tag:blogger.com,1999:blog-4565845984512808077.post6651692477340284442..comments2024-03-19T21:41:42.835+01:00Comments on Poemas del río Wang: Edge of the Old WorldStudiolumhttp://www.blogger.com/profile/06377777909296284368noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-4565845984512808077.post-6742844200837074592013-01-17T00:01:25.313+01:002013-01-17T00:01:25.313+01:00The link here http://www.psmsl.org/train_and_info/...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.<br /><br />It says, inter alia: <br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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).Canehannoreply@blogger.comtag:blogger.com,1999:blog-4565845984512808077.post-58382669811528540432013-01-16T17:05:57.937+01:002013-01-16T17:05:57.937+01:00Thank you for the interesting follow up informatio...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. <br /><br />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. <br /><br />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 <i>“Since 1785, during the reign of Emperor Joseph II, who succeeded Maria Theresa in 1780, there has been an instrument in the Ponterosso Channel</i> (Idrometro) <i>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.)”</i>. <br /><br />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.<br /><br />Finally, I particularly appreciate knowing about the meridian marker in Bratislava, which I will certainly visit the next time I am there.Lloyd Dunnhttps://www.blogger.com/profile/08215855379263509739noreply@blogger.com