How to map a reversed cone water tower?

It’s possible to map correctly this water tower?

The pillar should have a height tag, the reverse cone tank should have min_height and height, but what about the cone shape?

There was very similar problem about sphere shape here.

For me, your example could be tagged as single point with tags like:

height=15	<= height of cone top
min_height=10	<= height of cone base 
radius=5	<= radius of cone

But be aware that it is only idea and none of application do not render this right now.

Very interesting solution!

But this building (and probably the most of buildings like that) is a truncated cone, then what about tags like:

shape=cone (or truncated_cone)
height=15	<= height of cone top
min_height=10	<= height of cone base 
radius=5	<= radius of cone at the top
min_radius=2	<= radius at the base

_______	<- radius and height refer to here
\     /
 \___/	<- min_radius and min_height refer to here

In such a case don’t matter if the cone is reversed or not.

I think this is going in the right direction. However, I don’t think a single point is the best representation. Instead, I would use a normal building:part way representing the widest circumference of the shape.

This would produce a much better fallback for renderers that don’t understand this tagging. With your solution, they wouldn’t render anything at all if they don’t understand the tagging.

Additionally, using a way would be compatible with the various suggested approaches for adding attributes to only one wall section of a building part (e.g. when the southern part of the cone is red and the northern part is green).

This would also allow us to omit tagging the larger of the two radii.

We have the same situation with the cone in other direction.
Probably one possible way coud be to draw both shapes with the information what is the top and what is down.
In this case we had to use a relation for the both.
This solution could generally render more detailed complicated shapes like:—wikimedia-commons-uh3qsika.jpg

or this:

I don’t think relations are necessary or desirable here. It is true that we could model truncated cones by putting the top and bottom area into a relation. However, I don’t think this would be preferable – it would not work for circles, for example, and I also doubt that it would actually work for the shapes from the photos linked in Marek’s post.

I’m still in favor of using tags like the one several posters have already suggested.

We have unfortunately no circles in OSM. We ose instead “cirlce like” multipolygons with n-points. They should have the same number of points.
Of course, when you have only 2 shapes, you don´t need relations. What about situations with more shapes?
In such case the shapes may have as attributs number or height, which would be more comfortable for the users.

Examples with tagging suggestion comes soon.