Re: Adventerous Activity Permit

On 17 Jan, 08:53, marc <initial.surn...@xxxxxxxxxxxxxx> wrote:

Bollox, if you have a line .25 mm thick that represents 6m on the map,
and if you think that every contour line is accurately plotted you're
dreaming!

The centre of the lines (that represent features) as printed on the
map, are indeed printed to that sort accuracy.

You need to blow the map up many times and use a mm precision
referance point, and surveying equipment, but its entirely possible to
check a representative sample.

As to contours, they are drawn by hand, using the same stereographic
technique used for other features, but referanced to a 5M 3D grid
generated from aircraft by laser altimeters.

I recall that the accuracy of this 3D grid is well sub metre and this
is the digital height data used by memory map.

Follow the cursor on a contour (in memory map) and you will see that
the height varies plus or minus maybe 2M, its the contour that is more
accurate, the inconsistency is caused by flaws in the interpolutation
from the 3D digital grid referance.

I did not say what the error of the height of a contour line was (its
centre) but its very likely less than 3-4M.

Do the practical work and check for yourself, if you dont believe me.




.

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