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example image The image shows as example a small loop by car, recorded by two Garmin Dakota 10 devices. It seems both devices have had difficulties caused by the surrounding buildings, the "blue one" seemingly had better reception. A lot of the recorded coordinates seem to be more calculated/interpolated than measured.

We want to match the track to the (OSM) road network, and want to calculate driving speed. This proves difficult because of the scatter and interpolation. We would prefer missing over wrong positions.

Are other devices better suited for this task?

Additional info: One device was hold in the hand, the other one was laying by the window. WAAS off on both devices. This is in the middle of a longer tour, so the devices are already recording for more than an hour. We are "guest" on the cars, so mounting an external antenna is not an option.

  • Did they have the same options enabled? (WAAS activated in one and not the other) And they were in the same car together at the same spot? (heard of tinted windows doing something) Otherwise it just shows the problems in urban environment with multiple reflections. – Matte Mar 23 '16 at 13:51
  • An Inertial Measurement Unit (IMU) can improve accuracy. en.wikipedia.org/wiki/Inertial_measurement_unit – Kirk Kuykendall Mar 23 '16 at 13:54
  • How much time did you wait before starting to log (from turning on the devices)? – Giacomo Catenazzi Mar 23 '16 at 14:21
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There are a couple solutions I can think of, but based on what I was able to find on the Dakota 10, neither is applicable because the device doesn't support them.

We would prefer missing over wrong positions.

This can be done in more advanced GPS units by position filtering. GPS uses a measurement called Dilution of Precision (DOP) to measure the potential inaccuracy of a given GPS position observation. The lower the number, the better the accuracy. In some GPS units, you can set a DOP number to use as a filter, any positions with a DOP higher than your filter are discarded, which is exactly what you want to do in this case. But the Dakota 10 doesn't support filtering as far as I can tell. It may not even record DOP values.

The other solution would be to get an external antenna and mount it higher on the vehicle. If your position readings are poor because of surrounding buildings, moving the antenna position higher should help. But again, the Dakota 10 doesn't have a connection for an external antenna.

  • This sounds promising. So most flexible would be to record DOP values and to filter afterwards? Which device would allow this? – Redoute Mar 23 '16 at 15:56
  • @Redoute That would be a good way, though in order to do that you'd need to get and manipulate the raw NMEA data from the GPS unit rather than a GPX tracklog. Even then, not all GPS units record or output the DOP values you would need in their NMEA output, it's something you'd need to confirm before buying anything. Again, the Dakota 10 appears to be lacking: it doesn't even have an NMEA interface. – Dan C Mar 23 '16 at 16:09
  • I think we may try the Columbus V-900 or V-990 data logger. Here is a sample what a logfile looks like. – Redoute Mar 23 '16 at 18:32

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