Altitude (from GPS) optimizations

[deniswal]

Hello all,

I am a long time user of Locus and I certainly appreciate it. I have (Mainly) two classes of needs with it. Hiking and gliding. Could you add a preference in the track profiles to use or disable SRTM ? When I use Locus to record a glider session, I certainly do not want to be "mapped" to the ground. I would suggest GPS + sensor  only in this mode, and low filtering since the sky visibility is good in a plastic box).

Thanks,
Denis

[tommi]

Hi Menion,
I understand Altitude manager better now.
Just one point regarding *calibration* of pressure sensor: can I use SRTM data for this purpose?

As you didn't mention it: are the other issues I reported here fixed?

Thanks,
Tommi

[Menion]

Hi Tommi,
  fine, I'm glad it's better - "one screen (tab) to rule them all"

 - hmm SRTM for pressure sensor - no it's not possible. I'll add it to last column where you define exact altitude for current pressure, this will work right?

 - and fixed mentioned problems ... I was working on charts. Gradient now display correct values, not rounded. You may also compare charts not (for example with Locus Pro from store). All charts now do not display exact values, but all values are little bit filtered. Just a little, but it's much better to watch them now ...

 - I was also think about filters ... maybe I'll make them all little bit stronger, because from your testing seems that "light and medium" are completely useless

 - not refreshing time in dashboard - I forget on this, so tomorrow

[tommi]

- hmm SRTM for pressure sensor - no it's not possible. I'll add it to last column where you define exact altitude for current pressure, this will work right?

Yes, something like "automatic cslibration according to SRTM".

- I was also think about filters ... maybe I'll make them all little bit stronger, because from your testing seems that "light and medium" are completely useless

This was my impression, not sure about others'.

Thanks again,
Tommi

[tommi]

Hi Menion,
sorry for being besetting but I fear it could be forgotten since some time passed and a new test version is available.

- hmm SRTM for pressure sensor - no it's not possible. I'll add it to last column where you define exact altitude for current pressure, this will work right?

Yes, something like "automatic calibration according to SRTM".

And few more reports in this topic from my side regarding value display in dashboards...

[Menion]

hello Tommi,
  I have two things to discuss

1. auto-calibration by SRTM. I was thinking about it now and I'm not sure this feature is needed. When you want to use pressure sensor, there is expected you'll have enabled GPS as well. SRTM data are good, but both knows that they're also not precise, mainly in hilly areas. So I'm not sure if this feature will bring any benefit. And also, Altitude manager is unfortunately too complicated, so I think rather about "what to remove" then "what add"

2. time when you lost GPS - I reorded a little values in list of possible values for dashboard. "Time" at begin is time of last fix, not current time. I don't expect, that current time will be needed as it's always in top system bar

[tommi]

hello Tommi,
  I have two things to discuss

o.k, let's discuss . I'm happy that such things aren't simply disregarded.

1. auto-calibration by SRTM. I was thinking about it now and I'm not sure this feature is needed. When you want to use pressure sensor, there is expected you'll have enabled GPS as well. SRTM data are good, but both knows that they're also not precise, mainly in hilly areas. So I'm not sure if this feature will bring any benefit. And also, Altitude manager is unfortunately too complicated, so I think rather about "what to remove" then "what add"

I agree with you in hilly areas and as well it might also be wrong for our avionic freaks (I'm fascinated they can use such amateur instruments (this is not against Locus which makes the best out of a smartphones sensors) but is this really typical? When do you typically switch on GPS? I guess many more than 50% of all tours (bicycle, hiking) will start at a location like at train stations, parking areas, meeting points, from at home where you can expect quite accuate SRTM data. Automatic GPS correction typically gives me much worse values (e.g. 5..10m instead 0..2m for SRTM).

I try to sum up my understanding of the Altitude manager and also to raise questions:
Tab SETTINGS:
- ALTITUDE OFFSET: this allows different kind of "offsets" offset for the GPS measured altitude to be set on the OFFSET tab
- PRESSURE SENSOR: this enables/disables the PRESSURE tab and has the comment allow to improve measured altitude by pressure sensor.
-> What kind of improvement?
-> Isn't just the pressure value used for calculation of altitude?
- ELEVATION DATA: tick Optimize GPS altitude with comment that optimization is done with offline elevation data.
-> What kind of optimization?

Tab OFFSET:
- Manual correction: Ok, this configures a static offset by calculating the difference between the manual value and the measured altitude for the location of GPS sync
- Automatic correction: Ok, uses the GEOID model, For typical usage (tracks of 1..ca. 100km) it is also more or less a static offset, isn't it (not sure if I understood this correctly)?
- Altitude NMEA correction: No clue what this really does

Tab PRESSURE:
- Automatic: Automatic calibration according to GPS altitude
--> My recommendation: Rename it to Automatic calibration according GPS
- Automatic calibration according SRTM
--> My recommendation: Add this choice
- Pressure at sea level. Ok, this is the classic approach, I won't use it, too much effort to get this data
-Altitude. Manual calibration. This is what I did up to now if I wanted to get accurate altitude data. Typically I used just SRTM information for this which is the reason why I recommend the choice for automatic calibration according SRTM which makes it much more comfortable.

My personal summary (it could possibly be different depending on answers to my above questions):
I think in tab SETTINGS it would make sense to allow either ALTITUDE OFFSET _or_ PRESSURE SENSOR. Phones where the pressure sensor works properly will probably like to use it as it gives the most accurate data for both ground and aviation usage (maybe there will be divers in future ;-)). For all other phones the offset method is appropriate.

