Speed drift at stop lights

Former Member over 10 years ago

Just got a Virb (basic) Attached it to my Motorcycle and started to make videos using Virb Edit, I import the GPX files from my Nuvi 42. I found a great way to Sync the GPX with the video by using Bing Map Sat view then zooming in and match a Video frame with a crosswalk marking on the street, The sync is dead on, The issue is when I come to a stop light the speed drops to 0 then starts to rise as I sit still. I looked for the cause and I think I found it.

In the GPX file when stopped the <speed> tag is not recorded and the GPS seems to start logging data points at a much slower rate (it may be a design feature to save memory when not moving) As I watch the video the speed overlay starts increaseing at a constant rate, Why, looks like the next data point is taken afer the light has changed and I am moving again and now the data point contains a <speed> tag. So it looks to me the Virb Edit software is taking the diffrence between the two data points, The one with no <speed> tag and the next one with a <speed> tag and displays the change over time this makes the speed displayed in the video to display the stop at 0 MPH then start to rise to the next data point that was recorded after the light changed and has a <speed> tag,

So how to fix this, I don't know if there is an option to not allow the GPS to go into a slow sample rate when stoped. If the sample rate was faster when stoped this would not happen.

I may try it with a .fit file from my Forerunner 620, I have not looked at the .fit files to see if the <speed> tag is recorded when not moving and if the sample rate changes when stoped. But I don't want to use my Forerunner on my motorcycle it would realy mess up my running PRs :-)

Any ideas, I don't think I am the only one who has this issue.

Thanks all

0 Former Member over 10 years ago

I thought of another fix, it woild take some software develpment to write a GPX fix up tool that would find the data points with no <speed> tags and place bougas data points with no <speed> tag every second tell it comes to a time stamp with a <speed> tag.

0 Former Member over 10 years ago

More info the virb manual says the Virb Elete stores a track point every second, So the speed drift would not be an issue with the Virb Elete. My Nuvi stores a trackpoint at diffrent rate depending on changes in direction. When I sit at a stop light no new track points are created, since there is no change in direction. When I take off from a stop light a datapoint is created only after a small change in direction. This auto trackpoint logging in the Nuvi is the cause of the speed drift. If I used a GPS that logged a tackpoint every second there would be no speed drift. So what GPS to use with the Virb, It has to be one that stores track points every second not one that only stores a trackpoint with a change in direction.

0 Former Member over 10 years ago

I Found a series of button pushes to put the Nuvi in Engineering mode (used for development I guess) I can now force the Nuvi to record a trackpoint every second. I will see if this fixed the speed drift issue.

A little research on Garmin GPS. The Nuvi and Zuno (Motorcycle GPS) seem to be set to auto adjust the trackpoint logging with no way to change this feature they only record a trackpoint when thre is a change in direction. Useful to save memory and batery life I guess. The Outdoor GPS Montana, Etrex, Etc have this auto feature but it can be adjusted to record a trackpoint by time (like the Virb) or distance. A nice feature that is not in the GPS units that are made for Roads (Nuvi and its famaly of road GPS units) the Sports GPS have a auto track mode but its a bit more usful since it can create .FIT files that are more compact than .GPX files. The Fenix GPS can be set to auto or log every second.

I will try the Nuvi in engineering mode and see if this fixes the speed drift issue. Also I am getting a Fenix 3 ariving Monday from REI I will have to try it too.

0 Former Member over 10 years ago

I got my new Garmin Fenix 3 Created a new activity called moto for my Motorcycle riding set the Fenix to record a trackpoint every second, I was able to sync the Virb vido with the GPX file from the Fenix Moto activity an it works great. No speed drift.
Bottom line, the Verb needs a GPS trackpoint every second to sync the GPS data to a Verb video