Monday, October 25, 2010

Red dot in Google Earth

In a thread at the ThaiVisa forum - which I only noticed because a friend of this blog guided the readers here for more detailed information - the question came up what the red dot in Google Earth found for every Tambon is supposed to mean.

The above screenshot shows the area around the office of the subdistrict municipality Wat Pradu (เทศบาลตำบลวัดประดู่), located between Surat Thani city and Phunphin. The yellow marker is from my collection of office locations, and as I have passed the office several times I can even show a photo of the office complex - though a not that good one as I was on the wrong side of the big highway. One can see well that the red dot for Wat Pradu is located almost exactly on the office buildings as well. And when looking at the same area in Google Maps, instead of the red dot the name of the subdistrict shows up at the same location.

If this were the case for every Tambon I would have no problem to get XML full with all the locations, and it would be boring to write about these dots. For another office close by, the TAO Bang Chana (องค์การบริหารส่วนตำบลบางชนะ), Google places its dot into a palm plantation in the north of the subdistrict, whereas the placemark in Longdo Map points to an actual building in the very south. Though it is close to the city Surat Thani I haven't been able to verify that location yet, but in this case I believe the Thai map of Longdo much more than Google.

Another thing is that it seems Google has exactly one red dot per subdistrict, so they cannot correspond to the local government units exactly - there are subdistricts with two of them, for example Khanom subdistrict, Khanom district, Nakhon Si Thammarat has both a subdistrict municipality and a TAO, and the red dot points near the district office but nowhere near neither of the two local government offices. and for Talad subdistrict within the city of Surat Thani the red dot is within a private villa, even though there is no administrative office for this subdistrict at all. I'd be curious where did Google get the coordinates for these placemarks at first...


Patrick said...

As far as I know Google Maps had wrong lat/long-data for some time until they fixed it several years ago. I am working on a Google Map project of my own and found my data about Wats on Koh Samui - all points were at the correct spots in 2005 - now they are some metres away. I guess the same happened to those red dots.

Andy said...

If they were systematically off by some meters it'd be a different story, my point is that sometimes they point to the office of the corresponding local government unit within the Tambon, sometimes they seem to point to a random place, and they are even present in Tambon which have no local government unit by themselves.

Besides a few metres is what the satellite images jump around anyway, quite notable if you use the historical imagery function. For example the Lak Mueang of Surat Thani jumped 6 meters to the southwest between the 2005 imagery and the current one. Not sure if the current ones are the more accurate ones, or it is all within the standard error margin - standard GPS also has the same error margin.