[OSM-talk] Big Lakes

[Christoph Hormann]

On Tuesday 17 February 2015, malenki wrote:
> I am working on Lake Nasser* and can predict that after enhancing
> it's shore the resulting MP will be quite big.
> Based on what I have done so far I'd expect an Multipolygon (MP) with
> about 10.000 Members and an outline of 14.000 km length. A relation
> of this size is no good idea in hindsight of maintainability and
> conflicts due simultaneous edits.

10000 ways each with 2000 nodes would be 20 million nodes.  Evenly 
distributed on 14000 km outline means a node distance of 70cm - your 
average node distance seems to be more in the range of 10-20m - i 
suppose something is wrong here, for comparison the world coastline is 
only 33 million nodes.

> What do you think is the better way to map an updated Lake Nasser?
> Make another MMP (Monster MultiPolygon) or
> map it as coastline (which is discouraged in the wiki)?

Please no re-opening of the moratorium on newly tagging lakes as 
coastline.  If what is tagged as coastline changes this always means 
additional work for anyone processing the data.

Without knowing what exactly is wrong about your number above - based on 
the level of detail of your current mapping relative to the previous 
one i would estimate it to be not that much larger than other big lakes 
(Great Slave Lake is currently ~300k nodes).  From an absolute 
standpoint this is not really that big but i know editing such a beast 
in JOSM is no fun.

Area data type anyone?

> Regarding the Big Lakes:
> At the moment they are mapped with coastline /and/ partly as MP.

Last time i looked all land enclosed waterbodies (including the Caspian 
Sea) had multipolygon relations.  I did not check if these are valid 
and complete though - at least for the Great Lakes they are probably 
not.

Technical things aside - i hope you are aware that the water level of 
Lake Nasser varies quite a lot and when you map based on Bing images 
you probably map different water levels in different parts of the lake.  
There currently is no established rule what water level to map as 
natural=water in such a case (average/maximum/minimum) or how to tag 
separate mappings of different levels.  In any case you might want to 
consider that mapping both the minimum and maximum based on lower 
resolution data (like Landsat images) would be ultimately more useful 
than mapping a fairly undefined in-between state in higher resolution.

In any case nice to see improvements to such more remote lakes.  When 
you are done with Lake Nasser you could think about continuing with the 
Merowe Reservoir - which is currently a serious aspirant for the title 
of the most broken lake polygon in the OSM database:

http://www.openstreetmap.org/#map=16/18.9860/32.4292

;-)

-- 
Christoph Hormann
http://www.imagico.de/