It's an older Episode, but the awkward mishap could happen today: As the new Rhine bridge at Laufenburg is nearing completion, discovered by a surveyor during a routine check that the two halves of the Bridge in the would not be meeting in the middle at the same height. It would have built more, would have resulted in a level of more than 50 centimetres in height. There was still time for corrections to the structure, so that the cross-border bridge was opened at the end of 2004 without a step.

the builders here and there were off by half a Meter, astonished. After all, the tunnel builders had the puncture of the old Gotthard tunnel, with a difference in height of five centimetres to hit – and that was in 1880.

The calculation range in Laufenburg was quickly resolved. On both sides of the river, the planners had used the respective basis of Measurement in the country. For Germany and Switzerland, sea-level zero, however, is not the same. In Switzerland, the land surveying relates to a zero point of the Mediterranean in Marseille, Germany takes on a level of the North sea in Amsterdam as a base. The difference is 27 inches, what was taken into account in the case of the run Burger bridge. Unfortunately, were not added to the 27 centimeters of the Swiss value, but subtracts, and so the error increases instead of corrected.

height is zero everywhere else

That two countries have different measurement bases, is still the rule. Worldwide, there are dozens of levels, which are considered to have a height of zero. In Europe alone, 15 the reference level to be used. Spain looks to Alicante, Italy to Genoa, the United Kingdom is to Newlyn in Cornwall. A European standard value of Switzerland differs by 23 inches, Belgium, to 232 centimeters.

Even within countries there can be multiple zero points, for example, in Germany, where the GDR-values must be taken into account, which are based on the Baltic sea.

Before the measurement was federally regulated, in Switzerland almost every Canton has a different measurement basis, the difference between Geneva and lake Constance was about 3 meters. Each country holds on to its Definition of the sea level, has practical reasons. You would switch to the European system of reference, would have to be created all of the maps and plans.

is non-Zero everywhere on the same height. The level of the Mediterranean is lower than the level in the North sea.

began As the Survey of the countries that had the scientists, the idea that the sea surface had a distinct and unchanging Basis for the determination of the heights. "Meters about the sea" was to the term. "You was confident that you can determine with ease a common point of reference would be," writes Wilko Hardenberg from the Max-Planck-Institute for the history of science.

Soon, the observers noted, however, that the measured values of various level stations agreed. Zero is not anywhere on the same level when taking about the barometer measurements for comparison. The level of the Mediterranean is several inches deeper than the level of the North sea. In the 1830's was designed in the UK, the first level, recorded the readings automatically.

Several coastal States built up their own level networks. From a large number of measurements an average was calculated and the zero point is explained. The natural movements of the sea surface due to tides, waves and storms were statistically ironed out, an artificial zero level, which has been formed from the height measurements in the country were carried out. "The mean sea level, you can never see, it is a scientific construct," explains historian Wilko Hardenberg.

Important level measurements

level stations still play a role, although a satellite to measure the oceans. In an international network of water level stations, the measured values of around 300 stations to be collected. Several other networks use level information, for example, for the Tsunami warning system, for the calculation of tide times, or for the planning of the port buildings. The data are stored in multiple archives, so studies on the long-term development.

But it is missing in the view of many professionals in the coordination of the differently designed level systems. Often, the money for maintenance is missing. Several measuring stations are obsolete, were destroyed by Storms, or because they are on very remote Islands, for repair work is hardly accessible. The readings are from such locations is particularly interesting for the research. While the satellite periodically every ten days over a point in the past, can be recorded a level almost continuously, but only locally.

The Definition of a zero point for the land survey was for decades a controversial issue among experts. Today, the sea level is observed, especially, because his movements are an indicator of climate change. "The sea level is rising all over the globe are not distributed evenly as in a bathtub, but show regional differences", said a brochure of the scientific associations, the German climate consortium and the consortium of German marine research. To observe this global development, are satellite.

satellite with errors

Since 1993, satellites measuring the rise in sea level around the globe. The missions, which for reasons of cost, were only possible because of American and European projects have been placed together, the hot Sentinel/Jason. For November, the Start of the first of two Sentinel-6 satellite is planned. Its predecessor companies have determined that the sea level of the global average increases annually by 3 mm, these measurements should be continued.

The calculation of a global rise in the sea surface is suitable as a benchmark for climate change, but not for the individual case.

thanks to a better spatial resolution Sentinel is-6 also changes in smaller geographical areas. Wind, waves, eddies and currents are also recorded. The most important Instrument, the altimeter is. Sentinel-6 will be measured with Radar and micro-waves, the distance from the satellite to the water surface to the centimeter.

Also on Board is a positioning system using navigation satellites and the necessary correction of the program provides the precise Position is. The satellite navigation provides good information about the Position in longitude and latitude, but for the height. Even the manufacturers of navigation devices to warn, to assume of the altitude, sight unseen, you can be up to 100 meters in the wrong. So, increasingly, levels are combined stations on earth using satellite navigation.

the future of the Wadden sea

That the sea level does not rise uniformly, such as in the bath, has a variety of causes. The largest contribution to the expansion of the volume of water supplies due to global warming. This factor must be weighted in each Region separately. There are many mass movements. For example, the water sinks into the depth, if the salinity increases, what creates currents. Also the Wind can move water. For an accurate height measurement of the gravity field of the earth must be taken into account.

The amount of the increase in water levels is very different, it also depends on the terrain shape. Corresponding differences in the expected consequences are. In Germany, for example, the researchers are concerned about the future of the Wadden sea, this is a special habitat for animals and plants are sensitive.

The calculation of a global rise in the sea surface is suitable as a benchmark for climate change, but not for the individual case. Here is the satellite can provide a rough guideline, a level measurement is targeted. It is not rare to not find a location that the sea rise is equivalent to the global average of 3 millimeters per year, but 7 mm is.

Created: 08.03.2020, 17:38 PM