Totor
You are correct in that in general the coastline should be at the "mean
high water spring". However it may be that areas of mangrove need
special consideration because even at high water the mangrove trees are
above the water and therefore (a) when viewed from above you don't see
sea; and (b) its very difficult to navigate even a small boat through
the thickest parts of them; and in in this respect these areas are more
like land than sea.
When many years ago (2007), I imported the coastline from PGS for large
parts of the world I remember in particular coming across this dilema
when working on northern Ausrtralia. I believe we had a discusion on
the talk-au list and decided to map the mangrove / open sea boundary as
coastline, and if you look at
http://www.openstreetmap.org/#map=12/-12.5837/130.8784 you will see
that despite the number of years between my import and the present day,
and the large number of mappers who have since contributed to Australian
mapping, the coastline still remains at the mangrove / open sea
boundary. So there is a precedent for breaking the general rule the
coastline should be at the "mean high water spring".
I think that rather than stick to a defined rule as stated in the wiki
this is one instance where the local community should decide what they
believe is the most appropriate mapping approach, and then that
consistent approach should be adopted for the whole country. From what
you write it would seem the most consistent approach, where it is
possible from imagery or other sources to locate the dry land / mangrove
boundary to treat that as the coastline.
Regards
David
On 30/04/2016 03:40, Totor wrote:
Hi David,
I understood the coastline should be at the "mean high water spring".
The mangroves I have seen in the Philippines, were always outside that
coastline, in the sea.
So I map them as in your method 2.
When tracing from low res or unclear sat imagery, i usually include the
mangroves in the land area (because i dont know if it is a mangrove) but then I
do not tag the mangrove at all.
Just my opinion...
Cheers
Totor
On April 30, 2016 10:14:43 AM GMT+08:00, David Groom wrote:
There are two different approaches used in mapping mangrove areas in
OSM
1) Treat the boundary of the mangrove and the openwater sea as the
coastline, and then map the area between that line and the "dry" land
as
wetland. This means that the wetland symbols are rendered over the
white colour of the land, and that at zoom levels 12 and lower the
mangrove areas simply get shown as white, with the sea outside them.
2) Treat the boundary of the mangrove and the "dry" land as the
coastline, and then map the area between that line and the openwater
sea
as wetland. This means that the wetland symbols are rendered over the
blue colour of the sea, and that at zoom levels 12 and lower the
mangrove areas simply get shown as blue sea.
Early today I added some mangrove areas and followed approach 2 because
the coastline had been accurately mapped along the mangrove / dry land
boundary, as so I simply added the mangrove area outside this, as it
seemed the existing mapper had cleary thought the coastline should be
at
the dry land boundary.
However at http://www.openstreetmap.org/#map=14/9.7497/125.6105 both
these approaches have been used. Approach 1 has been used for Lamagon
Island, where the boundary of the mangrove area and the sea is tagged
as
coastline. But Approach 2 has been used for the island immeditately
south, where the boundary of the dry land is tagged as coastline.
On further investigation I see at
http://www.openstreetmap.org/#map=14/9.7003/125.6415 that Approach 1
has been used.
Has this issue been discussued before within the Philippine OSM
community, with any recommended way of mapping mangrove areas being
decided upon?
Regards
David
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