I made this point clear somehow and I'm interested what you think about the issue.