(39) http://upload.wikimedia.org/wikipedia/commons/a/ab/
WorldMapLongLat-eq-circles-tropics-non.png. GNU-FDL.
(40) http://en.wikipedia.org/wiki/Image:USGS_map_Albers_conic_tall.gif. Public
Domain.
(41) http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Brujula.jpg/
800px-Brujula.jpg. GNU Free Documentation.
(42) http://www.nps.gov/brca/planyourvisit/swampcanyontrail.htm. Public
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(43) http://erg.usgs.gov/isb/pubs/booklets/symbols/graphics/part2.jpg
http://erg.usgs.gov/isb/pubs/booklets/symbols/reading.html. Public
Domain.
