Anomaly in track distance shown in Basecamp Software

Hi all,

Apologies if this has been asked elsewhere, but I haven't been able to find posts on same topic.

When I finish a walk, before switching it off, I take note of the distance showing on my GPS64MAP's screen.

When I subsequently upload the track to Basecamp software, I can then confirm that the distance shown on the GPX track's "properties" tab is the same distance as was shown on the GPS device itself.

However, when I then look (in Basecamp) at the "graph" for the walk. the graph invariably shows a final distance which is sometimes markedly different to what both the device's trip-computer screen, and the Basecamp "properties" page had said.

A typical example: -

I recently did a walk, which, upon my return to the car, was showing as 11.9 miles on the GPS64MAP's screen.

I subsequently uploaded the track to Basecamp, and on the track's properties - tab, it also showed the "Distance" as 11.9 miles.

However, when I then looked at Basecamp's graph for the track, the end point was labelled as 11.3 miles. - a difference of more than half a mile!

Exporting the 3054 leg-distance measurements to an MS Excel spreadsheet, I was able to sum the totals, and the total amounted to 59600 feet.

Dividing that number by ( 1760 x 3 ), to give miles, gave an answer of 11.29 miles - which would be consistent with the graph's stated distance, which was presumably rounded up to 11.3 miles.

My question is, why, if the leg-distance measurements add up to 11.3 miles, does the track properties summary page show the distance as 11.9 miles? (and why did the GPS64MAP also show 11.9 miles on the trip computer screen?).

I would be extremely grateful if anyone could provide the answer to this query.

Best regards,
Mike Valentine

0 BTLAAKE over 7 years ago

A device finds a position solution in 1 second intervals. The trip computer uses this 1 second interval to compute the data fields on your device. It has some degree of intelligence and will ignore most updates when you are stationary.

The actual track log will not be a 1 second interval unless you have set it up that way. When you download the actual track log, it will contain the Trip Computer data followed by the recorded track log. You can export a track gpx file to see what it looks like – it is a text file and can be read with Notepad.

When you click on the graph tap, BaseCamp uses the actual logged data to create the graph.

Since GPS locations contain errors, the errors accumulate and the more points collected the larger the error – assuming curves aren't cut.

Imagine walking a long straight line. If positions were exact, the data would show a straight line. But, since there are errors, the data will show you straying to the left or right of the actual path resulting in a distance greater than the true distance.

Deleting one track point will force BaseCamp to recalculate the Summary date from the remaining track points. Alternatively you can cut and rejoin a track to keep all of the points. This will create one duplicate point (same logged time) that you can delete. Cut the track near the beginning to make it easier to find the duplicate point.

0 LancashireMV over 7 years ago

[FONT=Times New Roman]Hello BTLAAKE,[/FONT]

[FONT=Times New Roman]Thanks for your detailed response – much appreciated.[/FONT]

[FONT=Times New Roman]If I’m understanding what you say correctly, the device trip computer collects position data every second, adds up the total distance travelled, and that is the distance shown on the trip computer’s data field for distance walked.[/FONT]
[FONT=Times New Roman]The track log only records data once every several seconds, so unless you’ve walked in a perfectly straight line between each two consecutively recorded track-log points, the log will very slightly, but continually, be under-recording the distance travelled.[/FONT]

[FONT=Times New Roman]That does sound entirely logical. – It’s effectively recording the straight line distance between each consecutive pair of track log points, whereas in reality the walker wouldn’t have walked in a perfectly straight line between the logged points.[/FONT]

[FONT=Times New Roman]In my case, the GPSMAP64’s device settings are left on auto, so each “leg-length” recorded on the track log varies considerably, as does each “leg-time”. (anything between 1 second and 50 seconds in the case of the example I gave in original post).[/FONT]

[FONT=Times New Roman]In that particular example, a total of 3053 track “legs” were recorded, and the walk, (a mountain hike in winter conditions), took 10hrs.06min. to complete.[/FONT]

[FONT=Times New Roman]The “average” time for each track log leg 36360(seconds) /3053(legs), thus being approximately 12 seconds. – Giving plenty of time between any two recorded points to meander away from a dead straight line between the two.[/FONT]

[FONT=Times New Roman]Once again, many thanks for your answer – my curiosity is fully satisfied![/FONT]

[FONT=Times New Roman]Best regards,[/FONT]

[FONT=Times New Roman]Mike Valentine[/FONT]

0 LancashireMV over 7 years ago

I must apologise for the very small text in the above reply.

I created the reply in MSWord, and copied and pasted it into the above post.

I have tried several times to change the font/font size via the advanced editor facility, but each time I submit the edit, the screen locks up with the "working" icon continually showing.
I am then unable to navigate away from that screen other than by logging out of the forum - which leaves the small sized text response unedited, and still showing above.

Regards,
Michael Valentine.