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-   -   Negative Elevation (http://www.blackberryforums.com/gps-mapping/184974-negative-elevation.html)

lloydsfm 04-08-2009 02:44 AM

Negative Elevation
 
Hello,

I know questions about this have been posted before, but the threads are now closed. I have a Blackberry Bold and am running TrekBuddy and a few other GPS programs. All of these programs report the elevation as negative and the problem seems to originate in how the Blackberry interpretes the data. This negitive elevation is also recorded in .gpx track files that the GPS programs create, which then confuse external applications like GoogleEarth and GlobalMapper. I have seen it reported that other models of Blackberry have this same issue. Does anyone know of a fix or if RIM has acknowledged the problem and is working on a solution?

Thanks.

Dubdub 04-08-2009 06:37 AM

Altitude has never been an accurate part of GPS, as it also depends on barometric data which GPS doesn't have. I assume that the map application uses the GPS position to place you on a map and then uses that spot to interpolate the elevation. Not real accurate, but better than nothing I guess.

Not sure if it is still the case, but in the "early days" of GPS, it assumed a standard day setting of 29.92 inches to determine altitude. There are very few standard days so the number never was real accurate. Military versions were much better however due to the usage and the need for more accurate data.

Not sure what your issue might be, but I would never trust an altitude readout from GPS.

lloydsfm 04-08-2009 04:15 PM

The negative elevations that I am experiencing seem more than just a GPS inaccuracy. I have had my Blackberry with GPS software running beside a Garmin Handheld GPS unit. When both units are set to read elevation from GPS signals only (not using barametric pressure to adjust), the Garmin unit would read an elevation of 1050m while the Blackberry would read -1050m. Other posts from other users have reported this similar negative elevation, where the whole number is reasonable but with a minus "-" symbol in front. This negative number is then recorded in the GPX tracks.

dankarlinski 04-08-2009 09:31 PM

well, i know the elevation goes off of sea level. so negative will be below sea level.

takeshi 04-09-2009 10:59 AM

(nm)

emacf1 06-15-2009 10:48 AM

I just stumbled over this issue myself... I received a GPSLogger log file from a skydiver... well and apparently the data recording said, that he went 4500 m below "sea level" just to reach to surface after a couple of minutes...

Since I am living in quite a flat area it was quite difficult for me to understand the problem - since there are other GPSLogger users who does not encounter this problem (I have also logs that where taken during flights with commercial air liners)...

First of all you have to know, that the altitude is provided in WGS84 - this means, that depending on your location on earth there could be negative values! (since the earth is not an ellipsoid)... but for sure values smaller then -500 are far off the possibilities...

A typical GPS device sends the data in so called NMEA sentence - and on my Storm (and I assume also on a Bold) the $GPGGA NMEA sentence only contains negative values for the altitude - that's quite odd - but there is nothing that a software developer who is working with the RIM API can do about it...

All what can be done (and that will be in the next release of my GPSLogger) - to include an option to use only "absolute Elevation" values... It's correct, that this will not be correct in areas where the WGS84 will be indeed negative - but that will be not too many places...

So for your TrakBuddy problem you can kindly ask the developer if he might can consider a similar option in his application (to fix the problem for Bold & Storm users)... if you like you can additionally suggest the developer can contact me in order to get further details about this problem.

Matthias

aldogoreng 09-16-2009 11:27 PM

Hi, I have pict that show you that some blackberry have problem in measuring elevation.


I try to check elevation on the same place using huron (8800)
Both of them are hurron 8800
left
operator logo on handheld: cingular
carrier: indosat
elevation: negative

right
operator logo on handheld: indosat
carrier: indosat
elevation: positive (normal)

visortgw 09-22-2009 08:22 AM

This is a known bug in certain versions of the RIM OS. On my Tour, it was an issue in OS v4.7.1.40, but it appears to have been subsequently fixed in OS v4.7.1 53 (which, of course, was "released" by RIM or Verizon late last week but subsequently "pulled"). Matthias Marquardt (GPSLogger developer) and others have added a setting to their apps to optionally invert the altitude.


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