[Tagging] Using multipolygons to map bays in Alaska

Dave Swarthout daveswarthout at gmail.com
Thu Nov 15 05:32:39 UTC 2018

Since the long thread I started about multipolygons  [1
is sort of winding down, I thought it best to start a new one to discuss
something I've been doing since learning from that thread of the amazing
power of the multipolygon data structure. Alaska has hundreds of bays and
straits that have been added to OSM as nodes many years ago via an import
(Tiger?). The nodes contain useful information but because they're only
nodes cannot provide any visual representation of the size or extent of
bigger bays and straits. I've begun to use multipolygon relations to show
them more prominently using sections of coastline as the ways and then
drawing a connecting way across the mouth of the bay or strait to complete
the outer ring. This structure then defines the bay and when rendered it
shows up nicely on the OSM slippy map.

However, this method means that under certain circumstances some of those
ways end up as members of several multipolygons. For example, the coastline
might be part of a boundary, and in addition define a wooded or wetland
area so it might be confusing to new mappers who might be put off by the
complexities, as I was for a long time. That's one consideration. The other
is the question about using this technique to map truly large bays like
Cook Inlet. This is the reason I'm posting this question. Here is the first
paragraph of the Wikipedia entry for the Inlet:

Cook Inlet (Dena'ina: Tikahtnu) stretches 180 miles (290 km) from the Gulf
of Alaska to Anchorage in south-central Alaska. Cook Inlet branches into
the Knik Arm and Turnagain Arm at its northern end, almost surrounding
Anchorage. On its south end merges with Shelikof Strait, Stevenson
Entrance, Kennedy Entrance and Chugach Passage.

So, I could mouse along the coastline adding pieces of it to a new relation
until I have the entire Inlet surrounded, then draw a way from Elizabeth
Island on the east side to the other side to define its mouth. I copy the
tags from the existing node to the multipolygon and delete the node. But
with a water body the size of Cook Inlet, that's a lot of work. I mapped
Cook Inlet's Turnagain Arm [2
the other day and it involved adding 14 ways as outers. How many will a
multipolygon covering the entire Cok Inlet require?

I was thinking it would be much easier and perhaps even better to just draw
an approximate shape consisting of maybe 20 or 30 nodes, big enough to
define the area and cause it to render, but easy to draw and without
involving any multipolygons. The issue here is admittedly one I am pursuing
to get these water bodies to render in a manner proportional to their size
and I suspect that many will be against it on that basis alone. Still, I
thought it worthwhile to mention my idea here and see what you think about
it as a "shorthand" solution. Besides, I'm betting some other useful
information will issue from the discussion.


[2] https://www.openstreetmap.org/relation/8921978#map=9/61.1074/-149.5541
Dave Swarthout
Homer, Alaska
Chiang Mai, Thailand
Travel Blog at http://dswarthout.blogspot.com
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