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

Steve Hill steve at nexusuk.org
Sun Apr 6 11:49:22 BST 2008

Sjors Provoost wrote:

(Much of the below has already been said by other people, but I thought 
I should express my views)

> 2 - estimate the altitude of each node (point) in Australia from the
> neighboring SRTM points

Remember to put a source tag on the data (maybe we need a source:ele tag 
instead to show only the elevation data came from SRTM?)

I am also concerned that elevation tags will become more inaccurate as 
nodes are moved - you should think about adding some support into the 
editors to warn people when they move nodes with elevation data attached 
and ask them if they want to remove the elevation data.

> 3 - add extra nodes along long ways with few far-between nodes

I'm unconvinced this is a good idea since you're introducing nodes which 
could well have very inaccurate lat/lon data.  Nodes are usually 
considered to be relatively accurate since they are (generally) 
positioned by using some real data (GPS track, photos, etc).  What you 
are proposing introduces a lot of nodes that have very little relation 
to the real world and for which the elevation data will become 
inaccurate if someone moves them to better represent the real world.

I'm convinced that it is preferable to keep the SRTM data separate and 
just calculate the intersection of the SRTM data with a way when 
producing an altitude profile.  The SRTM data could, of course, be 
pulled from a server rather than having to keep the data set locally.  I 
just don't think polluting the OSM database with many potentially 
inaccurate nodes is a good idea.

> * apply to the whole planet

If you are adding extra nodes this will massively increase the size of 
the OSM data set.

Also, consider how you are going to handle nodes which already have an 
ele tag - are you going to leave them alone?  What about comparing the 
height you calculate with the existing ele tag and do something smart if 
they are grossly different (e.g. tag the node with a warning that the 
elevation should be checked)

> * update render tool to show when roads do not intersect at equal hight

I'm not sure what this means - a junction is a node which is a member of 
multiple ways - that node is going to have a single elevation tag so all 
the roads using it *must* have the same elevation (there is no other way 
of expressing the junction).

If you are talking about ways crossing without a junction (i.e. no 
shared node), I'm unconvinced the elevation data is high enough 
resolution to make a meaningful judgment as to whether the roads are at 
the same height as each other.


  - Steve
    xmpp:steve at nexusuk.org   sip:steve at nexusuk.org   http://www.nexusuk.org/

      Servatis a periculum, servatis a maleficum - Whisper, Evanescence

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