[talk-ph] areas of mangrove

[David Groom]

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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