If an enlargement is made then one would need to know the scale of the
enlarged image of course. I would go for the 'edge of a piece of
paper' method... it's fiddly, but cheap and resonably effective. And
as another member said, you'll never measure the true distance on a
map in any case (fractals). In general, the smaller the scale of map,
the shorter the distance will measure because of the effects of
cartographic generalisation.

D.--- In ordnancemaps@..., "Nicholas Verge"
<njverge@d...> wrote:
>
> Dear Ann,
>
> Your reader could probably increase the accuracy of using a map
> wheel/string if she photographically enlarges the map (if she is
allowed).
> Or, if she enlarges the map she could probably also divide the rive
course
> into liner segments, and measure these using a conventional
calipers+rule
> (this is effectively what would be done if the map was a digital
drawing.
> The accuracy of the latter approach would depend on the numerber and
> length of segments created.
>
> But, I think the most important thing you reader has to do is decide
> determine the precision she needs to measure to. Whatever method is
used
> be it digital or physical, it will be an approximation. So long as
she is
> consistent in her measuring then it should not be a mojor problem.
>
> This is my ha'penny's worth.
>
>
>
>
> On Fri, 04 Aug 2006 11:01:08 +0100, Anne Taylor <aemt2@c...> wrote:
>
> > Dear all
> >
> > A reader (who is doing an 'A' level project) has asked our advice
on the
> > best way to measure stream/river length on 1:10,560 or 1:10,000
scale
> > maps.
> > The project is on river sinuosity so the measurements have to be
fairly
> > accurate.
> >
> > We have suggested a map measurer (the rather old one we had
slipped too
> > much); the cotton/string approach; marking the sections of the river
> > along
> > the edge of a piece of paper.
> >
> > None of these was thought to be overly satisfactory.
> >
> > She has seen adverts for the 'MapWheel' - details at
> > http://scalex.com/pdf's/MapWheel%20Flyer.pdf Before she spends GBP
> > 20-plus on one I said I would ask if anyone has experience of
using one
> > and
> > might comment (off-list if you prefer) on its accuracy.
> >
> > Any other suggestions gratefully received. She does not have
access to
> > GIS
> >
> > Thank you
> >
> > Anne
> >
> >
> > Anne Taylor
> > Head of Map Department, Cambridge University Library, West Road,
> > Cambridge
> > CB3 9DR
> > Tel: 01223-333041. Fax: 01223-333160. email: aemt2@c...
> > http://www.lib.cam.ac.uk/maps/Home.htm
> >
>
>
>
> --
> ----------------------------------------------------------------------
> Nicholas J. Verge BSc. FGS
> Henley-on-Thames, Oxfordshire, UK
>
> Geologist, amateur meteorologist & stormchaser
> Member of European Storm Forecast Experiment (ESTOFEX)
> http://www.estofex.org/
>