I installed a Rux watchface that has an altimeter feature. I am getting an altimeter reading of -355 feet. Can’t understand the

• sign and the measurement of that height.
Can anyone shed some light on this?

The altimeter value in Facer is based on the atmospheric pressure reading provided by the hardware barometer in your watch. If the current sea level atmospheric pressure is 1013 hPa then the altimeter value should be correct. However, air pressure varies by around 12 hPa every 328 feet so if the local sea level air pressure rises to 1026 hPa the altimeter will read 355 feet less than it actually is. That is, if you are at sea level and the barometric pressure is 1026hPa, the uncorrected barometric altitude will be -355 feet.

I hope that helps.

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Mike… thank you very much for your explanation. To give you a practical situation the altimeter reading one my Rux watchface is 145 ft right now.No minus in front of that figure. There is also a figure of 1007 hpa. How would that breakdown to an actual height?
Is there anyone who would actually find these figures,the 1007 hpa and 145 ft or other figures like these shown during the day important in their personal life. Looking forward to your reply.

John

Hey John, in order to determine the correct altitude one needs two independent pieces of information, the current sea level barometric pressure and the current barometric pressure reading at the relevant altitude. The watch only gives the second. Without the sea level figure the altitude will only every be approximate. Unfortunately Facer does not currently have access to the sea level figure so the barometric altitude cannot be automatically corrected.
Cheers
Mike

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Mike…after reading your response repeatedly I have to ask myself what purpose does the altitude figure shown on a watchface serve? Does anyone profit from seeing that figure and,if so, how? One of my watchfaces now shows -221 ft . Seeing that minus sign really throws me for a loop! Can you enlighten me on that? My first thought was I’m 221 feet below sea level but that doesn’t make sense.

As I mentioned above, an uncorrected barometric altimeter will, in general, provide an estimated altitude with a significant uncertainty ( ie. error ) about the nominal value.

What use, well that is really up to the user…

It could for them have simple novelty value or if you take note of the altitude and then climb a hill and read the altimeter again you could determine the height of the hill be subtracting one value from the other, plus others uses I suppose …

It would be better if watches could make use of GPS signals to estimate altitude but I am told that most ( all ? ) smartwatches only produce lat/long with no altitude output.

Mike…I would like to point you to a non-Facer w atchface I have. It’s Futurom Travel Plus. It is the most comprehensive watchface i have ever seen. It has such a plethora of current weather and future weather. Do you want a watchface that will show your location? This one not only showed my state,my community and my exact address!! There is a feature that shows my altitude (GPS). Would you think this is an accurate figure? Currently it shows 14 ft (GPS). What should I make of this?

I am sorry but I don’t know the face you mention and I don’t know where it gets it altitude data from so I can’t really comment on the values it gives.

Mountaineers have been carrying barometric altimeters for years, long before smart watches and GPS existed. It is very useful aid on multi-day excursions into the wild without communication to the technological world outside (yes, there still are remote areas on our planet without cellphone reception and/or access to power to recharge; even GPS reception is not always reliable and yes, some of us deliberately venture there for our pleasure ).

To a mountaineer, position (in 3D) as well as weather are of critical importance.

In stable weather a mountaineer primarily uses a barometer as an altimeter and navigational aid, but in unstable weather it switches over to being a weather gauge. It is a bit of an art, because you are tracking interrelated variables while on the move.

As Mike already explained, you need to regularly calibrate the barometer at a known altitude for it to be accurate as an altimeter.

While moving, you need to keep an eye on the weather while tracking your altitude and compare your perception of progress with that of the barometric reading. If the change in air pressure does not correlate with your change in altitude, the weather is changing.

Knowing your altitude in mountainous terrain makes it easy to plot your position on a contour map (assuming you roughly know on which slope you are and can use common sense to read the land around you).

In unstable weather, a barometer is less useful as altimeter, but can be used to inform your own weather forecast. You always take a reading before going to sleep and again when you wake up. Changes in air pressure at a fixed location indicates a change in weather approaching.
Overnight spots are often lower down in valleys with obscured visibility in the direction of approaching weather. The barometer is your friend alerting you of an unseen approaching storm.

By the way, most advanced GPS navigation devices include a barometer as well to improve altitude accuracy since GPS altitude can be very inaccurate in mountainous terrain.

Sorry for the long (off topic) post; hope this is of use.

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Thank you for your input!:As I mentioned to Mike I used several Rux watchfaces that had an altimeter feature. The values given were extremely foreign to me. Some of the values showed minus signs. That really threw me for a loop. One of them was -314 ft. I live in a four story building on the 3rd floor so I knew something didn’t make sense. I have a non-Facer watchface that showed an altimeter reading of 15 ft (GPS). From what you wrote a GPS value is not accurate. The altimeter in my Gear S3 now shows 50 ft. I think that is closer to an accurate figure.
Any thoughts on these figures?

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GPS altitude can be accurate, but it depends on the visible satellite distribution available to the GPS receiver.
In a canyon (including urban canyons created by skyscrapers) only the satellites directly above a receiver is “visible”. This is OK for a 2D position fix, but not for 3D (fuzzy intersection of signals, making for poor triangulation). For a good 3D (i.e. altitude) fix by GPS, you want satellites positioned at opposing horizons.

Negative altitude reading is possible with a barometric altimeter that was last calibrated under weather conditions that differ from the current weather conditions (i.e. different atmospheric pressure due to the change in weather, not due to the change in altitude).

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Google will find lots of info on the subject.
Here are some:

https://www.gps.gov/systems/gps/performance/accuracy/

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How would I add the current pressure to the string provided by facer or what is a good equation to use for correcting altitude display?
Facer pre-populates with: (round(#ALT#)) \$#UNITSYS#=IMPERIAL?ft:m\$

The principle of the altimeter and sources of error

The altimeter works on the principle that the pressure within a column of air varies in a known way with height. The mathematical relationship that relates them is:

z = ( RT / gM ).LOGe( p0 / p )

where z is the height difference between the starting height and the measurement height, R is the gas constant, T is temperature of the air measured in Kelvin, g is the acceleration due to gravity, M is the molar mass of the gas (in this case air), p0 is the atmospheric pressure at the starting height and p is the atmospheric pressure at the measurement height.

In the above:

R = 8.3144598
g = 9.80665
M = 0.0289644

so,

``````z = 29.2697 x T x log ( P0 / P ),  where z is in metres
``````

P you can get from the watch, it is given by: #ATM# ( in hPa, which is the same as mBar )

The current value for P0 you can get from the nearest weather station with a known altitude ( make sure this is in hPa or convert it if needed )

The current value for T you likewise can calculate from the temperature provided by nearest weather station: T = 273.15+temp, where temp is in Celcius

Z then gives you the approximate altitude difference between the weather station and your current position. Adding the altitude of the weather station to Z then gives you your approximate altitude.

Note that the result is only approximate and can be significantly different than your true altitude due to local weather conditions, horizontal distance from the weather station and/or non-standard temperature variations with altitude ( eg. temperature inversion layer )

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Thank you again for your help! Much appreciated.

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