Land based cartographers were respected for their diligence and discipline that their trade required. Bench marking is another expression still used where a reference point is needed. It came from land surveyors and map makers. Particular heights above sea level were transferred inland, further and further, to give mappers some idea, as it couldn’t be guessed with any accuracy otherwise. Indeed, twenty years ago, these benchmarks were heavily relied upon, where presence of any other reference, was absent.

Now though, local authorities and surveyors use GPS or Global Positioning System. Even local civil engineers have access to this technology, and use it for projects such as small housing and commercial estates to anything bigger. So traditional benchmarking system, which was little more than a ground anchored “pad” of stone or concrete, has already been replaced by something that can view an area in a larger relative context, a satellite.

A satellites’ height above ground, or sea if you like, affords it a referencing advantage. There is no longer a need to physically walk to sea (sea level), and determine levels, thereafter. The sea is water, to some degree, and while earth is round also to some degree, water was “the great leveller”. It has also been replaced by various liquids that are more visible, and have less tendency to obscure inside of levelling chambers. Some have anti-freezing properties. You see, any accuracy relied on visual clarity of waters position within its’ clear container, and of course mappers understanding of parallax error. Parallax error is mainly a human one, where something is not viewed, correctly, or in cases like this, not viewed at 90 degrees exactly.