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From: George Huxtable (no email)
Date: Sat Apr 01 2006 - 17:34:36 EST
Doug Royer asked-
| This question is for those on the list who reside/hike in Brittan and the
| nations in Europe.
|
| What I'd like to find out from anyone who uses terrestrial projections in
| these countries/areas is what type projections are used there. What are the
| main datums used on these projections and what coordinate system is used in
| the norm while using a projection while traversing cross country?
|
| I could most likely look this up on the internet but I would really like to
| hear from people who use this stuff what they use.
|
| George lives in England and he will tell me what is used there. Anyone else
| on the list who lives on the continent and hikes?
======================
I'm not the best one to answer this question, not being much of a walker, if I can avoid it. Britain
is well-served by Ordnance Survey maps, a popular choice for walkers being the Landranger series at
1:50,000, or about one-and-a-quarter inches to the mile. They show contours, at close intervals,
which is my preference for showing relief, rather than any sort of shadowing. They are marked with
the "Ordnance Survey Grid", at kilometre spacings, which are straight lines, precisely horizontal
and vertical, right across the chart. They also show lat and long at the outer edges, but these are
widely spaced, and not joined by lines across the chart, so would not be easy to use for
coordinates. Lat and long correspond to OSGB datum (Ordnance Survey of Great Britain), which differs
slightly from WGS84.
Admiralty charts of British waters used also to be to OSGB, but are in the process of shifting to
WGS84, by adjustments of a small fraction of a mile usually. That shift may by now be completed, for
British waters. Most of my own charts are old and out-of-date, and to the old datum.
Back to landmaps. Britain being long (in the North-South direction) and thin, it has been mapped to
a transverse Mercator projection, based on a line that runs North-South up the middle of Britain.
That implies that OS grids do not follow exact N-S and E-W directions, particularly in the far
corners of the British Isles. On the maps, the grids are exactly rectangular, and aligned to the map
edges, so true North is not exactly vertical. However, at some point in the margin all OS maps show
the direction of true North, which can be 2 or 3 degrees off from the grid, and also magnetic North.
Those OS maps are often referred to as "one-inch" maps, because that's the scale they used to be at.
They can be readily picked up, for the local area, at bookshops and stationers. They cover every
square-inch of Britain. There exist other maps intended specially for walkers, in certain areas, but
I can't offer information on them.
If Doug is planning to hike in Britain, I hope he will give me a ring, so that we can, at least,
enjoy a pint in a pub, if he ventures anywhere near Oxford.
=============================
In France, it's a bit different. Their excellent state mapping is by the IGN (Institute Geographique
Nationale). I am familiar with their Blue series maps (sometimes labelled "Top 25", at 1:25,000,
which my wife and I have used to go poking around the remoter parts of Brittany in search of obscure
megaliths (ancient standing stones). France being much larger in area, its map grid covers two
separate zones, the Northern half of France, and the Southern including Corsica, and (of course) the
two grids don't match-up precisely where they meet.
On these French land-maps, it seems that North is always exactly up, and East-West is exactly
horizontal, as with true Mercator projection, and it's the kilometre grid that is slighly skewed.
The maps state that they are to the Lambert conformal conic projection, whatever that means.
There's a confusing variety of scales around the edge There are markings for lat and long (relating
to the Greenwich meridian), but only at 10-minute intervals. There are a few sparse grid lines
drawn right across the chart, which are (would you believe it?) lat and long in Grads, with respect
to Paris not Greenwich, at 0.1 Grad intervals! There are 100 Grads in a right angle, for anyone that
wants to know. There's a choice of 3 possible kilometre grid numbers, corresponding to zone 1
(appropriate to this Northern half of France), for zone 2 (for some reason) and for some unified
European grid system. Of these, the zone1 markings are shown all over the chart, wherever theiy
cross, as a grid of "+" marks at 1 km intervals. In practice, these are the only marks that you can
use for taking coordinates off the chart.
My Top 25 maps are more than 10 years old now, and it may be that the map coordinate system has
since been somewhat rationalised.
For driving along French country lanes, I like to use the IGN Green series (Serie Vert) at 1:100,000
=============================
If it's allowed, I should say something about the use of a GPS navigator with these coordinate
systems, because neither the English nor the French maps are arranged for convenient use of latitude
and longitude scales. My receiver came from a French firm, MLR, which has since been bought up and
closed down by Thales, who now own several brands and seem to want to eliminate competition. It
allows a choice of coordinate system, so it's possible to display position, not as lat and long, but
in the British grid system or in either of the French zone grids, Lambert1 or Lambert2. That's the
mainreason why I chose it. It may be that other GPS devices now offer the same choice.
As well as choosing the coordinate system, it's necessary also to choose the correct chart datum for
the mapping. For all newer Admiralty charts of British waters, WGS84 should be chosen, as noted on
the chart. For older charts of British waters, and for Ordnance Survey landmaps, choose OSGB36
(Ordnance Survey of Great Britain 1936).
For charts of waters around France, Europe 1950 - 02 is presumably the right choice. This is the
mean European datum which covers Austria, Denmark, France, Germany, Netherlands, and Switzerland.
The French IGN landmaps state they are to the Clarke 1880 ellipsoid. I'm a bit out of my depth here.
I usually use the Europe 1950 - 02 for those maps without being confident that it's the right
choice. Others may know better. Does it matter?
===============
I'm not at all sure what information Doug is after, but if I haven't supplied what he needs, no
doubt he will ask further.
George.
=================
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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