Is there time data on tracklog?

[Somnolent]

When I record a track to an SD card in my Legend HCx.... is there any time data attached to the trackpoints?   If so, how can I extract it ?

Reason I ask is that I competed participated in an endurance event (on foot not bike) last w/e.  It was supposed to have chip timing, but at several of the intermediate checkpoints the weather put paid to the electronics. Although I recorded the time of arrival at most of the checkpoints manually, I didn't note the departure time so I don't know my speed over each leg of the course.

[Greenbank]

Can't speak for the Legend HCx exactly, but the tracklog from my bog standard yellow eTrex looks like this when downloaded as a GPX:-

<trkpt lat="52.770102024" lon="-0.377032757"><ele>8.789185</ele><time>2009-07-26T14:58:28Z</time></trkpt>
<trkpt lat="52.772355080" lon="-0.376732349"><ele>9.750610</ele><time>2009-07-26T14:59:03Z</time></trkpt>
<trkpt lat="52.774500847" lon="-0.376625061"><ele>11.673220</ele><time>2009-07-26T14:59:38Z</time></trkpt>
<trkpt lat="52.776646614" lon="-0.376303196"><ele>12.153810</ele><time>2009-07-26T15:00:13Z</time></trkpt>
<trkpt lat="52.778813839" lon="-0.376431942"><ele>12.634400</ele><time>2009-07-26T15:00:48Z</time></trkpt>
<trkpt lat="52.781302929" lon="-0.376925468"><ele>10.711790</ele><time>2009-07-26T15:01:23Z</time></trkpt>
<trkpt lat="52.783770561" lon="-0.377268791"><ele>11.673220</ele><time>2009-07-26T15:01:58Z</time></trkpt>
<trkpt lat="52.786066532" lon="-0.376946926"><ele>15.037720</ele><time>2009-07-26T15:02:33Z</time></trkpt>
<trkpt lat="52.788641453" lon="-0.376431942"><ele>13.595830</ele><time>2009-07-26T15:03:08Z</time></trkpt>
<trkpt lat="52.791109085" lon="-0.376303196"><ele>16.479740</ele><time>2009-07-26T15:03:43Z</time></trkpt>
<trkpt lat="52.793705463" lon="-0.376303196"><ele>15.518430</ele><time>2009-07-26T15:04:18Z</time></trkpt>
<trkpt lat="52.796237469" lon="-0.377011299"><ele>14.557010</ele><time>2009-07-26T15:04:53Z</time></trkpt>
<trkpt lat="52.798619270" lon="-0.378062725"><ele>14.076420</ele><time>2009-07-26T15:05:28Z</time></trkpt>
<trkpt lat="52.800743580" lon="-0.379457474"><ele>15.037720</ele><time>2009-07-26T15:06:03Z</time></trkpt>
<trkpt lat="52.803082466" lon="-0.380530357"><ele>16.479740</ele><time>2009-07-26T15:06:38Z</time></trkpt>
<trkpt lat="52.805206776" lon="-0.380980968"><ele>19.844360</ele><time>2009-07-26T15:07:13Z</time></trkpt>
<trkpt lat="52.807481289" lon="-0.380852222"><ele>18.883060</ele><time>2009-07-26T15:07:48Z</time></trkpt>
<trkpt lat="52.809884548" lon="-0.380101204"><ele>18.883060</ele><time>2009-07-26T15:08:23Z</time></trkpt>
<trkpt lat="52.812137604" lon="-0.379264355"><ele>21.286380</ele><time>2009-07-26T15:08:58Z</time></trkpt>
<trkpt lat="52.813897133" lon="-0.377976894"><ele>24.170290</ele><time>2009-07-26T15:09:33Z</time></trkpt>

On the bog standard yellow eTrex however, If you choose to the save the active log then it drops the timestamps and only records the position/elevation.

Someone will be along to explain how to get it out of the Legend HCx sometime soon...

[frankly frankie]

The only way I know of to record a track onto the SD card on that model, is to have it log automatically.  (edit - this is not the default setup.)  These logs are .gpx files and do include the timestamp (UTC) as in GB's example above.

The filenames indicate the date each logfile was written.

You can just remove the card and put it in a card reader and copy the files to your PC, then do whatever with them.   Or you can connect the GPS and push it into 'data storage mode' to see the card as an external drive.  Just connecting in the normal way, you won't see the card.  If you don't know how to do 'data storage mode' then its easier just to use a separate card reader.

Your 'Active Log' may also contain the information you want (if it was a recent event and you haven't done much since) and that is very easy to download just through the normal connection methods, because it exists in the GPS 'user memory', not on the SD card.

If you have 'saved' your track on the GPS - that also is easy to download because again that is in 'user memory' and not on the card - but it won't have the timestamps you need and all in all is a rather poor record.

All this applies to the model mentioned (Etrex C type) and others of the same generation (60 series) but not to the newer Edge, Colorado, Oregon, Dakota models which have their storage organised differently.

[Somnolent]

OK I got that - but ploughing through 5000 lines looking for where I was "stopped" is not something I look forward to.

I got the data into an Excel s/sheet so I can analyse - thinking I can calculate speed between each trackpoint and that hopefully will show up times when I was moving/stopped/ as good as stopped.   Appreciate it's not going to be that accurate because of positional accuracy of the unit, but is this sensible approach ?

From a quick look at the borders of the OS map in question (lake district) I estimate a degree of latitude to be 111 km and a degree of longitude to be 64.5 km     Is this about right?

[Panoramix]

From a quick look at the borders of the OS map in question (lake district) I estimate a degree of latitude to be 111 km and a degree of longitude to be 64.5 km     Is this about right?

