Basecamp Rounding Errors

If I import a GPX track into BaseCamp with a trackpoint like:

<trkpt lat="33.9856342413" lon="-120.0267465413">

  <ele>36.19</ele>

  <time>2012-03-22T17:20:47Z</time>

</trkpt>

When I export the track from BaseCamp as GPX the same trackpoint is now:

<trkpt lat="33.985634241253138" lon="-120.02674654126167">

  <ele>36.189999999999998</ele>

  <time>2012-03-22T17:20:47Z</time>

</trkpt>

This makes me reluctant to use BaseCamp to store and organize my tracklogs; it also causes problems with other software (e.g. Garmin Connect).

I'm now using BaseCamp 3.3.2, but had the same issue with previous versions. Any chance this will ever be fixed?

0 Former Member over 13 years ago

Not sure what the problem is.

Are you worried that 36.189999999999998 isn't close enough to 36.19? The difference is way smaller than a consumer GPS can even measure whether the units are feet or meters.

0 POPEJ. over 13 years ago

Actually the difference between original gpx and BaseCamp is less than a hair's breadth :)

0 Former Member over 13 years ago

But the real issue is, why is Basecamp changing the values at all?? There should be no reason for any sort of conversion in this use case.

...ken...

0 Former Member over 13 years ago

But the real issue is, why is Basecamp changing the values at all??

This has to do with the way computers represent numbers. They operate using binary (0/1) and perform math in base 2. Since humans do math in base 10 ("decimal") computers represent numbers in IEEE floating point format. Conversion to/from human readable text and IEEE representation often introduces small errors.

PS: In my youth I owned a TI-30 calculator. If you divided 2 by 3 you'd see

0.6666667

If you then multiplied by 3 you'd get

1.9999999

Now any math teacher will tell you the real answer is 2 but again it's a side effect of how computers do math.

0 POPEJ. over 13 years ago

This is not so simple. BaseCamp is probably using double precision floating point with about 15-17 significant digits. Precise output should be like:
lat="33.985634241299998" lon="-120.02674654130000"

0 Former Member over 13 years ago

BaseCamp is probably using double precision floating point with about 15-17 significant digits.

Agreed.

Precise output should be like:
lat="33.985634241299998" lon="-120.02674654130000"

Depends on what else they do with the numbers once they're in BaseCamp. Also the conversion from ASCII text to double will introduce some "error".

Bottom line is that just because you can show "15-17 significant digits" doesn't mean they're all meaningful (or correct).

0 POPEJ. over 13 years ago

Depends on what else they do with the numbers once they're in BaseCamp.

Yes, this was the question that KGANSHIRT asked.

Also the conversion from ASCII text to double will introduce some "error".

No, these example values include errors from all conversions.

0 Former Member over 13 years ago

Man, I really love this place... there are some well educated boys and girls who hang out here :D