[OSM-dev] Altitude data & (cycle) route profiles

Andy Robinson (blackadder) blackadderajr at googlemail.com
Mon Mar 31 17:28:46 BST 2008

>From: MilesTogoe [mailto:miles.togoe at gmail.com] wrote:
>Sent: 31 March 2008 3:40 PM
>To: Andy Robinson (blackadder)
>Cc: 'Sjors Provoost'; dev at openstreetmap.org; 'Karl Newman'
>Subject: Re: [OSM-dev] Altitude data & (cycle) route profiles
>what is the accuracy on regular GPS elevation ?  +- ?  Here in mountain
>territory we deal with a lot of roads and trails that may change 100's
>of feet (or meters), thus even a low accuracy of +- 5-10 ft would still
>be a help.  And likewise to show off big things like the Chunnel or the
>high bridge in France....

For regular GPS receiver picking up just a handful of satellites the
accuracy from GPS alone isn't much better than +/- 50m (or worse) and it
doesn't get much more accurate when you have the maximum set in view. Don't
ever expect GPS alone to deliver good vertical height data. Horizontal
position can be established much more accurately. This is why some upper
market GPS's have barometric pressure support, but as another has pointed
out these need calibrating each time you go out to be reasonably accurate.

Survey GPS systems work a better but still struggle to get reasonable
accuracy in anything other than open space with a clear view to the maximum
set of satellites.



>If accuracy is worse, well I guess you have a good point.  Otherwise, we
>could use it with a notice about the accuracy levels.   At least it's a
>start and accuracy can always be improved by manual editing.
>I really see the value more in the near future when our tools are more
>3-D but we are seeing the beginnings with Google Earth (and some SoC
>projects are adding some 3-D to Inkscape.   I would think(hope) GPS
>elevation instruments might get more accurate as well.   Re: grades
>along ways, it's a good idea from the data perspective, but it may not
>be too practical for people to measure in the field as they are hiking
>or riding and their GPS is just collecting data.

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