4

I'm trying to figure out the latitude and longitude of rail stations that are contained in this WMS feature list. I'm completely new to the GIS field, so please forgive me if I use the wrong terms below. I've done quite a bit of reverse engineering and fiddling to get this far.

The first station has the following location:

<gml:Point srsName="urn:ogc:def:crs:EPSG::27700" srsDimension="2">
  <gml:pos>450077 356089</gml:pos> 
</gml:Point>

After some research, I've concluded this position is relative to offsets and I've used the EPSG site to look up code 27700. This shows me:

Latitude of natural origin: 49° N
Longitude of natural origin: 2° W
Scale factor at natural origin: 0.9996012717
False easting: 400,000 (metre)
False northing: 100,000 (metre)

I'm guessing the values in my station location have had 400,000 and 100,000 respectively added to them. So if I subtract those values I would get:

50077, 256089

But I'm not sure what these values represent. They are minuscule, so they can't be metres from the latitude/logitude point (0,0).

Can anyone point out where I've gone wrong? My goal is to eventually convert this gml:pos value into latitude and longitude.

  • depending on the language you are using some version of proj4 will do all the calculations for you. – Ian Turton Aug 5 '16 at 8:13
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You are almost right:

The false Northing is -100000, so the "real" Y coordinate is 456089 meters.

The point is 50 km East and 456 km North of the natural origin, that is 2° West and 49° North.

Here you see a grid of EPSG:27700 together with the origin and your railway station:

enter image description here

  • Ahah! It seems so obvious when you point it out :-) Thanks for unblocking my investigations... – Duncan Jones Aug 4 '16 at 18:38

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