That is correct, 1 minute of latitude = 1 nautical mile
1 degree = 60 minutes = 60*1852 = 111.12 km

For longitude I think that you multiply by the cosine of latitude as a degree of arc decrease as you go North

111.12 * cos 54.5 = 64.45km.

I have looked into this recently and found that the magic word to google is harversine formula.

[Craig]

OK I got that - but ploughing through 5000 lines looking for where I was "stopped" is not something I look forward to.

I got the data into an Excel s/sheet so I can analyse - thinking I can calculate speed between each trackpoint and that hopefully will show up times when I was moving/stopped/ as good as stopped.   Appreciate it's not going to be that accurate because of positional accuracy of the unit, but is this sensible approach ?

No. The sensible approach is to load the track into some software which will plot it on a map and a graph of speed, let you select different parts of it, and tell you the speed for each, without any messing about with spreadsheets.

I recommend SportTracks for this: SportTracks
Or you could use something like MapSource or Memory Map etc.

[Somnolent]

Well I did it the hard way with the spreadsheet....I'll look at Sportracks another day.

I got one very weird anomaly though.....
Event started with a 6k loop.   Elevation data at start is correct around 146 m
As loop progress elevation on tracklog climbs up beyond 400m (on ground and on OS map it never rises above 200 m)
As I get back to start point after 55 mins, the tracklog shows I fell off a 250 m cliff !
(I would have noticed I think!)

I know the units like the Vista with barometric altimeter are more accurate than the Legend- but this is more than a "blip"
I've got at least 50 datapoints over about 10 minutes that are "off"  but then another 18 hours of recording with no anomalies I can see.
Anyone else seen anything like this?

[frankly frankie]

It can take quite a long time after switch-on for the vertical position to stabilise - 20 minutes has been quoted - which would easily cover your 6km. 
It pays to switch the GPS on half-an-hour before starting the ride, if practical.

And I'm not convinced the Vista would do any better in this instance, unless you go to the length of manually calibrating the barometer before starting.

[Greenbank]

It can take quite a long time after switch-on for the vertical position to stabilise - 20 minutes has been quoted - which would easily cover your 6km.

"on foot not bike"

I'm guessing the drift is more to do with the orientation of the GPS. The Legend (as with other eTrexes) works best when near horizontal. Sticking it vertically in a backpack or bag will make it more prone to spurious readings.

[Somnolent]

Unit was switched on in horizontal position, allowed to stabilise for about 5 mins and then mounted vertically (on rucksac strap) for full duration of al that 55 minute loop.

I've since checked the horizontal and vertical accuracy for that loop - everything is good for the first 45 minutes - (so I doubt it's a "taking time to stabilise" issue)

Then for the next 8 minutes the horizontal position drifts off (up to about 250 mtrs off the track marked on the OS map) - the vertical position ditto.   Inaccurate position (and elevation) is maintained for about 2 minutes (about 10 trackpoints). Then suddenly between 2 trackpoints (13 seconds apart) my "elevation" drops 264 mtrs and my horizontal position "jumps" 254 mtrs.

Here's the track overlayed (in puple) on the OS map.



The route is (anticlockwise) Dalemain, Stainton, Dacre, & back to Dalemain, all on Public Footpath

And you see the "jump" on mapsource:
 

[andygates]

Drift is a known annoyance in the etrexes.  It's a bug somewhere in the device range - not a fault with your unit or your way of working.  You get occasional, sporadic moments of insanity.

I *think* it's a rounding error that only manifests when certain satellite configurations are in the sky, like all three having close angles - you've got three good sats and high confidence, but the maths engine chokes a little.  That's my guess, anyhow.

It's only a problem for us log-obsessives

[andrew_s]

OK I got that - but ploughing through 5000 lines looking for where I was "stopped" is not something I look forward to.

Open the gpx file in Mapsource
On the "tracks" tab, right-click on a track and "show selected track on map"
Zoom to the vicinity of where you stopped
right-click on track using the arrow tool, and select "track properties"

Track properties gives each trackpoint with the distance (and speed if you have time data present) since the previous trackpoint.
The current trackpoint is highlighted both in the properties and on the map. Scroll up & down using arrow keys.

[frankly frankie]

Then for the next 8 minutes the horizontal position drifts off (up to about 250 mtrs off the track marked on the OS map) - the vertical position ditto.

Were you travelling quite slowly (eg up a steep hill) at that point?  The 'H' etrexes have a well-documented problem at low 'walking' speeds.  As a cyclist, its never bothered me.
The sat constellation shape, as Andy says, would surely also play a part - your body is masking half the sky straight away, it only needs a bit of a bank ahead, and an unlucky arrangement of sats, for you to be down to 3 or less 'visible' (though you should then get a bleep and a prompt).

[Somnolent]

Were you travelling quite slowly (eg up a steep hill) at that point? 

For first 40 minutes we'd been averaging 6 kph over rolling terrain.
then 5 minutes jogging (about 9 kph) on slight downhill
The "drift" off track started about the same time as we slowed to a walk again - but on easy level ground where we'd be doing about 6.5 kph   & good all-round visibility of sky throughout.

Weirdly the error corrected itself as we went through a brief belt of woodland which you would have thought might have degraded the accuracy rather than improved it.

And we never got the problem again over the next 18 hours of solid recording- despite some ferociously steep bits where speed dropped to 3 or 4 kph - and the terrain could well have masked half the satellites.

[andygates]

Space goblins.  It was the space goblins.

[frankly frankie]

6kph is well below the 'slow' threshold where some people have reported drift with the Etrex 'H' models.  Some GPS models are better at motion (routing and tracking), and some are better at giving a static 'I am here' fix.  The H models fall into the first category.
You find loads of bitching about it on Groundspeak where the forumites are mostly more interested in accurate static fixes.