Scottish tourist Geoff Mumford tries to make sense of Bluff's suspect signpost.

The famous signpost at Bluff is just that, a bluff.

The Invercargill City Council is set to change the famous Stirling Point sign after it was revealed Wellington and Cape Reinga were pointing in the wrong directions, while distance calculations and co-ordinates were incorrect.

University of Otago School of Surveying senior lecturer Dr Paul Denys was called in to investigate the sign, after a Wilderness reader questioned its accuracy.

The Bluff signpost is all over the place.

John Hawkins/Stuff

The Bluff signpost is all over the place.

“The council maintained it was all correct, but I did a few calculations and I think they are not quite right.”

Council roading manager Russell Pearson confirmed the council was investigating “if we need to build a new one or modify the old”.

A sign at the site dated back to 1955 and despite several replacements “it’s probably time to make changes”.

Denys said the calculations were easy to get wrong “if you didn’t know what you were doing”.

“People don’t give these things much thought … we live in a two-dimensional plain world for most of the time.”

He found the signs pointing to Wellington and Cape Reinga were around the wrong way, and the co-ordinates for the sign itself were out 185 metres.

He suspected those co-ordinates used “old geodetic datum, which became obsolete in 2000”.

And assuming the direction to the equator was correct, it meant a correction to other signs was required.

That included the equator, which on the sign was 5133 kilometres, but he calculated it to be a distance of 5186km.

Correct distances included: New York with 15096km (sign says 15008), Cape Reinga 1404km (1401km), Hobart 1707km (1680), Sydney 2023 (2000) and Oban 36km (35km).