comparing Philippine road data

A few weeks ago geofabrik announced free openstreetmap data download in shapefiles. This is a cool service because I can now compare the “accuracy” of openstreetmap data from existing government data for Philippine roads.

I have in my old ~/gis_data/road some shapefile of the roads of Quezon City, Philippines. The proj file says something like:
REM NAD-27 Latitude/Longitude, Scalars in Degrees of Latitude
REM LL
PROJECTION GEOGRAPHIC
UNITS DD
PARAMETER

So I set my projection in QGIS to:
+proj=longlat +ellps=clrk66 +datum=NAD27 +no_defs

I also added a gpx layer of my recent trip in the area.

The resulting image is here (red=QC roads, yellow=gps trace):

Note that the GPS trace does not match. Hmmm.

I then added the openstreetmap data of the same area. The proj file says something like:
GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.25
7223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]

So I set my projection in QGIS to:
+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs

The resulting image is here (blue=OSM roads, yellow=gps trace):

GPS trace matches the road.

Now I overlayed both data:

Why such a huge "offset"? Usual suspects would be wrong coordinate system and datum shift. So I tried everything under my sleeve, but nothing seems to work.

I then asked other people for my mailinglist on the problem. The conclusion, the government road data is crap data. It looks like the road data was "rubbersheeted" grossly.

If this is what we get from the government (data paid by my taxes by the way), why should we trust them in generating reliable geodata?

Might as well contribute to openstreetmap.

About these ads

One response to “comparing Philippine road data

  1. Pingback: user:maning on OSM Philippines - 2008 « 4253·

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s