[talk-ph] better ways to coordinate coastline mapping?

Eugene Alvin Villar seav80 at gmail.com
Tue Jul 6 04:57:01 BST 2010


To clarify things, the sawtooth detection script is quite naive. It simply
detects if there are at three or more series of nodes where each pair of
adjacent nodes have the same latitude or longitude. This will also detect
any three linear nodes that all have the same latitude or longitude like
this: o----o----o  (A bit silly, I know, but the middle node should probably
be deleted anyway.)

The idea of the webmap is to highlight where there are coastlines derived
from the raster SRTM data. 99% of our coastlines were derived from SRTM and
any "sawtooth" is an indication that this should be cleaned up to better
match the actual coastline (like Landsat or PGS).

It's ridiculously easy to defeat the script: just nudge nodes. But the point
is not to defeat the script but to check out areas in need of correction. I
would assume that anybody moving coastline nodes would do so to correct or
refine the data and not simply to defeat the script, right? Right? :-D

Another clarification, the webmap is still just an initial map and it
doesn't currently do any updates (so maning's recent work on Mindanao's
eastern coast won't be picked up yet). I'm working with maning to devise a
way to have the map update on a near-daily basis by tying into his OSM-PH
Garmin Maps workflow. Wait in the future for this.



On Tue, Jul 6, 2010 at 10:17 AM, Ed Garcia <eppgarcia at gmail.com> wrote:

