Edge of the Old World

The post on the meridians pointed out a few important prime meridians and waited for information on further local prime meridians. From the description of the post it is clear that there are two main types of them.

The absolutely local meridians essentially all fix on the floor the trace of a light beam projected through a hole in the dome or high ceiling, a camera obscura, thus demonstrating the passing of time throughout the year. Such was the one made by G. D. Cassini in Bologna, but they designed similar ones almost everywhere, where extensive closed spaces were available. It was certainly a pleasant feeling to follow on the floor the path of the little light dot, the circular image of the Sun, passing through the line of the meridian, indicating both the noon and the place of the months and days.

The camera obscura designed in the vault of the Basilica di San Petronio in Bologna, and the image of the Sun at noon

The much more important local meridians were marked practically in every major observatory, since the basis of any observation is the precise setting of the local noon.

Accordingly, a large number of local prime meridians are known. As many observatories, as many local meridians. But only a few of them had the privilege to be accepted by the international society of cartographers as a generally valid prime meridian. Besides the already mentioned London and Paris meridians, the so-called Ferro meridian, that is, the meridian passing through El Hierro enjoyed a certain priority, which is also worth to mention because of its Central European reference. Indeed, the Austrian Empire, the Austro-Hungarian Monarchy, and the inter-war military maps all used it as the of their geographical coordinates.

How was this location picked out? Already in Ptolemy’s system of longitudes and latitudes, the westernmost point of the then known world was chosen as a prime meridian, thus providing a positive value to all the meridians in the continent. During the geographical discoveries it turned out that the Canary Islands are located much further west than any part of Europe. Thus they defined the prime meridian here, at the farthest point of the Old World – due to the inaccuracies of the measurement, round twenty degrees west of Paris.

The cartographers, by following the traditions of their place of operation, widely used the Ferro meridian as well as the generally accepted London and Paris meridians.

One of the earliest maps produced in Hungary was the work of students-engravers in Debrecen who, under the direction of Ézsaiás Budai, designed and printed the first school atlas in Hungarian language. Both the Paris and Ferro degrees were indicated on the edge of the maps: below that of Paris, and at the upper edge that of Ferro.

Frontispiece of the Oskolai magyar új átlás (New Hungarian school atlas) and the map of France in it

Germany and the Austro-Hungarian Monarchy (as well as its succession states) officially published all topographic maps with the Ferro meridian as long as the end of the Second World War.

1:200 000 scale general map of the Monarchy – TRIEST 31°46°

The 1:200 000 scale map of Trieste, for example, bears in the name the Ferro latitude and longitude running through the midle of the map section, and only that. After the war they at least started to mark with guard lines on the edge of the sections the Greenwich degrees as well.

The 1:75 000 military map of Sopron from the interwar period. The frame already indicates by guard lines the Greenwich degrees, too

But in order to see a printed country map which, in contrast to the international custom, was printed with a local longitude system: in 1753 András Frisch designed his map of Hungary on the basis of the data of Sámuel Mikoviny, the renowned 18th-century engineer and cartographer, with a longitude system based on the Pozsony (today Bratislava) prime meridian.

TABULA NOVA INCLYTI REGNI HUNGARIAE juxta nonnulas Observationes Samuelis Mikoviny. Concinnata Ab Andrea Erico Fritsch Posonii 1753

And once we spoke about Trieste…

Trieste is an important location in the Hungarian geodetic and engineering practice. As many prime meridians, as many zero sea levels to specify any geographic altitude. You only need a sea to it. But where, when which one? The zero sea level of the Austro-Hungarian Monarchy was for example established and fixed with a “golden nail” in 1875 on the basis of the observations of many years of the mareography of Trieste’s Molo Sartorio, using it as a starting point for altitude measuring throughout the Empire.

The sea level observation stations in Trieste and Kronstadt

In addition, because of the great distances, the exact altitude above the sea level was recorded, by way of multiple measurements, in a granite quarry of Nadap above the Velencei Lake in Hungary, assuming that granite is relatively stable, and the altitude fixed there will not change during the slow movements of the Earth’s crust.

