The Straight Dope: Do maps have "copyright traps" to permit detection of unauthorized copies?. www.straightdope.com [online]. [cit. 2009-11-22]. Dostupné v archivu pořízeném z originálu dne 2008-08-08.
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The Straight Dope: Do maps have "copyright traps" to permit detection of unauthorized copies?. www.straightdope.com [online]. [cit. 2009-11-22]. Dostupné v archivu pořízeném z originálu dne 2008-08-08.