Changing time offset in GPX?

During my walking holiday in the Pyrenees I forgot to change the
'Offset' on my Garmin GPS 12. However, no problem, after downloading
the tracks to my GPS Utility program on my PC I can adjust them there,
adding an extra hour. But when I then save the file in GPX format and
re-open it in Memory Map, the time remains unaltered; no added hour.

I gather that this is a characteristic of GPX files, i.e. they handle
only UTC (GMT?) times, no offsets.

So:
Q1: Anyone know if Memory Map has some setting somewhere that I've not
discovered that will allow me to adjust the time?

Q2: Failing that, can I hack the GPX file in my text editor (albeit
laboriously) to get same result?

--
Terry, West Sussex, UK

Indeed, GPX files record the UTC (=GMT) time. If you want to display the
local time on your PC (in Oziexplorer, Mapsource, ...) you have to put the
PC in the right time zone (so for the Pyrenees that is GMT +2).

Hi Terry,

The fourth line of a GPS Utility file saved in .txt format is ..

S Timezone= 00:00 for UTC
or
S Timezone=+01:00 for BST

If you add the extra hour in GPS Utility and save the file in .txt
format, then use a text editor to change this line back from
S Timezone=+01:00
to
S Timezone= 00:00
and then re-save it, when you next open it in GPS Utility it will
think the times are in UTC. You can then export it in .gpx format
with the times you want.

Eddy [UK]

Many thanks Eddy, that's very helpful.

I'm still hazy about a few aspects though. I think one reason I've got
myself confused is that I looked for such an indicator line previously
but couldn't find one. After some further experiment this morning I
assume the reason is that it only appears if Options|Data
Set|Timezone|Localtime is enabled.

My Garmin GPS 12 is set to an Offset of +01:00 (for BST). I recorded a
few seconds of track, starting at 08:16. Then I downloaded it as usual
to my PC with GPS Utility. The Time column, headed 'UTC', shows
trackpoint 1 as 07:16, i.e. the UTC time (old-fashioned 'GMT' to me).
After saving it as Test.txt, my editor shows the first 5 lines as:

H SOFTWARE NAME & VERSION
I GPSU 4.20 01 REGISTERED to 'Terry Pinnell'
S DateFormat=dd/mm/yyyy
S Units=S,M
S SymbolSet=2

But on checking, I saw that GPSU had Localtime unchecked. After
enabling it and setting it to +01:00 and saving again, that 4th line
you described duly appeared.

I now realise that I've naively been assuming that GPSU
*automatically* records the data recorded by my GPS 12, including the
Offset. Even now I'm still not entirely clear about the effect of
setting my GPS 12 Offset? Does it change the actual data recorded? Or
- as it seems from my experiments - does that remain at GMT
regardless? IOW, requiring me to change it via GPSU's Offset setting
if I want anything other than raw GMT? That would imply that the only
value in setting it is to ensure its screen displays local time?

As mentioned previously, in France I should have changed the GPS 12
from +01:00 to +02:00. Another complication muddying the water is that
my tracks were temporarily stored on my Pocket PC during the holiday.
After each day's walk, because of the 1024 trackpoint limit, I used
G7toCE to download the track to my Pocket PC. Then I saved it (for
safety) in two separate formats, 'Fugawi Trk' (.TRK), and 'Ozi Plot'
(.PLT). Then, on return home, I synced them to my PC and opened them
in GPSU. (BTW, the Fugawi type prompted GPSU to display a message that
rattled me a bit: "PCX5 file. Change File Saving format to PCX5?" But
I don't think a Yes or No answer made any difference to the trackpoint
times.)

The bottom line appears to be that I should have:
1) set my GPS 12 to +02:00
2) taken care that G7toCE wasn't altering or overriding any offsets or
other time settings in its Configuration menu
3) set GPSU to +02:00 before I opened the files on my PC.

Given all this, it's tempting to work in GMT throughout! But then all
my photos (taken on a camera with time correctly set to local time!)

--
Terry, West Sussex, UK

Terry Pinnell wrote:


The time in GPS data from satellites is always GMT. Your handheld is
reading that and, for your convenience, converting it for display on the
GPS as per your settings. The track data will always have the time in
GMT.

When you download the track to GPSU, there is a setting that takes the
GMT time from the track and displays it in your local time or any time
zone you prefer in the track data listings.

I'm not sure what happens from that point on if you save the data in
files or other formats but I suspect it is always converted back to GMT
to provide a universally understood starting point for time.

But if GPSU uploads track data (from any source or file) back to the
GPS, it converts the times back to GMT time in the process and then your
GPS settings determine what you see displayed for time.


That is exactly what aviation, military, communications, and other
people do in order to have a commonly understood time base. When
someone says something will occur at 120600Z AUG 2006, no matter where
you are in the world, you have a mental understanding of your
relationship (+ or - so many hours) to that time (and maybe even two
clocks showing them both).

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)