Newsgroups: comp.infosystems.gis
Subject: Re: Confused about digital scales
From: Peter Halls <PJH1@VAXA.YORK.AC.UK>
Date: Wed, 14 Oct 1992 08:10:00 GMT
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU
Via: uk.ac.york.vaxa; Wed, 14 Oct 1992 08:10:02 +0100

I've been reading these postings with interest - partly because most of the
issues - scale vs accuracy - are conceptural problems that I often find to be
'inter-confused' by many users of 'paper cartography', let alone GIS.

In my experience, in the UK, possibly a majority have a grasp of map scale
meaning that, at 1 : 50000, 1 cm represents 50000 cm; 1 inch represents 50000
 in.
Quite a lot have difficulty in expressing this ratio in terms of larger units
of distance, to which they relate far more easily.  Since our Ordnance Survey
switched from the 7th Series 1 inch to 1 mile (1 : 63360) maps - the handle on
which most people grasped being the '1 inch to 1 mile' - to the 1 : 50000
series maps I have found that many people fail to be able to sort out distance.

I think that even fewer people understand that there is an accuracy, or perhaps
'minimum representable size', element to the scale ratio.  I have watched the
surprise on the faces of some (highly intelligent and articulate) people when
I have pointed out the 'size' of a 1 mm wide object on a 1 : 50000 scale map
if that object were represented accurately!

We have TWO distinct problems, threads, here, I think.  One relates more
specifically to the GIS / Digital map provider: that of the accuracy of the
data.  In this case I want to know ALL the digitising / collection parameters -
the stability of any base material, the accuracy of that material and its scale,
the accuracy of digitising, any correction factors / rotation / projection
conversions that have been applied, etc.  All of these may have a significant
impact on the ACCURACY of the data with which I am dealing and which I may be
offering to an 'End User'.  The second relates more to the USE to which the
end user wishes to put the data.  Here accuracy DOES matter, scale(s) of
reproduction matter - so also do the collection parameters of any additional
data to be displayed with the digital map data.  This includes sampling
techniques for statistical data, accuracy, etc., for locational data and so on.

YES, it all MATTERS - but it also matters how we present this information to our
end users.  Whilst I would, at times, dearly love to tell a user that 'the data
you are working with has different scale / accuracy parameters, I will not let
you use them together' such is not a productive thing to do!  The user may have
no choice (because of availability of data, for example).  I think that our
duty, as GIS providers, is to ensure that our users UNDERSTAND what they are
doing (after all, if they (truly?) understand the statistical techniques and
accuracy of the related data that they often use to create their composite
maps, they ought to be able to grasp cartographic error!) and so make allowances
for the limitations of the cartographic information AS WELL AS their other
data AND THE TECHNIQUES THAT THEY ARE EMPLOYING.  Experience shows this to be
far easier said than done ... and the temptation is to restrict the work to
those who 'have been properly trained' and so maintain an air of 'mystique'.

That is fine for research workers.  Some of us, I know, are providing GIS for
'Joe Public' - and politicians and the like ... yet another education problem?

Peter
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PPPPPPH      H  | Peter Halls - University of York Computing Service -
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Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU