[Imports] Antarctica coastline/shelf-ice import

[Christoph Hormann]

On Wednesday 06 March 2013, Martin Koppenhoefer wrote:
>
> probably it would be there if there was more activity in Africa in
> OSM. Usually we don't accept deletions because a feature is not yet
> mapped elsewhere. I am opposing the idea to remove the "default"
> landcover.

Note the current main glacier multipolygon consists of the GADM 
coastline ways and a single inner ring at the Norwegian Troll station 
marking an ice free area (which we intend to keep - see import plan).  
So removing the multipolygon won't actually remove any data except for 
the information of the default.

We have briefly discussed the idea of splitting the polygon into smaller 
areas, but the only logical way to do this would be to split along 
drainage basins and unlike in case of river systems drainage patterns 
are not so well defined in case of ice.  To get the individual areas 
small enough to be good to handle you would need to introduce arbitrary 
splits and we did not consider this to be a viable solution (it would 
violate the basic rule of mapping that some feature in the data sould 
always represent something in reality and it is ugly to handle when 
making edits at those splits).

To give some numbers: The current ice sheet polygon contains 129 ways 
with ~28000 nodes - this is not large at all compared to multipolygons 
elsewhere.  With the data to be imported this would be more than 650 
ways with ~120000 nodes.  The number of inner rings (ice free areas) 
that could be mapped based on currently available satellite images goes 
into the millions.  Even if we split the ice sheet into something like 
6 major drainage areas those would still be large enough to make 
editing and using the data quite difficult.

> I generally like the idea to import data for the Antarctic and am not
> against deleting very rough coastlines or other data there (with the
> intent to replace it with more accurate data by import and with the
> old data coming from previous imports), but you shouldn't delete
> "correct" mapping and you should pay attention to the (maybe
> existing) manual refinements and mapping in the area (I.e. keep it).
> It is a really huge region and while there might be a lot of fun and
> none sense objects there might also be some treasures.

We do not plan to delete any data that has been significantly improved 
from GADM or newly mapped in better quality than the import data.  All 
detailed mappings of research stations will stay untouched anyway.  
Some of the islands at the Antarctic peninsula are somewhat tricky 
since they have been partly mapped in more detail but often with a 
significant offset.  Therefore we plan to do the import for the islands 
separately and will look at each of those comparing with satellite 
images to decide if to replace the existing data or not. 

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