Loss of altitude data whilst tracking

There are no English-language forums for handhelds other than iR devices. There are German forums for handhelds.

I am unable to tell just where in the track the problem occurred. There are a couple of places where GPS reception might be challenging. Because of the geometry, the error in altitude would typically be 2 to 3 times the error in horizontal position. However, this would not typically manifest as the type of discontinuity shown in the altitude chart. If you are using barometric pressure to determine altitude (I forget how the settings work on 66 series), this would suggest a problem with the barometric sensor. For example, partially blocked air inlet. (I would not expect an actual change in pressure due to something like a weather front to manifest as a horizontal line.) But I'm just guessing.

Thanks for taking the time to reply - Yes... Barometric Altimeter. 
I will try and go out for a short test route soon and so some analysis.. 
Fingers crossed its not a fault on the sensor!

Sorry I have no technical help but it sure looks faulty - to lose elevation for over 1km and then to resume normally.  I just wanted to thank you for the pleasant diversion to have a look at the area via Google Earth from the other side side of the world.  Where the elevation started to flat-line is about where the attached waypoint is.  Pretty rugged terrain and good photos there too.  Thank you.  Hmm, could not attach a .kmz file here - only a URL - so here are the coordinates copied from Google Maps: 53.10364976186486, -4.014684938483783

Thanks - Yes - I am concerned its a fault... 
Link to the files to look further..
Hopefully someone from Garmin may be able to shed some light on the issue..

https://www.dropbox.com/sh/i0swej7in9p5g1h/AADo-QGuajAp8PsTVuBgUKS2a?dl=0

At least on the 66i, the air inlet is on the mounting spine. So if you have a belt clip ('biner or whatever) mounted, the inlet will be behind it as well. Although the clip does not itself interfere, it does not take much to block the pinhole.

This is a peer to peer forum, not a support channel. Occasionally, someone from Garmin will comment. But if you want assistance, open a support ticket.

Yes - done that.. Thanks.. Is it the small hole at the top between the power and the antenna?
Im wondering if some dust or dirt got in it!

I don't think so. At least on the 66i, that one is an LED.

On the 66i (no idea about the 66s), it's a slightly smaller hole at the top center of the rear mounting spine. If in the same location, it would in the area circled below. This is the back of the 66s, but the figure on the Garmin sales site is not clear enough to see a pinhole of this size.

ah - I see it - it appears to be bottom left on my clip.. 


Tiny hole - which does get fairly obscured with the carabiner clip..

mmm.. maybe it did get blocked whilst in side of rucsac.. - Thanks for this suggestion.

Just an update to assist others if they experience this..
Garmin came back with this link..
https://support.garmin.com/en-GB/?faq=FhOYuggxmV6Atph276U4h8&identifier=3316539800&searchQuery=elevation&tab=topics

Over the next few trips I will monitor this and change some settings slightly. 
The weather was fairly stable and it dropped out whilst on a plateau so there was no huge difference in elevation..  

Anyways- annoying.. but hopefully a one off!