Talk:Admiralty chart

External links modified

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Transverse Mercator Projection

The article states: In contrast, coastal charts use the transverse Mercator projection, in order to depict the shape of the shoreline with greater accuracy.[2] This does not sound right at all. In general standard and transverse Mercator (SM, TM) maps should have the same amount of distortion, but differently distributed. For SM, the scale is constant along a line of latitude (E-W), but changes as you go N or S away from that line. For TM, the scale is constant along the central meridian (N-S) but changes as you go E or W away from that line. So TM has an advantage for areas that have greater N-S extent than E-W. This is the case for the National Grid (Great Britain) which uses a rectangle that is 1300 km from N to S and 700 km from E to W, and the grid uses transverse Mercator for that reason.

The reference cited in the article (The RYA navigation handbook, Bartlett) states that Admiralty large scale maps such as harbour plans use the TM projection, but does not give a reason. It's unlikely to have anything to do with distortion, which is minimal for a map covering a few km whether SM or TM is used. I suspect that these large scale charts use Ordnance Survey mapping, and so just use the same projection as the OS maps on which they are based. I haven't found a source to support (or refute) this yet. I'll keep looking. Kognos (talk) 13:00, 2 September 2019 (UTC)

I've checked in both the 1938 and 1987 editions of the Admiralty Navigation Manual, which has answered some of my questions. I've modified the text accordingly and added references Kognos (talk) 21:15, 20 January 2020 (UTC)

History

I've added a history section, dealing with chart production and surveys over the years since the establishment of the Hydrographic Office. I've removed the list of survey ships - this topic is covered by the separate article List of survey vessels of the Royal Navy. I've removed the chart of Somalia, which doesn't seem to connect to anything. I've not changed the section on features except for an introductory sentence. This could use some expansion, but it would need to be by someone whe knows more about electronic charts than I do. All comments welcome. Kognos (talk) 20:33, 23 December 2021 (UTC)