Auto elevation calibration

Chat about Hammerhead's Karoo in general.
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nnovod
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Auto elevation calibration

Post by nnovod » Tue Nov 27, 2018 12:51 pm

Does anyone understand how this works?

Does auto-calibration just set the elevation at the start of the ride based on barometric pressure? Does it do anything else? After the elevation is set at the start of a ride, either via an auto or manual setting, will the # of feet/meters of ascent/descent be different if elevation is set to manual vs. automatic for the rest of the ride? If you know the elevation at the start of the ride and can set it manually is there ever an advantage to using auto-calibration?

Confused,
Nathaniel

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Steve
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Re: Auto elevation calibration

Post by Steve » Tue Nov 27, 2018 4:30 pm

Whenever I've tried auto calibration it is hopelessly off. My impression (which could be completely wrong) is that auto calibration just uses the barometric pressure sensor to decide what it thinks the elevation is, then uses that. I'm pretty sure it doesn't do anything fancy with mapping data (i.e. correlating your GPS location with an elevation map to see what the elevation should be where you are).

nnovod
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Re: Auto elevation calibration

Post by nnovod » Tue Nov 27, 2018 5:12 pm

I just finished a long dialogue with HH support about this. Ashish from support said:
- Auto-calibration only takes place 1)At the start of the ride. 2)If WiFi is active.
- When auto-calibration does occur it is based solely on the GPS coordinates (I assume they do a query to a server that has elevation numbers based on GPS location - that's why they need the WiFi)
- Once a ride starts auto-calibration does not take place and all ascent/descent statistics are based on the barometric sensors.

So I guess the real test of auto-calibration is to start a ride with WiFi connected and see what elevation number comes up. My numbers have been way off, but I have not been connected to the WiFi when I start the ride so I don't know if auto-calibration is working for me.

For me the take-away from all this is that if you know the elevation at the start of your ride then put it in manually.

dthempel
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Re: Auto elevation calibration

Post by dthempel » Tue Nov 27, 2018 8:43 pm

nnovod wrote:
Tue Nov 27, 2018 5:12 pm
- Auto-calibration only takes place 1)At the start of the ride. 2)If WiFi is active.
Might be one of the dumber things I've heard about the Karoo in recent weeks. Why would any *GPS* device need WiFi to determine its location and elevation? Colossally stupid...

EDIT: Garmins from not that long ago (like the 510) only have BT and manage to figure out their elevation just fine. Not sure why this is annoying me so much other than that it's another case of them not "getting it". A Garmin Etrex that I used 10 years or more ago to geocache with the kids will tell me my elevation accurately. Not even sure WiFi was invented then... :/

JohnJ80
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Re: Auto elevation calibration

Post by JohnJ80 » Wed Nov 28, 2018 2:15 am

Actually, GPS is horrible at elevation. Have to be more accurate to look it up.

Scummer
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Re: Auto elevation calibration

Post by Scummer » Wed Nov 28, 2018 6:57 am

dthempel wrote:
Tue Nov 27, 2018 8:43 pm
nnovod wrote:
Tue Nov 27, 2018 5:12 pm
- Auto-calibration only takes place 1)At the start of the ride. 2)If WiFi is active.
Might be one of the dumber things I've heard about the Karoo in recent weeks. Why would any *GPS* device need WiFi to determine its location and elevation? Colossally stupid...

EDIT: Garmins from not that long ago (like the 510) only have BT and manage to figure out their elevation just fine. Not sure why this is annoying me so much other than that it's another case of them not "getting it". A Garmin Etrex that I used 10 years or more ago to geocache with the kids will tell me my elevation accurately. Not even sure WiFi was invented then... :/
It's not that easy. Barometric elevation needs to be calibrated as well, as the air pressure changes with the weather. And the GPS elevation data is too inaccurate to be considered.
On my Garmin 520 I had many elevation problems. Especially when I made a cake stop for about 30 min. and all of a sudden, clouds or bad weather would start coming in and with it changes in air pressure. All of a sudden during my cake stop I sunk like 300 feet into the ground.
So I'm assuming that the Wifi connectivity for calibration is needed to lookup the actual elevation data from an internet feed with the acurate GPS position provided by the Karoo GPS system.
On the Garmin you always had to put in the actual elevation number manually to calibrate it.
So calibration is either done automatically over Wifi, like the Karoo seems to be doing or putting the number manually by user input.

dthempel
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Re: Auto elevation calibration

Post by dthempel » Wed Nov 28, 2018 2:32 pm

Using the "raw" GPS number can't be worse than the situation I'm seeing on most rides, where I'm sitting ocean-side and seeing an "elevation" in Death Valley or higher than the highest point in a 30 mile radius. GPS fixes are X,Y *and* Z. It may not be accurate enough for flight navigation, but it's good enough for a cycling computer.

