[mkgmap-dev] [PATCH v2] Multipolygon: Improved role handling

Sun Mar 7 13:18:38 GMT 2010

>> I have found that some multipolygons use nested inner polygons.
>> Example:
>> * A forest (outer) with a lake (inner) that has an island (inner) which
>> is not a forest. (example:
>> http://www.openstreetmap.org/browse/relation/331402)
>>
>> By definition of the multipolygon relation
>> (http://wiki.openstreetmap.org/wiki/Relation:multipolygon) one has to
>> create two mps. But I see that it is a nice simplification lots of
>> mappers are using.
>>
>> Attached patch handles this situation.
>> The inner and outer roles are now used as follows:
>> * the outmost polygon must have role=outer (or role=<empty>)
>> * all outmost polygons with role=inner are not used (a warning is issued)
>> * all inside polygons with role=inner use their own tagging
>> * all inside polygons with role=outer use the mp tagging if available or
>> their own tagging if the mp is not tagged
>> * all inside polygons with role=<empty>  are categorized automatically as
>> outer/inner depending on the nest level
>> * all inside polygons with role=outer without a direct ambient inner
>> polygon are dropped with a warning message (outer in outer polygon)
>>
>> Most of the renderers do not support inner in inner polygons but I think
>> it is a very reasonable extension of the multipolygon handling.
>>
>> This patch also improves the error messages. I haven't seen any more
>> "are not processed due to an unknown reason" messages and I look forward
>> that the community will also not be able to generate any more ... ;-)
>>
>> WanMil
>>
>
> There have been different opinions about this patch.
>
> There is one controversal point:
> Should mkgmap allow nested inner polygons?
>
>   From my point of view: YES. Because it does not harm in any way. The
> nested inner polygons I analysed were all reasonable. Ok, they are not
> 100% compliant to the specification but this is common to lots of others
> things in (anarchic) OSM.
>
> At the moment I see two possible changes:
> 1. Issue a warning for any nested inner polygon but process it anyhow.
> 2. Reject nested inner polygons (and issue a warning). What a pity but
> they are not 100% compliant.
>
> Please let me know what's the opinion of mkgmap community and committers
> because I want the improvements of error messages to be committed.
>
> WanMil

The patch improves the handling of roles and warning messages in 
multipolygon.
- A reasonable warning message for nested outer and nested inner 
polygons is given
- nested inner polygons are processed anyhow

WanMil

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