Don't believe a word!

or: can you trust data recorded by your GPS?

A little rant about the "felt", "assumed" or "wanted" precision of GPS data...


All GPS data that you have recorded with your GPS device (no matter how good or expensive your GPS device is/was) are inaccuratePeriod.

So if you, as a "normal" human being, go biking or hiking through the landscape with your GPS device, keep the following things in mind.

1. Inherent inaccuracy

From the recorded data, you can not tell exactly :

- how far you went

- where (exactly) you where and when

- how much meters up or down you have travelled

This may sound like the Heisenberg (in)equation, but it's just technical limitations.


What you would need for "perfect", precise GPS data? Four things: 


2. Exact Timestamps

2.1. Subject: Timestamp

Known inaccuracy: can be ignored (for most use cases).

We (as a humanity) have been measuring time for so long that -by now- we're pretty good at it. Even in "consumer products". This one measure is pretty precise, no need to worry.


3. Exact Coordinates (x's + y's)

3.1. Subject: Latitude and Longitude

Known inaccuracy: somewhere between 10 - 100m for civil equipment (less only for military use)

The "x" and "y" coordinates recorded by your GPS are not very precise. They are mostly "off" by a couple of meters. And, they keep changing - even though you're not moving at all.


4. Exact Elevation data

4.1. Subject: Altitude

Known inaccuracy: 10-100m (in "common" weather conditions, otherwise more)

Most GPS devices measure altitude "indirectly", by measuring athmospherical pressure and "deducting" your current altitude from that pressure. Your GPS has NO IDEA of how "high" or "low" you REALLY are.

( have you ever stood on a beach with your GPS in hand, and it showed "-32m"? and you could still breathe normally? ;-)

(look here for more details on that subject.)


5. Conclusion

To get an exact "3d"-coordinate of WHERE and WHEN you where on your recorded trip, all four of the recorded coordinates (x,y,z and time) would need to be 100% accurate. But only the time comes somewhat close to being accurate.

So don't over-estimate things like "altitude diagrams", "speed graphs".


Every trip you took is only just as good as you felt it was -

(and it was good you took it in the first place!)