On my 5X I have added VMG to a data screen for an activity that uses direct routing using a bearing, i.e. not a course, to follow. This produces a course that ignores the roads and the initial starting point and leads straight to my destination waypoint from my current position. The fields for speed , bearing and distance remaining all appear correct and sensible values. The position is using GPS not Ultratrac. I test the route daily on my bus journey towards home using various modes on the watch while I get used to its features. The VMG field should represent the component of velocity towards the destination, so that if I am travelling at 30 mph directly towards the destination the VMG should be 30mph, whereas if I was travelling directly away it should show -30mph. I find that the VMG rarely shows a sensible or consistent value even along straight roads travelling at a steady speed. It often shows negative values when I know that I am moving in the rough direction of the destination. Sometimes even on a road that may be 10 degrees 'left' of the bearing to the destination it shows VMG of say 4 mph when my speed is 20 mph, and then when the road turns right , passing through the actual bearing to the destination the VMG rises slightly ( but does not reach the same value as my speed as it should) and the VMG shows say -4 mph when the heading is about 10 degrees 'right' of the bearing to the destination. It does not appear to be simply a function of being 'left' or 'right' of the bearing. Does anybody else see this behaviour?
You are correct in your explanation, thanks tommays56
The implementation of the VMG calculation by Garmin is incorrect.
I found that while travelling along at a steady speed towards my destination waypoint, as long as the device (12 o'clock position of watch face) was pointing towards the destination (using the red bug and local knowledge) then the VMG value equalled my speed. If the watch was rotated in a different direction then the VMG showed other values. When it was pointing 180 degrees from the bearing to the destination then it showed the negative value of my speed.
This is quite bizarre.
The VMG of the device should not be a function of the direction the device is oriented, it is a wearable device not fixed to a vehicle or boat or plane. Even fixed in a boat or a plane facing forwards, tides or winds could be moving the boat/plane sideways.
Garmin please note! The VMG calculation should be taking the direction of movement of the device towards the waypoint / destination to measure the component of velocity towards that point, even if the device is not moving directly towards it and it certainly it should not depend on the orientation of the device.
I compared the 5X with MotionX-GPS on my iPhone and the VMG on the MotionX-GPS app is showing sensible values, whereas the 5X values were meaningless.