JohnJ80
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Re: Auto elevation calibration

Post by JohnJ80 » Wed Nov 28, 2018 2:58 pm

Gps elevation is only accurate to 10-20m. I don’t consider that accurate at all. GPS in linear (x,y) is about 3m. The accuracy in altitude is no where near as accurate.

Garmin says it’s even common for it to be off by more than 100m. See:

https://support.garmin.com/en-US/?faq=Q ... yoxITW2vZ6

I agree with the design decision to try and look it up. How they have implemented that is a bit wonky. Gps elevation is really not accurate enough for our purposes in initialization.

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Psyclist
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Re: Auto elevation calibration

Post by Psyclist » Wed Nov 28, 2018 3:11 pm

Scummer wrote:
Wed Nov 28, 2018 6:57 am
On my Garmin 520 I had many elevation problems. Especially when I made a cake stop for about 30 min. and all of a sudden, clouds or bad weather would start coming in and with it changes in air pressure. All of a sudden during my cake stop I sunk like 300 feet into the ground.
That makes sense. I don't know why one would need WiFi for the Karoo's new way as the whole point of adding a SIM slot was to provide data connectivity without WiFi coverage, but OK, it seems like an improvement. It would probably need a bit better heuristics -- if you pause your ride and unpause it within say 100m of the last fix, it's very likely you stayed in the same spot rather than lugging your bike up or down a steep hillside, so if the barometer says otherwise after the stop it should be safe to recalibrate it to the last height.

nnovod
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Re: Auto elevation calibration

Post by nnovod » Wed Nov 28, 2018 4:07 pm

I too think what they did makes sense. GPS altitude readings are inaccurate. Initial barometer readings can also be inaccurate due to weather changes, etc. But GPS location readings are accurate. Consequently using the GPS location to look up the altitude in an on-line database (see https://www.freemaptools.com/elevation-finder.htm) is probably the right thing to do. Not using the GPS after a ride starts also makes sense to me. The barometer should be much better at recording changes in altitude.

I just sent in a question about whether "WiFi" was meant literally or any network connection (SIM card or WiFi) would suffice. Yes, they should only need a network connection to do the location to altitude lookup - it shouldn't matter if it's a WiFi connection or a cellular one.

JohnJ80
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Re: Auto elevation calibration

Post by JohnJ80 » Wed Nov 28, 2018 4:46 pm

Yep. Agree. Get the location in x,y and then look up “z” from someplace with a number in which you have confidence.

I’d bet that if you have the SIM slot loaded and connected, that it goes there if wifi isn’t available. HH are application programmers, not system developers. They are just using the normal Android networking stack so it’s going to work just like your phone. It will try wifi, if not available, it will try the SIM, if not then it takes the manual calibration when it should probably take the GPS elevation. Somewhere in that “no wifi no cellular” decision tree is where it gets wonky.

J.

nnovod
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Re: Auto elevation calibration

Post by nnovod » Thu Nov 29, 2018 12:34 pm

I verified with HH support: Both WiFi and cellular (SIM card) internet connections can be used to call the service to get an elevation from a location.

So now that I finally understand how "automatic" calibration is working I'm curious: Has anyone given it a try as intended? Specifically, at the start of a ride, if you're connected to the internet, is the elevation getting set correctly if you have auto calibration on?

JohnJ80
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Re: Auto elevation calibration

Post by JohnJ80 » Thu Nov 29, 2018 4:51 pm

Good point. What I have noticed is that it takes my Karoo a long time to settle into a stable GPS fix probably because I don’t have a SIM and most phones (which is the reference design behind the Karoo) use their cell tower fix to establish the base starting point for the GPS almanac calculation. So I think this would mean turn your Karoo on, verify that it has accurately fixed your position on the map and do this while you have connectivity (wifi or cellular) and BEFORE you start the recording. If so, then that means that the Karoo will have gotten it’s auto elevation calibration squared away.

The Karoo was so lethargic at establishing a fix that in one case, I had “ridden” a qtr of a mile before I ever left my 100’ driveway. So it’s pretty wonky until it stabilizes on the fix. Given that GPS altitude can be way, way off from actual (as much as 400’), I can see why some have reported “riding” below sea level. In point of fact, this happened to me a couple of weeks ago while riding in Florida on a low lying barrier island. We took our bikes by car, set them up and rode but the Karoo never had connectivity to a network so it could never get a decent elevation calibration over the internet. We were 10’s of feet above sea level and it reported that I was as much as 40’ below sea level. That makes sense now and had I either had a SIM or connected through my phone via hotspot this would likely have been much more accurate.

J.

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