[talk-ph] GNS town and city import (was: Hi Guys and Gals, Open Street Map)

[Axel Kollmorgen]

On 19.01.2009 11:50, Eugene Alvin Villar wrote:
> I think the place points for the towns and cities were imported from 
> the GeoNet Name Server. See this wiki page:
> 
> http://wiki.openstreetmap.org/wiki/WikiProject_Philippines/Data_import#Geonet_Name_Server
> 
> And it seems there are indeed errors. One town in the province of 
> Compostela Valley is placed out in the Philippine Sea!

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