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From: Catrike Ryder <Soloman@old.bikers.org>
Newsgroups: rec.bicycles.tech
Subject: Re: Garmin altitude problems
Date: Sun, 11 Aug 2024 21:00:04 -0400
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On Mon, 12 Aug 2024 07:43:11 +0700, John B. <slocombjb@gmail.com>
wrote:

>On Sun, 11 Aug 2024 16:09:37 -0400, Catrike Ryder
><Soloman@old.bikers.org> wrote:
>
>>On Sun, 11 Aug 2024 17:46:37 -0000 (UTC), Ted Heise <theise@panix.com>
>>wrote:
>>
>>>On Sat, 10 Aug 2024 14:12:12 -0400,
>>>  Frank Krygowski <frkrygow@sbcglobal.net> wrote:
>>>>  On 8/10/2024 12:21 PM, Jeff Liebermann wrote:
>>>> > 
>>>> > GPS is far more accurate measuring 2D surface distances than 
>>>> > 3D measuring altitude. "Grade, Elevation, and GPS Accuracy 
>>>> > FAQ" 
>>>> > <https://support.ridewithgps.com/hc/en-us/articles/4419010957467-Grade-Elevation-and-GPS-Accuracy-FAQ>
>>>> > 
>>>> > Garmin rates their GPS elevation accuracy at +/-400 ft and 
>>>> > their barometric elevation accuracy at +/-50 ft.
>>>> > 
>>>> > If anyone wants me to explain why in excruciating detail if 
>>>> > anyone is interested.  The short version is that GPS works by 
>>>> > measuring the Doppler shifts produced by multiple satellites 
>>>> > across the sky.  The motion of the satellite has to be mostly 
>>>> > toward or away from the GPS receiver.  For altitude, that 
>>>> > happens best when the satellites are near the horizon.  A 
>>>> > satellite directly overhead does not move toward or away from 
>>>> > the GPS receiver and is therefore useless for computing 
>>>> > altitude.  Since the horizon tends to be cluttered with 
>>>> > mountains, trees, buildings, reflections etc, the signals from 
>>>> > the best positioned satellites can't be used.  Just to make 
>>>> > things difficult, the better GPS antennas are designed to 
>>>> > suppress ground reflections which makes using low elevation 
>>>> > satellites even more difficult to use.
>>>
>>>Yep, and nice explanation.
>>>
>>>
>>>>  IIRC, Jobst explained that with some trigonometric detail many 
>>>>  years ago.
>>>> 
>>>>  And it's all irrelevant for Florida bike paths.
>>>
>>>LOL  Reminds me of a tandem rally in Sebring a few years ago.  At 
>>>one point we rode with a couple from Ft. Lauderdale and had a hard 
>>>time keeping straight faces when they complained about the hills.
>>
>>I don't ever ride with anyone, but I do hear from northerners at rest
>>stops complaining about the heat and humidity. I'm going to ride
>>Tuesday morning and I expect to see 90 degrees and 80+ humidity.
>
>Having have ridden in tropical countries for much of my life the
>solution is to start your ride at sun up and finish by 09:00 or maybe
>a bit later.
>
>Here  it is the rainy season - the coolest time of the year - and at
>the moment, 07:24, the temperature is 28 degrees C  (82 F) and it is
>expected to be 31 degrees (87 F) by noon.

I will start very early, but my rides last at least four hours, maybe
longer, so I'm going to experience 90 degrees or more.  I'll be ok as
long as I keep moving and generating a breeze and only stop in the
shade. I'm a prolific sweater and I'll be taking six 24 oz bottles of
Gatorade and/or water and there'll be places I can stop and fill them
up.