[OSM-talk] augmenting contour data with gps track logs

[Frederik Ramm]

Robin,

Robin Paulson wrote:
> the srtm contour data used in some osm renders is great, partic for
> cyclists and walkers. i've realised that it's not as high
> accuracy/precision as it might be though. i notice that most GPS
> devices also record the elevation within their tracklogs, and that it
> might be useful to extract this data, and use it to provide more data
> for calculating the altitude of a given point.

Pure GPS elevation is worthless due to bad precision (for technical 
reasons, Z measurement is much worse than X/Y measurement in GPSes). 
Some GPSes have barometric altimeters which are very precise but need 
calibration (to an extent they can be "post-calibrated" if you have 
visited a point with known altitude and weather hasn't changed on you 
too much during the trip).

The biggest problem however is that altitude data is an altogether 
different beast than what we're normally working with. The hill shadings 
and contour lines you see on our maps are basically derived from a 
giant, planet-wide bitmap which has one "pixel" for (approximately) 
every 100metre x 100metre square on the earth's surface.

We would need a mechanism that takes the spot heights measured with your 
GPS and somehow weaves them into this (or a finer-resolution) bitmap, 
*or* go for a true 3D surface model of the earth. Both will require 
different kinds of editors than we have now.

Simply adding altitude information to features we map will not do 
because most editors are unaware of that, and it is unclear what it 
means exactly. If you map a post box with altitude=100ft, and I later 
find that the post box is in fact on the other side of the road - does 
that mean that this side of the road is still 100ft, just the post box 
has to be moved, or does that mean that I have to move the altitude 
information together with the post box?

In my eyes, a separate project "OpenTerrainModel" or something like that 
is called for, the results of which could seamlessly replace our usage 
of SRTM data today.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"