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As average sea levels rise, will contour lines and spot heights on maps need to be recalculated? If so, when?

Simon Ritchie
Leatherhead, Surrey, UK

Height above mean sea level is only one of at least three measures of height. Two others are the height above a perfect ellipsoid that approximates the shape of Earth, and the height above an irregular shape called the geoid, which is another useful approximation of our planet’s shape. Modern GPS-based surveying equipment gives your position relative to Earth’s centre of mass. That is most sensibly expressed as geoidal or ellipsoidal height or both.

The mean sea level figure for the UK was calculated by mapping agency the Ordnance Survey (OS) in the early 20th century using a facility at Newlyn in Cornwall. For the purposes of making land maps, the fact that the value may no longer quite coincide with the mean level of the sea at that location isn’t a big issue.

It is essentially an arbitrary value – you can think of it as 80 centimetres above the geoid or as 1345 metres below the summit of Ben Nevis. In any case, if the OS did redefine it, the effect wouldn’t be visible given a paper map’s scale.

It is also worth mentioning that the meridian line marked on the ground at Greenwich Observatory in London to denote the line of 0° longitude is in the wrong place. That is due partly to continental drift, but mainly due to errors in the original calculation caused by factors such as deviations in the direction of local gravity.

Richard Kay
Coventry, UK

According to the

This rise will be combined with other ongoing natural changes in land elevation caused by glacial rebound, continental drift, seismic activity and changes in water tables due to groundwater extraction, reservoir building and river floodplain management.

Laser and radar altimetry, based on satellite surveys, seem likely to be more accurate, while enabling more regular updates of map contours, when compared with old-school surveying techniques involving measurements using theodolites.

Hillary Shaw
Newport, Shropshire, UK

If land is rising or sinking by a few millimetres a year, in a flattish area where the gradient is 1:500, the contours will migrate by a metre or so laterally per year. Over a few decades, this change would become clear on a 1:25,000 map.

In reality, contours need only be accurate vertically to up to 5 metres at their marked location. This is a NATO standard. NATO is interested in contours because ground-hugging missiles use them for navigation, so I guess civilian maps aren’t published with 100 per cent accurate contours either. We would have more worries than redrawing contours should sea level rise exceed 5 metres.

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