id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
20160	[patch] [rfe] British National Grid projection EPSG:27700	Adrian	team	"This proposal is in two parts. First, to add the British National Grid projection EPSG:27700 to the list of default projections. Second, to upgrade the definition of EPSG:27700 from the approximate version currently used by JOSM. The approximation produces misalignments of up to 5m depending on the location. The approximation is the one used by `proj4` at the time when the projection handling was added to JOSM. The definition used by `proj` was upgraded in `proj6`. It implements the official definition held in the EPSG registry.

The upgrade actually applies to the OSGB36 datum, used by EPSG:27700. This change affects 105 projections which use the OSGB36 datum, numbers 4277, 7405, 27700 and 102799-102900. It involves a grid shift file `OSTN15_NTv2_OSGBtoETRS.gsb`. The file came from https://www.ordnancesurvey.co.uk/business-government/tools-support/os-net/for-developers The file is also available here https://download.osgeo.org/proj/proj-datumgrid-europe-1.2.zip

A patch is attached. You will see that it includes an ugly workaround. I have updated the projection test files and the updates are in the patch. I have only updated the 105 projections affected. I used proj-4.9.3 - with a modification to the OSGB36 datum equivalent to the modification in the patch - when updating the test files. I have run `ProjectionRefTest` and `ProjectionRegressionTest` and they passed on my machine.

{{{
[junitlauncher] Test run finished after 59302 ms
[junitlauncher] [         1 tests successful      ]

[junitlauncher] Test run finished after 59390 ms
[junitlauncher] [         1 tests successful      ]
}}}

If you accept this rfe, and if you wish to credit me, my name is Adrian.

{{{
Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2020-11-21 14:56:29 +0100 (Sat, 21 Nov 2020)
Revision:17329
Build-Date:2020-11-22 02:30:52
URL:https://josm.openstreetmap.de/svn/trunk

Identification: JOSM/1.5 (17329 en_GB) Mac OS X 10.14.6
OS Build number: Mac OS X 10.14.6 (18G6020)
Memory Usage: 234 MB / 4096 MB (111 MB allocated, but free)
Java version: 15+36, AdoptOpenJDK, OpenJDK 64-Bit Server VM
Look and Feel: com.apple.laf.AquaLookAndFeel
Screen: Display 458669421 1920x1080 (scaling 1.0x1.0)
Maximum Screen Size: 1920x1080
Best cursor sizes: 16x16 -> 16x16, 32x32 -> 32x32

Plugins:
+ ImportImagePlugin (35567)
+ InfoMode (35543)
+ PicLayer (2a9aa7a)
+ apache-commons (35524)
+ cadastre-fr (35645)
+ editgpx (35562)
+ ejml (35458)
+ geotools (35458)
+ http2 (35427)
+ imagery_offset_db (35640)
+ jaxb (35543)
+ jts (35458)
+ log4j (35458)
+ measurement (35640)
+ opendata (35640)
+ photo_geotagging (35640)
+ poly (35640)
+ reverter (35640)
+ turnrestrictions (35640)
+ undelete (35640)
+ utilsplugin2 (35640)
+ waydownloader (35640)
}}}
"	enhancement	new	normal		Core	latest		template_report projection OSGB_1936 OSGB36