> Ah I get it ... So it detects node strings that form a "step" (a horizontal
> and a vertical combination).  It may be good to make mappers aware that such
> a combination will trigger sawtooth detection by Eugene's script.  For there
> are some coastline contours that are indeed on a step shape.  Good way to
> avoid it is to move the nodes just a bit so the longitude/latitude pairings
> would not be exactly the same.  I found this out on one of my Marinduque
> island edits.
>
> thanks!
>
>
>
> On Tue, Jul 6, 2010 at 9:31 AM, maning sambale <emmanuel.sambale at gmail.com
> > wrote:
>
>> @ ed,
>> The technique is outlined here:
>>
>> http://wiki.openstreetmap.org/wiki/Talk:WikiProject_Philippines/Coastline_Corrections
>>
>> Couple of suggestions:
>> 1. short link to the coastline correction wikipage
>> 2. enable link to editors only at higher zoom level (perhaps 15-17?)
>> 3. should be an osm-ph image of the week
>>
>> On Tue, Jul 6, 2010 at 9:20 AM, Ed Garcia <eppgarcia at gmail.com> wrote:
>> > Wow! amazing ... how do you detect "sawtoothness"?  can you teach this
>> at
>> > the skillshare?  OSM genius ka talaga!
>> >
>> > Anyway, I take note of the sawtooth patterns between Lucena and Pagbilao
>> > Quezon ... I have corrected this already but the sawtooth appears only
>> on
>> > higher zoom-out levels now.  Does this mean it is good as "done" now?
>> It
>> > still appears on yoursawtooth_coastlines map though.
>> >
>> > ed
>> >
>> > On Tue, Jul 6, 2010 at 1:04 AM, Eugene Alvin Villar <seav80 at gmail.com>
>> > wrote:
>> >>
>> >> HI guys,
>> >>
>> >> Here's an *initial* webmap showing where the jagged coastlines are as
>> of
>> >> July 3:
>> >> http://forge.codedgraphic.com/osm/sawtooth_coastlines/
>> >>
>> >> The larger the circle, the more jagged steps there are to clean up.
>> >>
>> >> Sorry, there's no link going to Potlatch or JOSM/Merkaartor. I'll add
>> that
>> >> up when I learn more of OpenLayers. But for now, this will have to do.
>> >>
>> >> Enjoy!
>> >>
>> >> Eugene
>> >>
>> >> On Mon, Jul 5, 2010 at 9:30 AM, maning sambale
>> >> <emmanuel.sambale at gmail.com> wrote:
>> >>>
>> >>> We missed the June target to to finish the 10 largest islands.
>> >>>
>> >>>
>> http://wiki.openstreetmap.org/wiki/WikiProject_Philippines/Coastline_Corrections#Priorities
>> >>>
>> >>> That's OK.  All we need is to finish mainland Mindanao and we can set
>> >>> our radar to the next 20 on the list.
>> >>> We are also working on some webmap to visualize the "sawtooth".  More
>> >>> on that when its ready.
>> >>>
>> >>>
>> >>> On Sun, Jul 4, 2010 at 5:48 PM, maning sambale
>> >>> <emmanuel.sambale at gmail.com> wrote:
>> >>> > I can load it in my public dropbox.
>> >>> >
>> >>> > On Sun, Jul 4, 2010 at 2:23 PM, Eugene Alvin Villar <
>> seav80 at gmail.com>
>> >>> > wrote:
>> >>> >> Hi guys,
>> >>> >>
>> >>> >> Once we are done with the large islands, the next step is to do the
>> >>> >> smaller
>> >>> >> islands. Ian Haylock has helpfully provided the PGS coastlines[1]
>> for
>> >>> >> the
>> >>> >> Philippines last year[2] which he edited to combine ways. PGS
>> itself
>> >>> >> was
>> >>> >> automatically generated by a software by analyzing the Landsat
>> >>> >> imagery.
>> >>> >> Unfortunately, Ian's file was hosted on a free file server and has
>> now
>> >>> >> expired but I think several people on the mailing list managed to
>> >>> >> download
>> >>> >> it.
>> >>> >>
>> >>> >> I tried opening the file last year in Merkaartor but my old laptop
>> >>> >> couldn't
>> >>> >> handle the data. Now that I have a 4GB-RAM laptop, loading the data
>> >>> >> into
>> >>> >> Merkaartor is very manageable. Using the PGS coastlines is much,
>> much
>> >>> >> easier
>> >>> >> than tracing Landsat by hand. I tried this with Homonhon Island and
>> >>> >> editing
>> >>> >> is way faster than actually uploading it[3]. Hehehe. What I did was
>> to
>> >>> >> delete the SRTM-derived coastline[4] and replace it with the PGS
>> >>> >> coastline[5].
>> >>> >>
>> >>> >> I think uploading the PGS coastlines is better for the smaller
>> >>> >> islands. I'll
>> >>> >> see what we can do to have the PGS coastlines (55MB) uploaded
>> >>> >> somewhere.
>> >>> >>
>> >>> >> [1] http://wiki.openstreetmap.org/wiki/Prototype_Global_Shoreline
>> >>> >> [2]
>> >>> >>
>> http://lists.openstreetmap.org/pipermail/talk-ph/2009-June/001071.html
>> >>> >> [3] http://www.openstreetmap.org/browse/changeset/5129873
>> >>> >> [4] http://www.openstreetmap.org/browse/way/4327906
>> >>> >> [5] http://www.openstreetmap.org/browse/way/65592780
>> >>> >>
>> >>> >> Eugene
>> >>> >>
>> >>> >>
>> >>> >> On Thu, Jun 17, 2010 at 8:20 PM, maning sambale
>> >>> >> <emmanuel.sambale at gmail.com>
>> >>> >> wrote:
>> >>> >>>
>> >>> >>> UPDATE. Only two islands from the top ten need some coasty
>> coastline
>> >>> >>> love.
>> >>> >>>
>> >>> >>>
>> >>> >>>
>> http://wiki.openstreetmap.org/wiki/WikiProject_Philippines/Coastline_Corrections
>> >>> >>>
>> >>> >>> Anyone working on Negros and Mainland Mindanao?
>> >>> >>>
>> >>> >>
>> >>> >>
>> >>> >
>> >>> >
>> >>> >
>> >>> > --
>> >>> > cheers,
>> >>> > maning
>> >>> > ------------------------------------------------------
>> >>> > "Freedom is still the most radical idea of all" -N.Branden
>> >>> > wiki: http://esambale.wikispaces.com/
>> >>> > blog: http://epsg4253.wordpress.com/
>> >>> > ------------------------------------------------------
>> >>> >
>> >>>
>> >>>
>> >>>
>> >>> --
>> >>> cheers,
>> >>> maning
>> >>> ------------------------------------------------------
>> >>> "Freedom is still the most radical idea of all" -N.Branden
>> >>> wiki: http://esambale.wikispaces.com/
>> >>> blog: http://epsg4253.wordpress.com/
>> >>> ------------------------------------------------------
>> >>>
>> >>> _______________________________________________
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>> >>
>> >>
>> >>
>> >> --
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>> >>
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>> >
>> >
>> >
>> > --
>> > website administrator:
>> > - www.waypoints.ph
>> > - reeflife.eppgarcia.com
>> >
>> > PADI Divemaster #491048
>> >
>>
>>
>>
>> --
>> cheers,
>> maning
>> ------------------------------------------------------
>> "Freedom is still the most radical idea of all" -N.Branden
>> wiki: http://esambale.wikispaces.com/
>> blog: http://epsg4253.wordpress.com/
>> ------------------------------------------------------
>>
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>
>
>
> --
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> - reeflife.eppgarcia.com
>
> PADI Divemaster #491048
>
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