Help Irrational car journey leg speeds

[Bob Hawkins]

Something occurred on my most recent long motorway journey that I have not encountered previously: I use Locus Map, a mobile navigation app, in which I have its track recording panel visible whenever I travel by car. I have found it to be very reliable until my most recent journey. I use cruise control as much as possible, which my first example file, Journey legs 1, demonstrates. I saw at one point, however, the maximum speed displayed was 80+mph. I see, upon investigation, 300 instances of legs up to 108mph and erratic changes of speed as in Journey legs 2. My similar journey of 7374 legs a few days earlier in the reverse direction shows a total of three irrational legs: one at 86mph and two at 104mph - that is all. It is difficult for me to understand that gps vagaries can apply so differently on two similar journeys. I wonder if something else might be the cause?

Journey legs 1

Journey legs 2

 

[mikedt]

Occasional loss of GPS reception possibly, and the log showing high speeds as it catches up to where you actually are. Things like urban canyons, mountains, tunnels, etc. that can disrupt GPS reception.

 

[Bob Hawkins]

mikedt
I thank you for your reply. I accept the limitations of gps, of course. This does not explain the difference encountered on two almost identical motorway journeys, merely travelled in opposite directions three days apart.

 

[mikedt]

GPS satellites will be in different positions on different days as they orbit, differing weather conditions, what was clear reception one day might not be the next time. The fact it's over a distance only what 265ft, that's very minor, and momentary loss or inaccuracy of GPS location data could cause that AFAIK. Is that a built-up area, tall buildings? Where did you keep the phone, top of the dash with a clear view of the sky, or down by the transmission or something? In fact looking at that log, the speed seems to be jumping all over the place every 2-3 seconds. If you were in a car, that really should be pretty much constant speeds shown. Average seems to be around 60mph, so I guess that was on a highway.


Valid GPS location, but I seemed to be going quite fast that day.

 

[Bob Hawkins]

I have checked the .gpx files of two motorway journeys earlier this year: no leg in either direction exceeded 73mph. I have one thought, irrational as it might seem: I changed the cable I use to connect my 'phone to the car's cigarette lighter input before my most recent motorway journeys because the previous one had become too loose a connection in my 'phone. I am replacing it with another and shall view future records with interest.

 

[mikedt]

Might also be the way the app is logging and calculating the data. The 41 mph for 3 seconds on a motorway is as wrong as the 95 mph. 60-70 mph is probably the average. I've never tried logging GPS journey data like that myself. That "836 km/h" I showed was only varying by a few km/h, plus or minus.

 

[Bob Hawkins]

mikedt
You are quite correct regarding "The 41 mph...", of course.