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BRITARCH  July 2006

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Subject:

Re: How far can you see

From:

Andrew Larcombe <[log in to unmask]>

Reply-To:

[log in to unmask]

Date:

Fri, 14 Jul 2006 15:50:06 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (43 lines)

[log in to unmask] wrote:
> After looking at a fortified site and attempting to spot another site of 
> the same era and failing miserable to see it.  I am left wondering, just 
> was is the range of the human eye mark 1 in respect to distances 
> ,intervisilibty, etc
> 
> Does anybody known of an work that has been done on this type of subject.
> 
> I am well aware that as one gets holder your natural eyesight like every 
> thing else get worse.

In "Assessing the level of visibility of cultural objects in past 
landscapes", Journal of Archaeological Science, 2005, Ogburn looks at 
the various influences on the range of human visibility paying special 
attention to the 'Psychophysical limits' and 'Environmental limits'. 
With this he puts forward a model which can be used in designing 
GIS-based fuzzy viewshed algorithms.

It's a complex area. Essentially there's a distance where there is a 
relatively constant high likelihood of an object being visible, but past 
this point the chances of it being visible drop off rapidly.

A 1 metre wide object can be visible at 3400 metres to someone with 
20/20 vision. However, many things can affect this figure. The colour of 
the object (red can be seen at further distances than green or blue), 
the contrasting shape of an object and its background (a white object on 
a green background being easier to distinguish than a grey object 
against a green background). Likewise rectilinear objects in a rolling 
landscape can be seen further away than circular objects. Some things 
though don't stick to these rules. One example he gives is the 
suspension cables of the Golden Gate bridge which are less than 1 metre 
wide and in theory should be visible up to a maximum of 6880 metres in 
optimum favourable conditions. However, they are apparently visible from 
the campus of Berkeley, some 20000 metres away, in sub-optimal 
conditions. Ogburn puts this down to two factors at play - the darkness 
of the cables against the sky, and the shape - ie a very long 
curvilinear feature.

Hope that helps,

Andrew
[log in to unmask]
www.andrewlarcombe.co.uk

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