The starting point of Hungarian altitude measuring, the Basic Sign of Nadap

At present, in Hungary they officially use the Baltic zero sea level, deduced from the sea fluctuations at Kronstadt for the Soviet system maps. When connecting the two basic levels, it turned out that the Adriatic values were approximately 67 centimeters lower than the Baltic ones.

And to satisfy the request of the previous post on meridians: the prime meridian of the above 18th-century map of Hungary, the Meridionalis Posoniensis has not passed without a trace either. The memory of this prime meridian has been immortalized under the castle, on the promenade along the Danube.

Memorial stone of the Pozsony (Bratislava) prime meridian, which points at the northeastern tower of the castle.

If the interested readers will report on the prime meridians known to them, would they also be so kind to tell whether there are any other zero sea level marks, and where? In Britain, in the Netherlands… Anywhere where they produce relief maps, and indicate on it the altitudes above sea level.

2 comentarios:

Lloyd Dunn dijo...

Thank you for the interesting follow up information to my own modest research in the previous post on meridians. Altitude markers make a nice addition to the impromptu collection.

When I wrote my post, I did not know that the word ‘meridiano’ in Italian also means ‘sundial’ (as one of my commenters pointed out), so my writing there tended to conflate the two ideas. I am quite happy to have been corrected.

When I was in Trieste (the source of inspiration for the earlier post) I made a photograph of bronze marker along the Canal Grande (which is not very near to the Molo Sartorio). The marker takes the form of a large plaque in Italian, German, and English. The English reads “Since 1785, during the reign of Emperor Joseph II, who succeeded Maria Theresa in 1780, there has been an instrument in the Ponterosso Channel (Idrometro) which measures sea levels and tides. It is called “Zero Ponterosso (ZPR)”. It is engraved in the stone and graduated in feet and Parisian inches (1 foot = 32,484 cm = 12 inches = 12 x 2,707 cm), and was probably used for occasional observations of the sea for vessels sailing in the canal. The canal was then used for mooring merchant ships. The altimetrical network of Trieste is still based on that zero set by the Vienna Military Geographical Institute (Z.I.G.M.V.)”.

There is also a diagram showing where to look in the canal to see it and, there is also a schematic of its markings. So it seems taht Molo Sartorio was for calibratings altitudes and Canal Grande for marking the tides, so they apparently had different purposes. Perhaps the connection is mildly interesting.

Finally, I particularly appreciate knowing about the meridian marker in Bratislava, which I will certainly visit the next time I am there.

Canehan dijo...

The link here http://www.psmsl.org/train_and_info/faqs/ is to the Permanent Mean Sea Level organisation, run jointly various oceanographic organisations under the auspices of UNESCO.

It says, inter alia:

Heights above sea level, such as mountain peak heights, have traditionally been defined in terms of a measurement of 'mean sea level' at one or more locations. The value of mean sea level, once determined at the location, was then carried around the country by levelling, using methods similar to those used by surveyors in the road or construction industry.

For example, in the U.K. the height above sea level is defined in terms of 'Ordnance Datum Newlyn' (ODN), which is the mean level of the sea at Newlyn in Cornwall in S.W. England in the period May 1915 to April 1921. This definition replaced an earlier Ordnance Datum Liverpool based on sea level in that port in 1844. So, a height of a Scottish mountain means height above the sea level at Newlyn many years ago. Similarly, Normaal Amsterdam Peil in the Netherlands is approximate mean sea level at Amsterdam and represented by a marker in the city hall. French heights are relative to mean sea level at Marseille at a particular epoch.

ODN was carried around the country by levelling. One can think of the levelling results by imagining a network of thin, and in some places very deep, canals across the country. The water level at each location, which determines the 'zero' level at that point, is such that the water will not flow in any direction. In the case of the United Kingdom, ODN determines the 'zero' reference level for the network.

In many countries with two coastlines, such as the U.S. or India, there are often two or more datums because the distance from the sea to the mountains can be great and errors creep into the measurements. In addition, sea level along coasts can be different in different places due to dynamic ocean effects, i.e. mean sea level is not a 'level surface' (see FAQ #1).