[talk-ph] Node position for towns

maning sambale emmanuel.sambale at gmail.com
Wed Apr 7 02:54:10 BST 2010


Last year, Axel proposed an approach to make the place nodes more accurate:
http://www.mail-archive.com/talk-ph@openstreetmap.org/msg00360.html

<snip>
this is because GNS specifies towns and cities (Feature Designation Code
(DGS) == ADM2 (second-order administrative division)) with a low 1 to 2
digits after the comma precision only. which makes sense, as these
entities can be quite big.

there are also much exacter (3 to 5 digit precision) coordinates of all
these towns and cities in GNS, the ones with DGS == PPL, populated
place. for example, for maasin in southern leyte:

ADM2:   10.2, 124.85
PPL:    10.133611, 124.844722

unfortunately, there is no easy way to relate the place with the exact
coordinates to the administrative division. which i guess is why the
import used the low precision coordinates and we got towns in the
Philippine Sea and coastal towns on mountain ranges now.

there might be some way, though. i've been fiddling around with this for
some hours now and got some results that don't look too bad. my basic
idea: get the name and some other characteristics (province, jog) of
ADM2 and find the PPL entry with the same name and characteristics.

there are some problems with this approach: ADM2 and PPL must have the
same name (doesn't work for "Ormoc City"/"Ormoc"), sometimes there are
more than 1 PPL with the same name and characteristics as ADM2 ("San
Miguel"), sometimes the PPL coordinates are not of better precision, ...
nonetheless, with some tweaking, i got precise coordinates for 1271
towns and cities, or 78% of the "136 cities, 1,495 municipalities" [1]
of the philippines.

for a visual comparision of the old import and my results, i uploaded 2
josm screenshots of leyte with landsat background [2]. first with the
old towns and cities, then with the better precision. note how the
coastal towns align nicely with the coastline in the second image.

comments? any interest in redoing the import like this?

ax

p.s. does anyone know of a way to automatically find all towns and
cities that have been edited / relocated after the initial import? these
  towns wouldn't have to be reimported.

[1]
http://en.wikipedia.org/wiki/List_of_cities_and_municipalities_in_the_Philippines
[2] http://picasaweb.google.com/axkosm/gns

</snip>

He is referring to a wikipedia's Cities and municipalities entries
which has a lon/lat coordinates.  I've checked on some of them and
indeed it seems to be more accurate:
http://www.openstreetmap.org/index.html?mlat=17.6&mlon=120.613889&zoom=12&layers=B000FTF

However, many of these coordinates were extracted from Google Maps
(there goes the debate again).  In addition, even if we are allowed to
use this data, it maybe even more harmful to re-import 136 cities and
1,495 municipalities again because many have been manually moved
already.

Re-visiting his message and looking at his screenshots:
http://picasaweb.google.com/axkosm/gns#5294006509150710466

Another way for those interested to do wholesale adjustment of place
nodes is to use Landsat image (in JOSM or Merkaartor).  Concentration
of settlements which is an indicator of the town's poblacion are seen
as gray-purpleish color.  You can adjust the nodes within that area.

For long distance travellers, you can also note the KM markers along
highways.  KM 0 is usually the town center.

On Wed, Apr 7, 2010 at 8:20 AM, Marloue Pidor <murlwe at mail2engineer.com> wrote:
> On that note, Maguindanao's Municipal Hall is located somewhere in the
> plains with only two large structures in sight the rest are insignificant.
> That's the center of the municipality and almost nothing around the area. I
> been there years before and still no significant structures around the town
> center except two big houses.
>
> murlwe
>
>
> <-----Original Message----->
>>From: maning sambale [emmanuel.sambale at gmail.com]
>>Sent: 4/7/2010 6:52:03 AM
>>To: talk-ph at openstreetmap.org
>>Subject: Re: [talk-ph] Node position for towns
>>
>>And of course there will be exceptions. Puerto Princesa City Hall is
>>several kilometers away from the traditional city center/poblacion.
>>Some towns are also relocating town halls away from the poblacion in
>>order to add more buildings and decongest the center.
>>
>>
>>On Tue, Apr 6, 2010 at 11:59 PM, Eugene Alvin Villar <seav80 at gmail.com>
>> wrote:
>>> I suggest to place the node in the town/city proper, which is not the
>>> geographical center and not always where the city or town hall is. The
>>> city/town proper is basically where the town plaza or poblacion area is.
>>> Usually, it's where the church or public market is.
>>>
>>> For example, the Las Pinas city hall is here:
>>>
>>http://www.openstreetmap.org/?mlat=14.4496929645538&mlon=120.982496738434&zoom=14
>>>
>>> But I placed the Las Pinas place=city node in the poblacion area, where
>>> the
>>> plaza and the main church (home of the famous Bamboo Organ) is:
>>>
>>http://www.openstreetmap.org/?mlat=14.4809997081757&mlon=120.981595516205&zoom=14
>>>
>>>
>>> On Tue, Apr 6, 2010 at 10:22 PM, Jim Morgan <jim at datalude.com> wrote:
>>>>
>>>> Ed Garcia wrote, On Tuesday, 06 April, 2010 10:05 PM:
>>>> > Glad you agree guys! Surprised to see so many +1s in barely an hour
>>>> > from posting. Okay then, I'll start with San Antonio in Zambales.
>> That
>>>> > town node is currently so far away from town proper ... it is
>>currently
>>>> > located on the mountains.
>>>>
>>>> Seems like a lot of these town / village / city markers are way off the
>>>> mark to begin with. I guess they were imported en masse from an
>>>> imprecise
>>>> source. They usually seem to be 1 or 2 km off the mark. Usually when I
>>>> find
>>>> them, I'll drag them closer to a known settlement -- sort of plonk it
>>in the
>>>> middle of the most obvious mass of houses on a satellite view, or near
>>>> the
>>>> most obvious confluence of roads. Then when someone with more local
>>>> knowledge finds it, they drag it closer to the City Hall ... and when
>>>> the
>>>> City Hall is actually on the map, then it's truly accurate.
>>>>
>>>> This process of gradually getting more and more precise seems to be what
>>>> OSM is all about really.
>>>>
>>>> Jim
>>>>
>>>> --
>>>> datalude: information security
>>>> e: jim at datalude.com
>>>> Philippines: +63 2 403 1311 / mob: +63 920 912 5830
>>>> Hong Kong: +852 6840 6693
>>>> w: http://www.datalude.com/
>>>>
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>>>
>>>
>>>
>>> --
>>> http://vaes9.codedgraphic.com
>>>
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>>
>>
>>
>>--
>>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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-- 
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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