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OS contours vs Anquet height data



 
 
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  #1  
Old November 12th 04, 08:10 PM
Rob Kingston
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Default OS contours vs Anquet height data

Does anyone have any ideas/opinions/baseless ramblings on the relative
accuracy of the height data in Anquet compared with the contours on
the maps? I know that in very bumpy areas the height data tends to
miss sharp ridges and summits etc because of the resolution of the
grid (50m IIRC), but in my relatively flat part of the country
(northern Hampshire) there are some interesting discrepancies - for
example flattish hills reading well over 100 metres, even as much as
115 or 120 metres over a wide area when the OS contours only admit to
90 metres (or 95 on the 1:25000).

In most places the heights agree very well, but I have found plenty of
places where this sort of thing happens - usually with Anquet heights
being "too high". Many of these places are forested, which makes me
wonder whether the heights were collected by radar measurement which
picks up the trees rather than the ground, although some are bare
grassland. Also, most forested areas show no corresponding jump in
height compared with bare ground.

Does anyone in fact know how the Anquet data is derived? It definitely
doesn't include building heights - no 100m + readings in London, for
example! - but I wonder if tree cover can throw it out somehow?
  #3  
Old November 12th 04, 08:58 PM
bob watkinson
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Default


"Rob Kingston" wrote in message
...
Does anyone have any ideas/opinions/baseless ramblings on the relative
accuracy of the height data in Anquet compared with the contours on
the maps? I know that in very bumpy areas the height data tends to
miss sharp ridges and summits etc because of the resolution of the
grid (50m IIRC), but in my relatively flat part of the country
(northern Hampshire) there are some interesting discrepancies - for
example flattish hills reading well over 100 metres, even as much as
115 or 120 metres over a wide area when the OS contours only admit to
90 metres (or 95 on the 1:25000).

In most places the heights agree very well, but I have found plenty of
places where this sort of thing happens - usually with Anquet heights
being "too high". Many of these places are forested, which makes me
wonder whether the heights were collected by radar measurement which
picks up the trees rather than the ground, although some are bare
grassland. Also, most forested areas show no corresponding jump in
height compared with bare ground.

Does anyone in fact know how the Anquet data is derived? It definitely
doesn't include building heights - no 100m + readings in London, for
example! - but I wonder if tree cover can throw it out somehow?


Set a route to follow a stream downhill and see how much uphill the stream
does on it's journey down


  #4  
Old November 12th 04, 09:26 PM
Paul Saunders
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Default

Rooney wrote:

Run the cursor over a lake and see the height fluctuate!


Or not, as is usually the case. You must have picked the only reservoir
in Britain to have been built after the height data was collected!

Paul
--
http://www.wilderness-wales.co.uk
http://www.wildwales.fsnet.co.uk


  #5  
Old November 12th 04, 09:28 PM
Paul Saunders
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Default

bob watkinson wrote:

Set a route to follow a stream downhill and see how much uphill the
stream does on it's journey down


Then look at the 3D view to see how the stream doesn't follow the line
of least resistance (due to the 50m resolution).

Try following the coast path from Langland to Caswell and watch the path
suddenly plummet to sea level and back up again as you round Whiteshell
Point.

Paul
--
http://www.wilderness-wales.co.uk
http://www.wildwales.fsnet.co.uk


  #6  
Old November 12th 04, 09:32 PM
Rooney
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On Fri, 12 Nov 2004 20:26:25 -0000, "Paul Saunders"
wrote:

Rooney wrote:

Run the cursor over a lake and see the height fluctuate!


Or not, as is usually the case. You must have picked the only reservoir
in Britain to have been built after the height data was collected!

Paul


No - I'm looking at natural tarns in the Lake District. The
fluctuation isn't large but it is there.
--
R
o
o
n
e
y
  #7  
Old November 12th 04, 09:48 PM
Paul Saunders
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Rooney wrote:

No - I'm looking at natural tarns in the Lake District. The
fluctuation isn't large but it is there.


Strange, most of the lakes I've looked at have identical heights all
over the surface.

Paul
--
http://www.wilderness-wales.co.uk
http://www.wildwales.fsnet.co.uk


  #8  
Old November 12th 04, 09:59 PM
Chris Morriss
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Default

In message , Paul Saunders
writes
Rooney wrote:

Run the cursor over a lake and see the height fluctuate!


Or not, as is usually the case. You must have picked the only reservoir
in Britain to have been built after the height data was collected!

Paul
--
http://www.wilderness-wales.co.uk
http://www.wildwales.fsnet.co.uk


Carsington reservoir in Derbyshire is another one.
--
Chris Morriss
  #9  
Old November 17th 04, 04:59 PM
W. D. Grey
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In article , Paul Saunders
writes
Rooney wrote:

No - I'm looking at natural tarns in the Lake District. The
fluctuation isn't large but it is there.


Strange, most of the lakes I've looked at have identical heights all
over the surface.

Paul
--
http://www.wilderness-wales.co.uk
http://www.wildwales.fsnet.co.uk


No good for Irish water skiing then :-)
--
Bill Grey
http://www.billboy.co.uk
 




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