My very quick guess, based on some years in print production for offset
lithography of maps, is that those are color check samples for process
or spot colors in the map run. It may have been used to check colors in
the map against a color swatch to make sure the color was printing
correctly in the run.
There may also have been some targets printed along other parts of the
sheet that helped id the plate alignment. All of these are press check
aids I think.
The color dot you see is related to what color plates were used in the
print run, so only photo revised maps would contain the purple plate.
Again, purely a guess.
David
On 2015-10-19 12:38, Emily-Jane Dawson wrote:
> Esteemed colleagues,
> My library has recently moved a number of maps from our off-site
> storage facility to our Central Library, and as a result we are
> interfiling our historical topographical maps. While interfiling, we
> came up with a question we haven't been able to answer, and I'm hoping
> you can help.
>
> We have come across several pairs of 7.5 minute USGS topographical
> maps that appear to be duplicates. They have the same original date,
> the same revision/photorevision dates, and the same notations in the
> lower left corner about source data etc. All the information around
> the edges of the maps looks identical, and the maps themselves also
> look identical, so far as we can see.
>
> But there is one difference. One of the pair of maps has some dots
> just below the map image, on the right side (near the statement
> "Interior - Geological Survey - Reston Virginia"). The second map in
> the pair has no dots -- or sometimes the second map has a different
> selection of dots.
>
> The dots we've seen are black, green or purple. Sometimes there is a
> black star next to the dots. I've attached a detail photograph of
> our 1969 Boyds, WA quadrangle sheet, which has a black star, a green
> dot, and a black dot.
>
> The purple dots seem to be present only on sheets that have been
> photorevised, with photorevisions noted in purple. But we haven't
> been able to puzzle out what the other dots mean, and a careful
> perusal of our copy of Morris M. Thompson's _Maps for America_ did
> not reveal anything.
>
> Can any of you help us out? Or, can you tell us where we should look
> to find the answer to this question?
>
> thank you!
>
> --
>
> Emily-Jane Dawson | _reference librarian_
>
> Multnomah County Library
> sun-thurs: Central Library [1] | 503.988.5728
> _ follow us: _facebook [2] | twitter [3]
>
> "There is no such thing as a moral or an immoral book. Books are
> well written, or badly written. That is all."
> – Oscar Wilde, author's preface, _The Picture of Dorian Gray_
>
>
> Links:
> ------
> [1] http://www.multcolib.org/agcy/cen.html
> [2] http://facebook.com/multcolib
> [3] http://twitter.com/multcolib
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