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/]
