There are many online resources for historical cartography. Many of these focus on 17th and 18th century maps. In this post, I will highlight two outstanding collections that include 20th century cartography as well, and have excellent - and free - online access. Additionally, some blogs on cartography are included in this overview.
Chris Harrison created this wonderful image of the density of internet conncetions around the globe. More images like this can be seen (and downloaded in high-resolution at his website.
One of the collections I like most is Persuasive Cartography - the PJ Mode Collection, held at Cornell University. Mr Mode is a retired laywer, and a student and collector of maps since the Eighties. He focusses on “persuasive cartography,” maps intended primarily to influence opinions or beliefs – to send a message – rather than to communicate geographic information." The website included over 800 high-resolution images of such maps, which are accompanied by extensive notes. The maps are very well indexed, according to subject, era, etcetera. Additionally, there is a very good search engine available.
David Rumsey Map Collection
This huge collection of maps is housed at Stanford University and includes over 150.000 maps, many of which can be explored at the online David Rumsey Map Collection Database. Online features include the wonderful Georeferencer, which allows you to overlay historic maps on modern or other historic maps.
The maps are extensively cataloged, and very high-definition images are available. Categories include Pictorial Maps, Children's Maps, Data Visualization, etc.
Tim Bryars Blog
Bryars & Bryars is a London based independent dealer of antiquarian maps, books and prints. They have an excellent collection of maps, including a great selection of 20th century and pictorial maps, as well as transit maps, including London Underground Maps. Tim Bryars writes very interesting and very well investigated blog posts providing in-depth information on a wide range of cartography related topics. Additionally, you can sign up for the monthly Rhumb Lines longread newsletter. Together with British Library lead map curator Tom Harper, Tim wrote the amazing book A History of the Twentieth Century in 100 Maps.
Map Dragons is a blog maintained by journalists Greg Miller and Betsy Mason. In 2014 they founded a cartography blog at Wired called Map Lab. In 2016 their blog moved to National Geographic, where it was renamed All Over the Map. On Map Dragons they publish a selection of previous blog post, but hopefully, they will expand the website with new material in the future.
They published the very well received book All over the Map in 2018.