province border between petchaburi and prachuab kirikhan is incorrect

as the title implies, this border is incorrect. I live in prachuap province but the osm map shows my place to be in petchaburi. I’ve made two screenshot (if required I can post them). One from osm and the other from google maps. the latter shows my place in prachuab. If you compare the two, you can see that halfway between Hua-Hin and the Miramar border, there is in google maps a “bump” in the province border. It does not exist in OSM.
I do occassionally edit OSM, but I do not have the correct info to do this. Some guidance is needed. Can I get that here, or can someone correct this?

Hello jplee,

thanks for pointing this out. You can upload and link your images for example using or a similar service.
Having a picture helps to better understand your topic.

The province boundaries had been imported from an available source a while ago. That source can be inaccurate or the boundary could have been moved (potentially by accident) by a mapper.

Can you also point to some coordinates of where the border is off? Maybe link to the ID of the relevant OSM element. This way we could check in the history, whether there was a potential edit.

Do you have access to a source reflecting the exact outline of the border? Borders are quite difficult to map and verify on the ground, as they frequently do not follow visible features.

I assume you refer to the area roughly between highways 3314/3316 in the north and 5062 in the south nearby “Huai Sai Ngam Reservoir”, right?
If so, it would require to acquire a data source of the boundary which we are legally allowed to use within OSM.


thx for the reply. I will check with the local government offices if I can find a good source. This might take some time. I might ask the “phuyai-baan”, but i do not think he will know.

yep, my address is along the 5062 route.

I’ll try to make another post with the links of the screenshots I made.

something else, strange came up: in the mapping, the border, as I stated is wrong, however, when going into edit mode, the border appears correct. When I zoom in to edit (in browser editor), the border disappears (probably not allowed). So now I am a bit more confused :slight_smile:

As someone who recently edited some administrative boundary, I noticed that the border that was imported a long time ago was pretty rough. Even tracing quickly from the law’s map is much better. So, if you know what’s going on on the ground, I think adjusting the boundary closer to the proper spot is fine, even if it’s not perfect.
If you want more precise map, you can search for the map that annexed with the law at (only in Thai), since the law is not covered by Thai copyright law.

thx everyone for the input :):slight_smile:
I’ve put the two screenshots on the link is
please check it out

nitinatsangsit, do you know which projections the mentioned maps are in?
Or is the accuracy so low, that this doesn’t matter at all? I expect at least that you have to reproject the bitmap from Mercator to WGS84 in JOSM.

this link gives you the image from osm when you start editting, but are not yet in the correct zoom for editting. The border is correct here!. if you zoom in, the old border is seen.

If you mean the source of the imported boundary, I’m not sure, and I think the accuracy is too low to determine. (Many kilometers away)
For Thai Government map used in law, it usually use the UTM Indian 1975 projection.

Yes, this I was referring to.

So if you want to use bitmaps of Thai maps as a background for tracing, you have to reproject from INDIAN 1975 (I think EPSG:24048) into the right projection before drawing from it.

OSM is in WGS84. Editors typically reproject to mercator to allow drawing from the typical web projections (like bing/maxar)

If you open with PicLayer
You should change Projection in JOSM to EPSG24048 before aligning the bitmap and starting to draw from it.

It’s a little bit complicated.

Thai Government uses RTSD base maps, mostly in these two series: L7017 and L7018.
L7017 uses Indian 1975 UTM, while L7018 uses WGS 84 UTM. Their differences are approximately 300-500 meters.
L7017 has been used since 1975 and may still be utilized in some cases, while L7018 was released in 1997 and is commonly used today.
So, if the law was passed before 1997, it certainly used L7017. If it was promulgated after 1997, you have to check.
For UTM, most of Thailand is in Zone 47. The majority of Northeastern, a piece of Eastern, and a part of Narathiwat are in Zone 48.

So, in conclusion, the Thai Government map falls into one of these four coordinate systems:
EPSG:24047 for Indian 1975 / UTM zone 47N
EPSG:24048 for Indian 1975 / UTM zone 48N
EPSG:32647 for WGS 84 / UTM zone 47N
EPSG:32648 for WGS 84 / UTM zone 48N