Search This Blog

Thursday, August 18, 2011

WGS84 to OSGB36

A transformation between WGS84 and OSBB36 requires a datum transformation. A 7 parameter transformation should give results accurate to about +/- 5m…

ArcGIS supports a number of 3 para, and 7 para transforms. CS2CS using Proj may also be used as follows :

cs2cs +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +nodefs +to +proj=tmerc
+lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy
+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 +units=m

(thanks to Paul from for settings)

However for a centimetre accurate transformation an NTv2 grid is required. The OS provide an online coordinate transformation service here (a site for bulk conversions is also available)

It’s also possible to download the grid in NTv2 format from here:

or directly from this link

Test for a point in Edinburgh (Scotland)

WGS84 –3.185, 55.95
OS Online NTv2 Conversion > 326097.07,673630.932 [taken to be correct conversion]
ESRI (3 par option_1) > 326097.86,673625.357
ESRI (3 par option_5) > 326095.042,673619.452
ESRI(petroleum option)> 326096.974,673630.405
CS2CS (not using NTv2)> 326096.98,673630.4

So it would appear that ESRI’s petroleum option and CS2CS (as above without NTv2) and the OS Online conversion tool all agree to within 1 metre, while the 3 parameter version are out by at least 5m. The options relate to which parts of the country the re-projection has been set up for… this test would suggest option_1 works better for Scotland.

Friday, August 12, 2011

SpaceBook Project

So we’ve arrived in Edinburgh and I’ve started working with the SB team… William’s away and kindly allowing me to use his office for the moment.

Useful Data sources (thanks to Guy):

MasterMap – building outlines from OS (downloaded thru EDINA)
LiDAR and 3D building (from DSM-DTM) polygons from

edin 3d from landmap

(The 3D buildings from Landmap SHP source)


GRASS – new viewshed function (ie not r.los)

r.viewshed (addon) – using faster algo (sweep method)

Install method for Windows summary – download GRASS source (all).. add r.viewshed source folders… change the MAKEFILE to incl r.viewshed – compile GRASS as per this page:

Using msys

the package error log goes into this folder:
\\OSGeo4W\usr\src\grass641\mswindows\osgeo4w as package.log