What is the effect?
Every coordinate changes by 130-170 metres - east and north, latitude and longitude
On a map, the shift depends on the scale:
- 1:250000 (“Army survey”) 0.6mm
- 1:20000 (Street Directory) 8mm
- 1:500 (site survey) 300mm
Not noticeable on small scale map - but on a site plan, a facility may disappear right off the page
And what of digital data, used at unknown scales?