[OSM-talk] re contours

[Steve Hill]

On Thu, 20 Mar 2008, elvin ibbotson wrote:

> Treating contours as shape files seems to me to be heavy on storage, 
> downloads and processing. I have made a proposal in the wiki at 
> http://wiki.openstreetmap.org/index.php/Relief_maps#a_proposal to use relief 
> shading as a background to mapnik tiles. I'm sure there must be good reasons 
> not to do this and look forward to hearing them.

I hadn't come across that proposal before, but my initial thoughts are:

Coloured relief as described is good for an at-a-glance idea of the 
terrain, but (IMHO) are less useful when you want to look at the map in 
more detail.  It could be sensible to use this system on the low-zoom 
tiles and the switch to contour lines on the more detailed high-zoom ones.

The proposed doubling of the intervals leaves them far too widely spaced 
at high altitudes which would render it more or less useless in 
mountainous terrain.  For example, a ski resort may have the town centre 
at 1100m and the top of the mountain at 3300m - on that map the only 
colours you will see are the 1024-2048m and 2048-4096m bands - 2 bands to 
cover up to 3000m of altitude difference is nowhere near enough to be 
useful.  On the whole I'm not convinced about reducing the band frequency 
with altitude anyway - if you're cycling (for example) at an altitude of 
600m, a 100m high hill is just as significant to you as it would be if you 
were cycling at sea level, but in the former case it wouldn't show up on 
the map at all whilst in the latter it would be very obvious.

With some modification to the banding intervals, it could be useful to use 
colours *as well* as contour lines on the map to make it easier to see 
which direction the gradient is (i.e. so you can tell the difference 
between a ridge and a valley without looking at the legend printed on the 
contour lines).

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

      Servatis a periculum, servatis a maleficum - Whisper, Evanescence