[OSM-talk] Lighthouses

[Steve Hill]

On Thu, 3 Apr 2008, Steven te Brinke wrote:

> Besides that, IMO starboard and port are not a good way to specify the type 
> of a buoy. That is because in the Netherlands buoys are placed in the 
> downstream direction, with green at the port side. However, at sea they are 
> placed towards land, with green at the starboard side. So in fact you don't 
> see the difference, only the definition is different. That's why starboard 
> and port are not well defined.

I think putting the lateral buoys in a relation would be a good plan - 
this means that to find the channel you don't need to know whether they 
are placed in the upstream or downstream direction, you just know that the 
bit between a port and starboard buoy within the same relation is the 
channel.

i.e. imagine a pair of channels marked like:

   S     P   S    P
   S     P   S    P
   S     P   S    P

(where S is a starboard marker, P is a port marker).

Without knowing the reference direction, it is a bit ambiguous - you could 
have 2 channels with the harbour to the South (so the port marker is on 
the port side when you're heading South).  Or you could have 1 central 
channel with the harbour to the North and the markers on the edges could 
be part of other channels.  If we surround them with relations it becomes 
more obvious:

   [S        P]        [S        P]
   [Relation 1]        [Relation 2]

Now, to work out where the channel is, we don't need to know what the 
reference direction is, all we care about is that we can tell the 
difference between the port and starboard markers - the bit between 2 
different markers in the same relation is the channel.  In the above case, 
we clearly have 2 channels.

However, tagging them port/starboard is still important since it lets us 
choose which symbol to render on the map (which must match what the 
physical buoy actually looks like)

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

      Servatis a periculum, servatis a maleficum - Whisper, Evanescence