GPS time?

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psmiffy
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GPS time?

Postby psmiffy » 26 Mar 2017, 11:03am

I have a GPS64s which sits on my handlebars and records where ive been when im touring

At present im writing up my journal from last year - Using basecourse and the GPS track to check whereabouts I took photographs - I painstakingly made sure the camera date and time were correct at all times and matched local times - The GPS at all time displayed local time without me adjusting it anywhere

Using basecourse to check locations of the photos - first time i've used the data like this - it immediately became apparent that track times are GMT - is that always true - or do other devices operate in local time - what does the likes of strava show?

psmiffy
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Re: GPS time?

Postby psmiffy » 26 Mar 2017, 11:23am

Having just joined Strava to have a look it seems that strava does not care :D - more interested in how I rated against the likes of Dick van den Berg - Im hoping that doesn't translate to something rude :D

What else can I look at to see if it shows tracks with real time?

psmiffy
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Re: GPS time?

Postby psmiffy » 26 Mar 2017, 8:55pm

No worries - I think ive managed to answer my own question - time stamps on the GPX file are always UTC regardless of what time the unit is operating in - Ive checked it this afternoon by setting it to Hong Hong time and going out tomorrow morning to get a track - be useful if the GPX file captured local time but never mind - I really ought to read the manual sometime

matt_twam_asi
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Re: GPS time?

Postby matt_twam_asi » 26 Mar 2017, 11:59pm

I discovered this exact problem recently when using Darktable to add GPS positions to some of my pictures from Switzerland. I *think* that the GPS clocks are set to GMT, then converted to UTC in the receiver, although they're pretty much interchangeable (give or take a handful of leap seconds).

I've also chosen to turn off the fancy daylight savings adjustment on my camera as this *also* messes up the matching between photos and gpx tracks. Less is more in this case.

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Cunobelin
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Re: GPS time?

Postby Cunobelin » 27 Mar 2017, 6:41am

psmiffy wrote:I have a GPS64s which sits on my handlebars and records where ive been when im touring

At present im writing up my journal from last year - Using basecourse and the GPS track to check whereabouts I took photographs - I painstakingly made sure the camera date and time were correct at all times and matched local times - The GPS at all time displayed local time without me adjusting it anywhere

Using basecourse to check locations of the photos - first time i've used the data like this - it immediately became apparent that track times are GMT - is that always true - or do other devices operate in local time - what does the likes of strava show?


Some programs like "GeoTag" have a slide bar to allow you to correct a time difference and this is a simple and easy way round the problem of misaligned camera and GPS timings

manybikes
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Re: GPS time?

Postby manybikes » 27 Mar 2017, 9:11am

Another issue (with the 60 versions) is that it writes up a gpx track at midnight and starts a new one. This occurred to me in Australia.
I saw the track truncated but once I realised I found I could join the split parts together using mapsource on the computer.

psmiffy
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Re: GPS time?

Postby psmiffy » 27 Mar 2017, 9:48am

manybikes wrote:Another issue (with the 60 versions) is that it writes up a gpx track at midnight and starts a new one. This occurred to me in Australia.
I saw the track truncated but once I realised I found I could join the split parts together using mapsource on the computer.


I have actually found that "issue" to be quite useful on the occasions that I have forgotten to save the days track before I rolled into my sleeping bag - another one of those things i really ought to look into how it works properly - my GPS use is not very sophisticated :D - In Australia did it save the track at midnight local time?

psmiffy
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Re: GPS time?

Postby psmiffy » 27 Mar 2017, 9:53am

Cunobelin wrote:Some programs like "GeoTag" have a slide bar to allow you to correct a time difference and this is a simple and easy way round the problem of misaligned camera and GPS timings


I looked at Geotag when I was toying with geotagging the photos - and a few other programmes - they all seemed to alter the Camera time permanently - I prefer to keep the metadata intact - for me it is no big deal when viewing the GPX data on basecourse to subtract 1 - is the Geotag slider a temporary adjustment?

psmiffy
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Re: GPS time?

Postby psmiffy » 27 Mar 2017, 10:00am

matt_twam_asi wrote:
I've also chosen to turn off the fancy daylight savings adjustment on my camera as this *also* messes up the matching between photos and gpx tracks. Less is more in this case.


This is something i might consider for the future along with keeping the camera time on GMT - although my preference is for photos to be at the correct (local) time and date - between the plethora of camera menu settings and all the metadata that there is within camera image files these days there ought to be room for both camera time and UTC - what do GPS cameras record?

Warin61
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Re: GPS time?

Postby Warin61 » 27 Mar 2017, 10:33am

psmiffy wrote:
manybikes wrote: In Australia did it save the track at midnight local time?


