Issues altitude diagram

Dimitrios Kanellopoulos

Stefan Kersting

@Dimitrios-Kanellopoulos ok then I have to wait more than 2-3 minutes…

Stefan Kersting

@Brad_Olwin now I did a hike and not a run in order to have enough time to follow the altitude on the watch. The interesting thing was that I started at an altitude of 5 masl and with a fast ascent to 30 masl and after a short time the altitude was at 13masl which was realistic. So that must be an issue with the satellite fix at the start. BUT I have never seen a negative altitude on the watch and when downloaded I see that the lowest point was registered with -15 below sea level. So now I don’t understand anything. The watch displays another altitude than SA…

johann.fuehrer

Hello, I’m surprised that this “my altitude does not match with my map reading” topic is coming up ever and ever again. I have written about this somewhere in the past, so here some information from guys who know what it is about: https://www.usgs.gov/faqs/why-dont-elevations-your-maps-agree-those-provided-my-gps-system-which-ones-are-correct?qt-news_science_products=0#qt-news_science_products

This is one of the reasons, Garmin brought in DEM data to Fenix models (I think mesh points in 30m grid)…

That is why I suggested a local offset which CAN be set manually and would shift the GPS altitude calculated by this offset for EVREY altitude value - right ones and wrong ones (bad GPS, abro i.e.). So also fused alti would work. It would not touch the GPS altitude reading and calculation directly, but after getting GPS altitude + fused alti calculations, it would add/sub the local offset of the GPS_geoid_to_real_map_alti difference which has been set manually. my5c

(which would mean: exact the same altitude profile but shiftet to real altitude due to offset…)

Fenr1r

@johann-fuehrer said in Issues altitude diagram:

TruFusedAlti. Do we know if Suunto uses the datum elevation or that from satellite triangulation? Or one and then the other?

my5c

Inflation?

Dimitrios Kanellopoulos

@johann-fuehrer afaik Suunto uses mean sea level. What am I missing ?

johann.fuehrer

@Fenr1r said in Issues altitude diagram:

@johann-fuehrer said in Issues altitude diagram:

TruFusedAlti. Do we know if Suunto uses the datum elevation or that from satellite triangulation? Or one and then the other?

Dont know. BTW: This was the thread & post I referred to, needed some time to find it again … https://forum.suunto.com/post/36919

Fenr1r

@johann-fuehrer Thank you. Very interesting and now my head hurts.

johann.fuehrer

@Dimitrios-Kanellopoulos said in Issues altitude diagram:

@johann-fuehrer afaik Suunto uses mean sea level. What am I missing ?

The physical model of Earth and the mathematical model of GPS geoid do not match. Not in altitude (and also not in horizontal offsets, but let’s forget this by now). Maps are not based on the GPS geoid but of some agreed MSL - but not to the same over the whole world (https://en.wikipedia.org/wiki/Vertical_datum)
There is lot of stuff about this on the internet, and to what funny results this led like tunnes planned by two different countries which did not met, bridges and other stuff. But the main conclusion is: GPS watches calculate the altitude based on a “ideal” world shape, which does not 100% match. For some parts of the world the altitude calculated match perfect, on others it’s way off (there is some map about the offset here: https://www.esri.com/news/arcuser/0703/geoid1of3.html) . DEM models have correction information based on the location for the GPS calculated values.

So for places on earth where the GPS and map data have a “natual” offset, you need an additional correction, if you want to be precise - or live with the altitude profile being OK but have a small or bigger offset.

johann.fuehrer

@Fenr1r said in Issues altitude diagram:

@johann-fuehrer Thank you. Very interesting and now my head hurts.

I’m sorry for your headaches…

Dimitrios Kanellopoulos

@johann-fuehrer I have a headache as well

Dimitrios Kanellopoulos

@johann-fuehrer at the same time I hear / read in other groups of garmin that the DEM is not ok. Am I wrong ? I am asking not to downplay Garmin, I love DEM, but I want to know in practice. At least if it solves issues like @johann-fuehrer has or even @Stefan-Kersting

johann.fuehrer

@Dimitrios-Kanellopoulos said in Issues altitude diagram:

@johann-fuehrer I have a headache as well

@Fenr1r @Dimitrios-Kanellopoulos

https://imgs.xkcd.com/comics/headache.png

Fenr1r

@johann-fuehrer @Dimitrios-Kanellopoulos

You see, the problem is with the inexact spheroidal shape of our heads vis à vis mean sea level, air pressure and Mondays. Also noses.

@Dimitrios-Kanellopoulos as for @Stefan-Kersting: so Suunto doesn’t use it? Seems like a good case to look at, I agree.

Stefan Kersting

One of my thoughts was that the problem is that I live 67 degrees north and the gps satellite coverage is not as good as in lower latitudes. The satellites are not as high above the horizon as eg in Central Europe. So the constellation results on lower precision. But the case I described in my last post can’t connect to that issue i think. There must be a missing link between the altitude displayed on the watch and displayed/recalculated in SA

johann.fuehrer

@Dimitrios-Kanellopoulos said in Issues altitude diagram:

@johann-fuehrer at the same time I hear / read in other groups of garmin that the DEM is not ok. Am I wrong ? I am asking not to downplay Garmin, I love DEM, but I want to know in practice. At least if it solves issues like @johann-fuehrer has or even @Stefan-Kersting

I have had the F5 and FR935 and DEM was not really great - as soon as you are in steep areas, the 30m grid is to coarse, even with interpolation. It made my altitude readings worse than without but you have had the choice.

However, I would prefer manual offset possibility in the watch, because here in Vienna I’m always about 8m to high (not really a problem, except you like to know - in watch mode - which altitude you are).

Fenr1r

@johann-fuehrer said in Issues altitude diagram:

here in Vienna I’m always about 8m too high

[Insert own joke here] Forward to Vienna Tourism.

johann.fuehrer

@Fenr1r said in Issues altitude diagram:

@johann-fuehrer said in Issues altitude diagram:

here in Vienna I’m always about 8m too high

[Insert own joke here] Forward to Vienna Tourism.

Need to take more care of my wording, it seems

Fenr1r

@johann-fuehrer Nah, it’s all very cool.

Stefan Kersting

When going to work this morning i calibrates manually to 4 masl, some minutes later checking in the office the watch displayed -25 meters.
Now I did a auto calibration, left the watch 15 min with GPS fix. After the auto calibration the altitude fell with 25 meters/minute before it got stable again. The absolute air pressure differs with almost 15 hPA: 1005 on the watch and 1029 at the official weather station.
Something is wrong here…