ArcGIS – How to Reproject TM65 to OSGB in ArcGIS for Better Accuracy

arcgis-10.0arcgis-desktopcoordinate systemraster

I have a TIFF file in the TM65 (Transverse Mercator) projection, and want to convert it to the OSGB (British National Grid) projection. I'm trying to do this in ArcGIS using the Project Raster tool.

I can select the input and output co-ordinate system fine, but it then asks me to choose a Geographic Transformation. It seems that I have to choose two – one to go from TM65 to WGS84, and then another to go from WGS84 to OSGB. My question is – which one should I choose? What are the differences between them? I have tried choosing some almost at random (and also tried matching them up – so I do TM65->WGS84_2 and then WGS84_2 to OSGB) and each time once the points are reprojected they are displaced from their true locations – the amount varies between fairly small (but large enough to be an issue) and quite large.

Is there any way I can get an accurate conversion between TM65 and OSGB? Am I using the wrong tool?

Best Answer

Currently, none of the supported transformations for OSGB36 cover the Republic nor Northern Ireland. TM65_To_WGS_1984_2 is probably the best choice for conversion to WGS84 as the older transformations have no known accuracy values.

You can find out about supported transformations in ArcGIS in technical article 21327.

If the data's offshore, there's a transformation from OSGB36 to ED50 and you could then piece together other transformations. Accuracies are several meters.

You might check the online EPSG Geodetic Parameter Dataset to see if they have any other options. Note: I'm on the subcommittee that maintains the dataset.

UPDATE - December 11 2017

EPSG:6946, TM75 to ETRS89 (3) (Esri name: "TM75_To_ETRS_1989_3_NTv2"), and EPSG:6947, TM75 to WGS 84 (4) (Esri name: "TM75_To_WGS_1984_4_NTv2"), were added to ArcGIS Desktop version 10.3.1. That version was released in mid-2015.

Related Solutions

[GIS] Setting spatial reference to OSGB_1936_British_National_Grid in ArcGIS Desktop

In ArcGIS 10, if you go into, for example the Data Frame Properties, and go to the "Predefined" co-ordinate systems, you can click on two co-ordinate systems you mentioned in the question - OSGB 1936, and British National Grid, if you click "Modify" on both you will see that they actually both use the OSGB 1936 Datum (click on "Modify" again on BNG to see the Datum).

You probably want to use the British National Grid Co-ordinate system, because it is the basis of all Ordnance Survey Grid references- basically the projection is a set of mathmatical formulae that maps real world positions onto the co-ordinate system that is used in the UK. Not entirely sure what the OSGB_1936 co-ordinate system would be, but the major difference between a geographic and projected co-ordinate system is that:

"Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions" (ArcGIS 10 Help File).

This obviously makes it much more useful for precision applications such as plans, although this accuracy will generally only be high over a relatively small area compared to a geographic co-ordinate system, which is suitable for describing a point with (relative) accuracy over the whole earth's surface.

PostGIS – Calculate LineString Length from WGS84 After Transforming from OSGB 36

The length functions work differently with 3D linestring geometries:

ST_Length - returns 2D distances for geometry types, and oddly 3D distances for geography types (but not in this question)
ST_Length_Spheroid - returns 3D distances for geometry types

Your example is in 3D, so it will calculate the 3D length with ST_Length_Spheroid and the 2D length with ST_Length on a geometry.

However, if you always want 2D lengths, you can force the geometry to a 2D linestring using either ST_Force2D or ST_Force_2D function (the name change in PostGIS 2.1).

SELECT ST_Length(geom) AS length_27700,
  ST_Length_Spheroid(ST_Transform(ST_Force2d(geom), 4326), spheroid) AS length_spheroid_2d,
  ST_Length_Spheroid(ST_Transform(geom, 4326), spheroid) AS length_spheroid_3d
FROM (
  SELECT 'SRID=27700;MULTILINESTRING((
      423216.279 574665.249 0,
      423206.315 574708.077 44.649,
      423158.458 574896.911 242.525,
      423132.406 574993.061 344))'::geometry AS geom,
    'SPHEROID["WGS 84",6378137,298.257223563]'::spheroid
) AS f;

  length_27700   | length_spheroid_2d | length_spheroid_3d
-----------------+--------------------+--------------------
 338.39264086563 |   338.515774984236 |   482.622196178051

Best Answer

Related Solutions

[GIS] Setting spatial reference to OSGB_1936_British_National_Grid in ArcGIS Desktop

PostGIS – Calculate LineString Length from WGS84 After Transforming from OSGB 36

Related Question