[OSM-talk] Geographic objects

[Christian H. Bruhn]

Hi!

If we look at the OSM-map (you can take each), there will be missing
the name of most of all geographic objects. You will not find the
Atlantic Ocean, the Alpes or anything else.

I think it is very important to give geographic objects names and
display them on a map. The map will look more complete and you can
work the information e.g. if someone tells you that he went hiking in
the Alpes and you don't know where they are, you take a look at OSM
and you will find it.

In the next breaks I will concentrate on water objects as an example.
But it is not limited to this. You can use it for every geographic
object.

Which tag can we use?

I thought about 'natural' or 'geographic'. But we use the
'natural'-key already to describe that there is water. 'geographic'
would be a new tag, but I think that we can you 'place' for this
purpose. We use it already for islands. So we can expand the tag with
different values like:

      place=ocean
      place=sea
      place=bay
      place=archipelago
      place=mountain_range
      place=low_mountain-range
      place=landscape
      ...
      place=deep_ocean_trench
      ...

As you can see there is no limitation for mapping all kind of objects.

How should these objects look like?

There is already a now for the Baltic Sea [1] (and it is tagged with
'place=sea') You might think that this is the most easy way to tag a
sea. You're right. It is easy but not so satisfying.

When you create a map on which is only a part of the Baltic Sea which
doesn't contains the node, you could not display the name. If you
have just the coastline, you have no information what kind of sea or
ocean it belongs. And in some non-OSM printed maps you have the name
extended to almost over the whole sea.

So we need to describe the shape ob the object. Luckily we have
already the coastlines-ways. Historically we were not able to handle
such giant object like a sea. But now we hvae the
multipolygon-relations which we already use for national borders. We
only have to create MP-relations with the tag place=* and name:*=* to
map geographic objects.

You can create overlapping objects. Oceans and seas are often divided
into different seas. Just create one the whole sea (like the
Mediterranean Sea) and then create a different MP-relation (like the
Adriatic Sea). It doesn't bother if the objects overlaps. For
rendering you can use the size and shape of the MP to display the
name.

One thing I forgot: Of course you have to create new lines where the
sea ends and goes over to another sea. And sometimes it is not easy
to tell exactly where one sea or bay ends. But it is like always in
OSM: Map the object as good as you can, and if somebody knows it
better he could improve the object.

I created an MP for the Mediterranean Sea as an example [2]. These
relation describes only the outer-line; the inner have to add later.
But the JOSM-plugin WaySelector crashed on my machine [3], so that I
had to click every single coastline-way.

The only thing I am not sure about is how to exactly tag this
'superrelation' as a MP. The example contains 7 MP-relations with each
about 500 ways. The relations are put together in one 'superrelation'.
But I think we should use mother- and children-relations for large
objects, otherwise the relations will to big and hard to work with.

Please feel free to improve the example, to create new geographic
objects, to test how to use them (renderer, namesearch, etc.) or to
bring in new ideas.

Christian

[1] http://www.openstreetmap.org/browse/node/413554195
[2] http://www.openstreetmap.org/browse/relation/1159204
[3] http://josm.openstreetmap.de/ticket/5394