-------- Original Message --------
Subject:        Re: Patron question regarding topo maps
Date:   Thu, 06 Mar 2008 22:30:48 -0800
From:   Virginia R. Hetrick, Ph.D. <[log in to unmask]>
Reply-To:       [log in to unmask]

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        Maps, Air Photo & Geospatial Systems Forum <[log in to unmask]>


Hi, Mary -

The answer is actually pretty simple.  If you think about the length of
a latitude's circle at 34 degrees north (in SoCal where I am) versus 48
degrees north (in Port Townsend, just north of where you are and where I
grew up), the length of the latitude line is much shorter than in SoCal
because the circumference of the Earth is much shorter there.  By
contrast, the height (longitude) degrees will always be the same because
the circumference of the Earth along a longitude line is always pretty
much the same, allowing for the fact that the Earth is not a perfect
sphere but an oblate spheroid.

Also, if the map were wider than it is tall, it is likely that the
horizonal grid would have to develop a slight curve in the latitude
lines on individual map sheets.  Such a curve is pretty manageable on a
1:24 000 or 1:63 360 sheet as they were laid out.  It would be more
difficult to manage on a sheet half again as wide or twice as wide.  The
splines I ran into when I was a cartography student wouldn't have
reached across that distance.

I was given to understand, when we went on a tour of the USGS back in
the 1960s, that the reason for the "shape" of topo maps was so that they
could be easily used in the field where a single person might need to
hold the map sheet in a wind.  So, having them taller than wider was a
good choice for a very human reason.  Of course, since the guy who told
us that was not around when the original orientation decision was made,
he could have been all wet.  Also, of course, he's probably long since
retired and therefore not responsible for telling us anything!  ;~)

HTH.

v
--

\ /     Virginia R. Hetrick, here in sunny California
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Oo     "There is always hope."
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