Recent satellite images . Here is a series of experiments by Richard Phillips at Nottingham Univ. By William A. Bowen of the California Geographical Survey. Atlas of the United States , Atlas of California , etc. Over 4.5 million maps, with 60,000 atlases and thousands of other items including journals, books and reference materials.
Online material is scanty.
Osher Map Library , University of Southern Maine. At UT-Austin holds over 230,000 maps. They also have a cartography glossary page. The University of California, Berkeley Earth Sciences and Map Library . Their map collection contains over 310,00 maps.