Re: [talk-ph] areas of mangrove

[Eugene Alvin Villar]

Hi David,

Mangroves and wetlands have been discussed here in 2009 and 2010 but no
definite convention or guidelines was agreed upon. Please see the following
two threads for the previous discussions:

https://lists.openstreetmap.org/pipermail/talk-ph/2009-April/000695.html
https://lists.openstreetmap.org/pipermail/talk-ph/2010-July/002398.html

Based on agreed practice in OSM, the natural=coastline is defined to be the
high water line. So mangrove areas would be normally at the seaward side of
OSM's coastlines. But I think the answer is between your approach 1 and 2.
Mangroves can actually extend to the land-side of the coastline as the
ground there would still be saturated with sea water even if the ground is
not submerged at high tide. The problem is, the high-water line will rarely
be visible on satellite imagery. So I think we just map using approach 2
when doing remote/armchair mapping and then hope that these can be refined
in the future using actual field surveys.

Regards,
Eugene


On Sat, Apr 30, 2016 at 10:14 AM, 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
>
>
>
> ___
> talk-ph mailing list
> talk-ph@openstreetmap.org
> https://lists.openstreetmap.org/listinfo/talk-ph
>
___
talk-ph mailing list
talk-ph@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-ph

Re: [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



___
talk-ph mailing list
talk-ph@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-ph


___
talk-ph mailing list
talk-ph@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-ph



___
talk-ph mailing list
talk-ph@openstreetmap.org
https://lists.openstreetmap.org/listinfo/talk-ph