[OSM-dev] A dilemma, problem, challenge

[Sandor Seres]

Is there anyone out there having a suggestion, a model for a solution to
the following typical mapping/cartographic problem:

-Assume we have given two sets of area objects. For simplicity, let us
call them L1 (large lakes) and L2 (small lakes).  Also, for simplicity,
let us use the topology area notion where an area is defined by one, and
only one, outer (enclosing, container) border polygon and a zero or
arbitrary number of inner (excluding, hole) border polygons. Anything
inside a hole-polygon, just like anything outside the container-polygon,
is outside the corresponding area. How L1 and L2 are created is irrelevant
but they may be of huge cardinality.

-Assume that there are no other than topologic relations between the
elements of L1 and L2. In other words, areas from the large and small
lakes may fully, almost or partly overlap; some of the holes may belong to
objects in L1 and L2 at the same time, many holes only in objects from L1
but not in objects of L2 and the contrary, and so on. 

So, the question/problem is - find/create a new set of areas L that covers
L1 and L2 but where the holes from L1 and L2 remain as holes in L?

Real-world examples of this issue may be found in OSM source data. Take
for the L1 lakes (or holes) from the planet_land (created from the
coastline data) and for L2 lakes from the natural_water data. Whatever
rendering model is used, some of the islands will disappear. Rendering a
new area class over L1 and L2 will never solve the described problem.

The dilemma is why some islands are present only in the coastline data but
not in the overlapping natural_water data and the contrary.

The described issue is obviously a problem in many OSM mapping
applications. 

Finally, the problem might be a challenge for researchers, mappers and
programmers. Note that even in a well founded theoretical solution a
program should probably handle hundreds of thousands polygons and hundreds
of millions vectors simultaneously.

Some more detailed specification and examples you may find in a white
paper here:

https://docs.google.com/file/d/0B6qGm3k2qWHqemFoeXpseTlndFk/edit?usp=shari
ng

 

Regards, Sandor.

 

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