[OSM-talk] Big Lakes

[malenki]

Christoph Hormann wrote:

>On Tuesday 17 February 2015, malenki wrote:
>> 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.

A subset of 92.331 km I just looked at has 8,764 nodes. That is not too
much imho. You should take into consideration that there are also some
islands which are responsible for the big growth in the count of
members.

>> 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

Imho nothing

> - 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?

Seems I forgot to mention this in my OP…

>> 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.

Not all of them, as I wrote:
>>  three MPs for three of the five lakes

>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.

I am aware of this and not too lucky about it.
The shape existing until my updates started I created in 2009 with
Landsat Imagery. Then it had 8000 km length and was quite rough.
For curiosity I had a look at the first version of Lake Nasser – the
shore was 1733 km long.

>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.

Imho the minimum would be empty ;)

>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

At least three times smaller then Lake Nasser – piece of cake. 
If only someone would pay me I so could do mapping all day. (: