[OSM-newbies] Mapnik rendering of Beaches

[Richard Moss]

Can someone help with advice on rendering of beaches in Mapnik?

I took my gps on holiday to the lovely village of Katelios in Kefallonia (Greek
island).  Looking at it on OSM, back in the Spring, I could see there was a lot
missing, and then about a month before I went, Graham Jones was nearby, and
added some initial details.

The extent of the island was/is defined by the "natural=coastline" way, which
had been imported from PGS in 2007 and is a fairly crude outline of the land-sea
interface.

Being the Med, of course, there is no significant tidal movement in this line. 
One of Graham's contributions was to indicate where the sandy beaches were. He
added more-or-less rectangular areas using the "natural=coastline" as the
seaward edge of the beach, and labelled an area around 30m landside of this as
"natural=beach".  An example (which I have only edited by adding a node where
the ferry picked us up) is http://www.openstreetmap.org/browse/way/116586186/ . 
Before I started fiddling, these areas were rendering in Mapnik as good
beach-looking yellow areas.

But now they seem to have disappeared.  For the beaches near where we were
staying, I walked the shoreline and back of beach with my gps, and have edited
the areas to reflect my survey.  An example is
http://www.openstreetmap.org/browse/way/127089185/ ;there are others to the left
and right.

They are not rendering as beaches in Mapnik (apart from one or two slivers which
seem to slip beneath the curtain of the coastline), though in Osmarender, they
are there OK.  I have raised this with Graham (they were his beaches originally)
but he is as mystified as I am.  What have I done to cause these beaches to
disappear in Mapnik?

I am very confused by the guidance in wiki e.g. at
http://wiki.openstreetmap.org/wiki/Tag:natural%3Dbeach it says:

<quote>

Beach areas should always meet with a natural=coastline way. Do not use this tag
for patches of sand/gravel which are not by a coastline; what should be used?
Note that the natural=coastline should ideally be positioned at the average high
tide line, which may mean the beach is quite small or not mapped at all in fact.

For seaside resorts use leisure=beach_resort.

alternate view

There are areas on rivers and lakes which are called "beaches". This is in
complete concordance with the Wikipedia definition of a beach, which meets a
"body of water". These beaches will not meet a coastline, and may be several
hundred km from one.
 
</quote>

What does "meet with a natural=coastline way" mean?  I have recently added nodes
where the edges of my beach cross the coastline way, but this doesn't seem to
have made any difference.

To try and understand if I've done something wrong, I had a look at another
beach area I'm familiar with, but which I haven't edited, at Wells-next-the-sea,
Norfolk (UK).  The beach here seems to be rendering fine, see 
http://www.openstreetmap.org/?lat=52.9689502716064&lon=0.840797424316406&zoom=13
with the areas of natural=beach straddling either side of the natural=coastline
and the area showing as beach on Mapnik is a good representation of the sandy
stuff that's there to enjoy.

So why doesn't that seem to be working at our beach in Katelios?

Apologies for this rather long post, but I'll be grateful to anyone who can
clear my confusion, and explain why Graham's original beaches rendered OK in
Mapnik, and my editing of them now doesn't.

Richard (user: richardm565)
 
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