Locus 330 altitude management

sl91 shared this problem 5 months ago
Solved

Hi,

Today, I made a hike (about 9 km). I recorded the GPX file using Locus Pro 330 installed on my Samsung Galaxy S8+ (Oreo) smartphone. My goal was to record of the most precise possible altitude. My altitude manager settings are:

-Parameters: No offset, SRTM: replace GPS values, use the pressure sensor, light filter

-Pressure: Automatic

I thought that this way, Locus will first use the altitude value extracted from the srtm file and from that point will use the pressure sensor to record some real time altitude values. Also, I read that locus will check each two hours the right value of the altitude from the srtm file and perform some correction in needed in order to avoid the problems related to the weather (and the related ambiant pressure) change during the day. Am I right ?

Well I recorded my hike and exported the gpx corresponding file. I opened it using one of my favorite softwares, i.e. MyTourbook in order to analyze the altitude data. I noticed 2 issues.

1. Locus pro generated a relatively huge jump in the altitude value after 1h20 of walk. This is shown by the green curve. The brown curve corresponds to the strm values.

2. One can also clearly see that the altitude value is really exact at the starting point and drifts too much with the time. Locus Pro reads correctly the first value but what happens after that ?

I have attached a picture that shows this effect

Best regards


sl91

Comments (8)

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By the way, I have also recorded the same hike using my Garmin 935 watch. It has also a pressure sensor. The recorded altitude is attached to this comment.


Regards


sl91

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Hello,

your setup if perfectly valid and is identical to what I use for my own recordings. Jump in elevation is really weird.

Pressure re-calibration is made currently once per hour, but change in made over 15 minutes, so no such jump should be visible.

Few ideas that comes over my mind:

  • are you using SRTM data from only a single source? Such jump and following lazily-changed elevation should be caused by different SRTM file used in second part of your trip.
  • are you walking this track more often? Should be usable to test it once more. Also take third attempt (in case of same second test) without a pressure sensor enabled.
  • if second test will be same, if may worth to remove and re-download all SRTM files again

Menion

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Good morning Menion,


Thank you for the answer. I will try with another smaller hike that I do very often. I will do a first record today if the snow (and the cold wind) decreases. For this first record, I will do it using the same setup of Locus than yesterday. I only use the SRTM files (.hgt) dowloaded together with the Lomaps as proposed by the Lomaps download process. I have 988 files in the srtm folder corresponding to several european, ,african and asian countries. But some of these files can be much older than the corresponding Lomap, when I buy a newer version.


Best regards


sl91

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Helo,

to be true, I'm not sure how old may be oldest HGT files we provided. There was not any serious update during last years, mainly on quite flat lands like I saw in your records, Anyway you may also try to delete them all and download only needed. You will at least save quite a lot of space in your device.

If there still be any problem, I'll create special version that will print raw received data for some manual post-process. Thanks and sorry for complications!

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Ok Menion. I will let you know what happened with the next record. Space is really not an issue. All these files are stored on a Samsung 256GB microSD card. When I travel, I feel really better if all the needed files are already stored.


Regards


sl91

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Menion,

Here is a pdf file containing altitude profiles as recorded and compared to srtm data. The first chart is related to Locus whereas the second one to my Garmin 935 watch (it has a pressure sensor). I don't see any jump but it is not really "perfect". What is your opinion ? It seems that the srtm files are downloaded by MyTourbook from this website: https://dds.cr.usgs.gov/srtm/version2_1

One can see on this profiles how the barometric sensor can be accurate. It is really amazing, at least for altitude gradients measurement.

A last poinbt regarding to the last version of LocusPro. It seems that the satellites windows never show less than 10 meters for the accuracy even in the case of an excellent GPS signal (21/22 satellites).


Best regards


sl91

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Hello,

not sure what you mean by "perfect", but for me, it looks really good. Measurement of Locus Map and Garmin are really close, which at least means, that there is not visible serious issue in Locus Map system (compare to Garmin). Fact that measurement does not match to SRTM elevations for around 10 meters is ok. The accuracy of SRTM is also not ideal. I do not have exact knowledge about possible accuracy, but 10 metres sounds good.

Unfortunately, it does not explain the weird jump in the previous measurement.

10 metres accuracy ... don't take this number too seriously. It's just guess and new method that since 3.30 Locus Map use, just return for the same precision little higher number.

So all "good" for now?

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Ok. Thank you. I don't see jumps anymore. It can be because the phone was in an internal pocket of a very thick jacket and it was raining. So may be the water vapor can have some impact on the pressure sensor reliability.

Best regards


sl91

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