2. time when you lost GPS - I reorded a little values in list of possible values for dashboard. "Time" at begin is time of last fix, not current time. I don't expect, that current time will be needed as it's always in top system bar

Hm, one reason why I use dashboard is that the values offered by Locus in the top panel as well as in the top system bar are hardly to read when e.g. riding a bike as these values are printed pretty small and don't have too good contrast (top system bar). Therefore I like to use current time in the dashboard. For me the GPS fix time is a value I don't really need in a dashboard which should mainly show live data.

Have fun with reading and keep up the good work going!

[locuscycling]

hello Tommi,
  I have two things to discuss

 SRTM data are good, but both knows that they're also not precise, mainly in hilly areas.

Few years ago i remember i tried Mountainbiking in the hilly forest  using Altitude not from GPS or barometer  but only from SRTM  ( one of the option in TwoNav)but after few minutes i stoped this, it was no sense. Altitude from Gps was much better.

But maybe it will be good to have in Locus possibility to correct Altitude  after finishing course.

I mean we finish a course, then save or export track and then correct Altitude using SRTM.
(It is possible for ex. in software like ''MyTourbook'' )

,,Momentary'' SRTM Altitude I don't know if it is a good Idea.

[tommi]

hello Tommi,
  I have two things to discuss

 SRTM data are good, but both knows that they're also not precise, mainly in hilly areas.

Few years ago i remember i tried Mountainbiking in the hilly forest  using Altitude not from GPS or barometer  but only from SRTM  ( one of the option in TwoNav)but after few minutes i stoped this, it was no sense. Altitude from Gps was much better.

But maybe it will be good to have in Locus possibility to correct Altitude  after finishing course.

I mean we finish a course, then save or export track and then correct Altitude using SRTM.
(It is possible for ex. in software like ''MyTourbook'' )

,,Momentary'' SRTM Altitude I don't know if it is a good Idea.

Maybe I was got wrongly. In case of reasonably working pressure sensor I only want to use SRTM for initial calibration of the pressure sensor.
In other case (no usage of pressure) I'm interested in getting answers to the question which kind of optimizations/improvements are made.

[locuscycling]

I only want to use SRTM for initial calibration .

I agree (if  i good understand).

You will start a course and want to have a possiility to set start altitude using SRTM ,,automatically'' ( not manualy -looking at maps ,touch a point to see altitude- and then adjust) ?

[tommi]

I only want to use SRTM for initial calibration .

I agree (if  i good understand).

You will start a course and want to have a possiility to set start altitude using SRTM ,,automatically'' ( not manualy -looking at maps ,touch a point to see altitude- and then adjust) ?

Right, either use offline SRTM data or if not available but internet connection use online SRTM data.

[Menion]

Hi,
  so discussion

firstly, I created this manual http://docs.locusmap.eu/doku.php/manual ... de_manager , so hope most question will be answered there.

secondly, thanks tommi for valuable feedback. I improved a little texts below some items, to make more sense.

and finally, things to discuss. I understand usage of SRTM for calibration, but I need to think more about it. For me, it's an unwanted settings. I'm rather thinking about merging this directly into "Automatic" system. So Locus should use SRTM if available or GPS altitude if not available. I'll think about it ...

And time value to dashboard - added "Current time" item

[tommi]

Hi,
  so discussion

firstly, I created this manual http://docs.locusmap.eu/doku.php/manual ... de_manager , so hope most question will be answered there.

Seems my silly questions have helped at least a bit to explain the stuff.
secondly, thanks tommi for valuable feedback. I improved a little texts below some items, to make more sense.

Seems my silly questions have helped at least a bit to explain the stuff.

and finally, things to discuss. I understand usage of SRTM for calibration, but I need to think more about it. For me, it's an unwanted settings. I'm rather thinking about merging this directly into "Automatic" system. So Locus should use SRTM if available or GPS altitude if not available. I'll think about it ...

Automatism is also ok with me but needs again explanation in man page to avoid obtrusive users like me asking a lot of questins about it

And time value to dashboard - added "Current time" item

Thank you!

[sherman]

Hi, my two cents:
I agree with tommi, I also would like to have a possibility to automatically calibrate altitude sensor using SRTM. It is one from reasons I still use ipBike for biking instead of Locus.
I think it would be more reasonable to leave the possibility for choosing the source of automatic calibration.
It would be also nice to have a possibility to autocalibrate using METAR data from nearest airport.When the airport is not too far away, this method is most accurate.

[tommi]

Aah, forgot few questions which might help to make the man page even better:
" Locus allow to define this offset by three methods
    Manual correction - basic method, in case you know exact value of this offset and want to define it manually"
My understanding is that the exact altitude (e.g. 500m above N.N.) and not an altitude offset (-50m) shall be entered here.

"Automatic correction - method that may cover 99% of your needs. Locus can automatically compute this offset for whole world from model of Geoid. This method is precise enough for mobile device and allow not to worry about this offset in case, you travel between different locations. (offset change with your location)."
Not sure if I understand right. This sounds like if I record a track from A to B that the correction might change during the record of the track due to different Geoid altitude of A and B. On the other hand how does this work in case of pressure sensor!?

"Altitude NMEA correction"
Maybe better write: Method is deprecated because there are devices which report wrong values.

"Automatic - if you enable GPS, Locus will use current altitude and current pressure and use them as base values. All next measurements will be related to these values. During usage, Locus will periodically check measured values and in case of need, automatically recalibrate pressure sensor."
What does "during usage" mean? During track recording?
Specify periodically.

I made few little changes in the manual where I think things could be misunderstood. Hope you are ok with these.