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Map Scale mystery

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Michael Gilligan29/06/2021 22:54:30
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Yesterday, we re-watched Prof. Jerry Brotton talking about Maps

**LINK**

https://www.bbc.co.uk/iplayer/episode/b00s2wvh

All good fascinating stuff, but it reminded me to ponder something:

The Cassini maps were made to a scale of 1 : 86,400

… But what is the logic behind this ?

So far as I am aware; the only common occurrence of 86,400 is that it’s the number of seconds in 24 hours … but I can’t see how this is relevant to a map scale.

MichaelG.

Ady129/06/2021 23:09:29
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It's pre decimal stuff, like TPI

It was on a scale of one line to 100 toises, i.e. 1/86,400.

**LINK**

All projections are a compromise, only the longitude origin is truly accurate at zero latitude

Said it took 3 generations to complete the map so a lot could change in that length of time

The guys who really impressed me were the early circumnavigators who

1-Didn't hit a rock and sink in uncharted waters or founder, (unparalleled seamanship)

2-Accurately Mapped huge chunks of the coastal globe, both land and sea (high levels of education and ability required)

4-Kept a wooden ship going in tropical waters for years (no mean feat when your ship is made of sugar candy)

3-Got home alive after being bitten and munched by every tropical bug and disease imaginable

It's difficult to comprehend the level of abilities and skills those people possessed, and how tough they were

Edited By Ady1 on 29/06/2021 23:26:20

Calum Galleitch29/06/2021 23:14:30
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The original scale was 1:100 'toises', one toise being 72 poises or 864 lignes...I'm not sure about the lopping heads off bit, but those Revolutionaries certainly had a point when it came to metrology. A toise was about 2 metres.

Michael Gilligan29/06/2021 23:18:39
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Eureka !!

Shortly after posting … I found this: **LINK**

https://www.davidrumsey.com/blog/2009/10/11/national-survey-of-france-1750-1815

[quote]

The Carte de France was one of the first national surveys completed on the same scale, 100 toises (a toise was equal to 6ft and the equivalent scale today would be 1:86,400), according to a specific plan.

[/quote]

which in turn, led me here:

.

a396b5ac-3549-4c8e-a080-0b681dc0d705.jpeg

.

So now I know smiley … nearly

84,600 or 86,400 ?

MichaelG.

Michael Gilligan29/06/2021 23:19:49
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Thanks, both … you got there before me blush

MichaelG.

Ady129/06/2021 23:36:15
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There was an interesting BBC prog on maps, I think it was a Dan Snow documentary

The tool which made surveying much easier was the Theodolite

Martin Connelly30/06/2021 13:27:45
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My daughter is doing research (museum related and about something from what is now Germany) into something from the early 1800s. She was reading about lines and inches used for measuring things and thought she had the number of lines per inch sorted out until I asked her which inch was being written about? She had to do some more research and found about 5 different inches and had to chose the one most likely to be the correct one for the time and area in question. So when you say a toise was equal to 6 feet I would ask how long was the foot that was being used then and there? SI units were brought about to get rid of all the various local units around Europe with a non-political scientifically derived basis.

Martin C

Michael Gilligan30/06/2021 14:14:19
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Just for the sake of clarity …

What piqued my interest was the exact form of Prof. Jerry Brotton’s statement, commencing at around 34min 30sec into the programme.

MichaelG.

Martin W30/06/2021 14:28:04
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Michael

Is that 34min 30sec east or west of the Greenwich Meridian as was. devil

Martin W

Michael Gilligan30/06/2021 14:34:03
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laugh

Martin Kyte30/06/2021 14:38:12
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Posted by Michael Gilligan on 29/06/2021 22:54:30:

Yesterday, we re-watched Prof. Jerry Brotton talking about Maps

**LINK**

https://www.bbc.co.uk/iplayer/episode/b00s2wvh

All good fascinating stuff, but it reminded me to ponder something:

The Cassini maps were made to a scale of 1 : 86,400

… But what is the logic behind this ?

So far as I am aware; the only common occurrence of 86,400 is that it’s the number of seconds in 24 hours … but I can’t see how this is relevant to a map scale.

MichaelG.

Doesn't seem strange to me. Meridians can be fixed by star sight. East west angles are specified in hours minutes and seconds with 24 hours being 360 degrees. Or am I being too simplistic.

regards Martin

Swarf, Mostly!30/06/2021 16:24:26
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Posted by Ady1 on 29/06/2021 23:09:29:

SNIP!

The guys who really impressed me were the early circumnavigators who

1-Didn't hit a rock and sink in uncharted waters or founder, (unparalleled seamanship)

2-Accurately Mapped huge chunks of the coastal globe, both land and sea (high levels of education and ability required)

4-Kept a wooden ship going in tropical waters for years (no mean feat when your ship is made of sugar candy)

3-Got home alive after being bitten and munched by every tropical bug and disease imaginable

It's difficult to comprehend the level of abilities and skills those people possessed, and how tough they were

Edited By Ady1 on 29/06/2021 23:26:20

Also quoted by ADY1:

The tool which made surveying much easier was the Theodolite

Back in the 1970s I went on a few diving holidays in the Isles of Scilly. One year, the Admiralty Hydrographic Department were surveying the area. There were two ships involved - one was HMS Hecate, I can't remember the name of the other.

What impressed me was the sight of a surveyor using his theodolite from a rock you'd have said was only just large enough for the tripod with the surveyor performing contortions to keep his personal centre of gravity vertically over terra firma!! Devotion to duty or what?!?!

Best regards,

Swarf, Mostly!

Swarf, Mostly!30/06/2021 16:26:10
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What's the proper way to get two (or more) quotes into the same post?

Best regards,

Swarf, Mostly!

Georgineer30/06/2021 23:38:10
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Posted by Martin Connelly on 30/06/2021 13:27:45:

...So when you say a toise was equal to 6 feet I would ask how long was the foot that was being used then and there? ...

According to a book I have, the French foot was longer than the English foot, being equal to 12.79 English inches. The toise was six French feet in length, said to be half the distance between the walls of the inner gate of the Louvre.

Calum, I believe the toise was described as 72 pouces, not poises. I was amused when I started French in the first form to discover that "J'ai cinq pieds et sept pouces" - I have five feet and seven thumbs - a statement which my wife is happy to endorse.

George B.

Calum Galleitch30/06/2021 23:57:42
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Quite right George, thank you. My keyboarding is becoming more divorced from my brain day by day!

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