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rob-murray / os-leaflet

A Leafletjs TileLayer using Ordnance Survey OpenSpace web map service
http://rob-murray.github.io/os-leaflet/
MIT License
24 stars 8 forks source link

Replaced OSGB36 +datum with +towgs84 parameter to proj4 string #5

Closed jhrrsn closed 10 years ago

jhrrsn commented 10 years ago

This seems to further improve on Bug #2. Something in the proj4 OSGB36 datum is miscalculating coordinate conversions, and using this alternative definition seems to move points to the correct location with my test cases.

rob-murray commented 10 years ago

Thanks, where did the full definition come from? I have seen the towgs84 param before but haven't looked enough at Leafletjs internals to know if it was required. I guess if Leaflet transforms all points from projected co-ords into wgs84 then into map points then it could be required.

I was using the same proj4 definition via Proj4js and also via Leaflet and getting different results which I blamed Leaflet but this could explain those errors too.

Will have a look later...

jhrrsn commented 10 years ago

My evidence is mainly experiential - it fixed the offset on the points I was plotting. It's an alternative one I found on the proj4 forums, unsure of its provenance...— Sent from Mailbox for iPad

On Fri, Mar 7, 2014 at 6:26 PM, Robert Murray notifications@github.com wrote:

Thanks, where did the full definition come from? I have seen the towgs84 param before but haven't looked enough at Leafletjs internals to know if it was required. I guess if Leaflet transforms all points from projected co-ords into wgs84 then into map points then it could be required. I was using the same proj4 definition via Proj4js and also via Leaflet and getting different results which I blamed Leaflet but this could explain those errors too.

Will have a look later...

Reply to this email directly or view it on GitHub: https://github.com/rob-murray/os-leaflet/pull/5#issuecomment-37052491

rob-murray commented 10 years ago

I have merged this into a new branch def_towgs84 on this repo for now...

ccb40edc85990462e4f9e078f28ae5be73281f01

tomchadwin commented 10 years ago

Look at the 27700 projection details in QGIS, and I think you'll see the towgs values. Using the datum parameter seems to lead to an offset of ca 200m. I recommend merging this into master.

rob-murray commented 10 years ago

OK - I haven't had a chance to think about this for a while but the towgs84 transformation param seems valid.

So, I think this change is an improvement and seeing as this repo is a bit experimental and doubt anyone is using it in the real world* (?) - I will merge.