C
Chris Malcolm
Guest
Peter <[email protected]> writes:
>Michael J. Klein wrote:
>> On Wed, 16 Jun 2004 21:12:27 -0700, Peter
>> <[email protected]> wrote:
>>>Do the recent versions allow you to plot the elevation
>>>profile using the altitude data from the GPS? I have
>>>version 2 and it uses the DEM topo data to generate the
>>>profile just using the horizontal position data
>>
>>>from the GPS tracklog. Unfortunately the DEM data is not
>>>very accurate.
>>
>> Neither is GPS alititude. If knowing the altitude is
>> important, use an alitmeter.
>No measurement is perfect, but I've found GPS-derived
>altitudes to be far better than the DeLorme DEM data. The
>DEM-derived profiles tend to show considerable up and down
>variation on roads that actually have a very even grade.
>The overall shape of the profile is pretty good but the
>cumulative climb is greatly exagerated. Here's a sample
>elevation plot using only GPS data from two bike rides over
>the same course on different days to check for consistency:
>http://home.comcast.net/~prathman/Image33.gif A barometric
>altimeter (I presume this is what you mean by 'alitmeter' -
>a GPS is also a form of altimeter) by itself is subject to
>considerable variation due to changing atmospheric
>conditions and is generally less trustworthy than careful
>GPS measurements unless frequent recalibration is feasible.
>Much better is using the two together with the pressure
>sensor providing short-term stability and the GPS
>correcting for longer-term atmospheric changes.
Which is exactly what the autocalibration feature of the
Garmins which have barometric altimeters does, e.g. Garmin
eTrex Vista & Summit, 76S, Geko 301, 60CS. Much more stable
and accurate altitude readings, plus altitude accuracy which
is maintained when GPS accuracy degrades, or when satellite
lock is temporarily lost. The downside is occassionally
ridiculous altitude readings for some time after switch on,
unless you can be bothered to check for miscalibration and
correct it manually.
--
Chris Malcolm [email protected] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]
>Michael J. Klein wrote:
>> On Wed, 16 Jun 2004 21:12:27 -0700, Peter
>> <[email protected]> wrote:
>>>Do the recent versions allow you to plot the elevation
>>>profile using the altitude data from the GPS? I have
>>>version 2 and it uses the DEM topo data to generate the
>>>profile just using the horizontal position data
>>
>>>from the GPS tracklog. Unfortunately the DEM data is not
>>>very accurate.
>>
>> Neither is GPS alititude. If knowing the altitude is
>> important, use an alitmeter.
>No measurement is perfect, but I've found GPS-derived
>altitudes to be far better than the DeLorme DEM data. The
>DEM-derived profiles tend to show considerable up and down
>variation on roads that actually have a very even grade.
>The overall shape of the profile is pretty good but the
>cumulative climb is greatly exagerated. Here's a sample
>elevation plot using only GPS data from two bike rides over
>the same course on different days to check for consistency:
>http://home.comcast.net/~prathman/Image33.gif A barometric
>altimeter (I presume this is what you mean by 'alitmeter' -
>a GPS is also a form of altimeter) by itself is subject to
>considerable variation due to changing atmospheric
>conditions and is generally less trustworthy than careful
>GPS measurements unless frequent recalibration is feasible.
>Much better is using the two together with the pressure
>sensor providing short-term stability and the GPS
>correcting for longer-term atmospheric changes.
Which is exactly what the autocalibration feature of the
Garmins which have barometric altimeters does, e.g. Garmin
eTrex Vista & Summit, 76S, Geko 301, 60CS. Much more stable
and accurate altitude readings, plus altitude accuracy which
is maintained when GPS accuracy degrades, or when satellite
lock is temporarily lost. The downside is occassionally
ridiculous altitude readings for some time after switch on,
unless you can be bothered to check for miscalibration and
correct it manually.
--
Chris Malcolm [email protected] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]