Yes .. it saves at midnight local time .. don't know it that includes daylight saving or not .. I'm not awake then, or if I am I'm certainly not looking at the GPS. I don't recall having to switch the GPS to local time .. I think as it knows where it is it just uses the local time.

GPS time link to explanation in much detail .. http://www.oc.nps.edu/oc2902w/gps/timsys.html

Don't have one of them cameras with GPS.. would make it interesting crossing a time zone .. the next photo could be taken at a time before the last photo :?

psmiffy
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Re: GPS time?

Postby psmiffy » 27 Mar 2017, 11:44am

Warin61 wrote:
psmiffy wrote:
manybikes wrote: In Australia did it save the track at midnight local time?


Yes .. it saves at midnight local time .. don't know it that includes daylight saving or not .. I'm not awake then, or if I am I'm certainly not looking at the GPS. I don't recall having to switch the GPS to local time .. I think as it knows where it is it just uses the local time.

GPS time link to explanation in much detail .. http://www.oc.nps.edu/oc2902w/gps/timsys.html

Don't have one of them cameras with GPS.. would make it interesting crossing a time zone .. the next photo could be taken at a time before the last photo :?


Having delved into the myriad of settings on the GPS I now know that there is two options - Automatic - which is what ive been using up to now and the facility to set the units time to whatever time zone with or without daylight saving - I will probably carry on with the automatic as it is good to have at least one reliable time reference when im on tour - on the occasions that I have forgotten it made shopping a bit more difficult - ditto i have a dual time watch - going to be uni time next time i go on tour - forgetting to put it back to the right time display after cooking the pasta has caught me out a couple of times :D

Difference between GPS UTC and ordinary UTC/GMT is so small as to be of no practical use (unless of course you are an airline pilot where i suspect 13 seconds or whatever might be dodgy) - the time zone thing might make filing the photos a bit more tricky - but not an insurmountable problem - :D

Moral of the story at the moment is that I ought to learn more about what the GPS 64 is capable of - On my last tour (my first with the 64) I was a couple of weeks in and it came as a bit of a surprise that with the City navigator mapping that i could find supermarkets and occasionally campsites quite easily using it :D

IanW
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Re: GPS time?

Postby IanW » 27 Mar 2017, 12:11pm

Timestamps are often (but not always) recorded as a number of seconds elapsed since the start of some arbitary start-of-epoc.
There are several different epochs used across modern accurate time-keeping / timestamping.
This URL might give you some context http://www.leapsecond.com/java/gpsclock.htm.
And this question on Stackoverflow http://stackoverflow.com/questions/20521750/ticks-between-unix-epoch-and-gps-epoch will give you an indication of how easily you can get errors in timestamping.
And this is much further complicated when timestamps are interconverted between some arbitary number of seconds since the start of some epoch and some local-time (+/- daylight savings time) representation that human beings understand.
Oh and to further complicate things we have "leap seconds" https://www.timeanddate.com/time/leap-seconds-background.html that get into some timestamping systems and not others.

manybikes
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Re: GPS time?

Postby manybikes » 27 Mar 2017, 2:34pm

My problem was that I thought the gps60 was showing the correct time but when I found out about the issue I noticed that, under settings, it was still recording under U.K. time. This is further compounded by the fact that I was frequently crossing and recrossing different time zones, some only 30 minutes change. Very close to 12 hours from U.K. hence the misinterpretation
My camera is new *( know don't take a new camera on a trip but this was replacing a stolen one) and I didn't consider changing the clock until half way through. Geo location is going to be a bit technical therefore so I'm relying on pen and paper, i.e my diary

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andrew_s
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Re: GPS time?

Postby andrew_s » 27 Mar 2017, 2:57pm

Warin61 wrote:Yes .. it saves at midnight local time .. don't know it that includes daylight saving or not .. I'm not awake then, or if I am I'm certainly not looking at the GPS. I don't recall having to switch the GPS to local time .. I think as it knows where it is it just uses the local time.

It doesn't actually do any save at midnight; after all, it's probably turned off.
What it does is save to a file named after the current date.

Had you been riding a DIY 600 km audax over the weekend, from Saturday afternoon to Monday morning, the file for Sunday would only have 23 hours worth of data in it due to the clock change.

GPS time is the number of seconds since midnight on the 6th Jan 1980 (the start of week 1), with no allowance made for leap seconds. The satellites also transmit the offset between GPS time and UTC, so your gps can show the correct time.

psmiffy
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Re: GPS time?

Postby psmiffy » 27 Mar 2017, 4:43pm

andrew_s wrote:Had you been riding a DIY 600 km audax over the weekend, from Saturday afternoon to Monday morning, the file for Sunday would only have 23 hours worth of data in it due to the clock change.


Not according to what ive been reading over the weekend - all the timestamps for the GPS are UTC - and i assume that a UTC Sunday has 24 hours in it same as the rest of the year