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Map-based visualization has been a distinct focus in cartography and geography since the early 1990s.Over that time,both the technology and the theory that underlies visualization have advanced dramatically.We have reached a point where advances in technology (particularly open source technology) and theory,coupled with ever-increasing volumes of geographically-referenced data,make it possible for a wide range of potential developers to generate map-based visualization tools that meet the diverse place-based needs of our society.But,these developers (and students learning to be developers) need guidance in leveraging the tools and the data to produce effective online mapping applications.Open web mapping with Leaflet,by Professor Nai YANG fills this need for developers and students anxious to generate effective mapping applications with open and easily extensible tools.

Open web mapping with Leaflet draws upon three decades of cross-fertilization between developments in cartography and visualization.In the early years (the 1990s),the visualization focus within cartography was on leveraging specialized advances in computer hardware and software to produce one-off mapping applications designed for expert users.In my own early work on cartographic visualization,I presented “visualization” as being at one end of a continuum of map use,with “communication” at the other.The key motivation in that work was to prompt cartographers (and other map makers) to recognize that maps can be more than presentation devices,that they have an important role in supporting thinking and knowledge creation.

The visualization applications created during that early period of map-based visualization supported experts who leveraged growing repositories of spatial data to (a) facilitate scientific research and (b) achieve narrowly focused solutions to information analysis needs within government and other organizations (e.g.,for regional planning,environmental management,or monitoring disease outbreaks).These specialized,map-based visualization applications continue to be important and are increasingly effective due to both the exponential increase in geographically-referenced data and a renewed realization that location matters (that in spite of modern technology eliminating some distance barriers,issues related to disease,the environment,livelihoods,and more continue to vary geographically).But,map-based visualization is no longer the exclusive domain of experts.It is now possible for a wide range of potential map users to access spatial data and mapping technologies online (and on mobile devices) to think with and create knowledge from geographic data as well as to communicate that knowledge.Additionally,a wide range of individuals (not just professional cartographers) now have the potential to develop specialized online mapping applications that leverage diverse geographic data sources.Open source technologies are a key factor in these developments.

Now is ideal time for an online mapping text that organizes knowledge and techniques in one place in order to instruct would-be map creators on how (and why) to leverage open technologies to build their own custom,online mapping applications.Open web mapping with Leaflet,by Professor Nai YANG,is thus poised to have a substantial impact in training the next generation of web cartographers in the basics of map-based visualization and in how to utilize open source technologies to build novel and effective maps.

During 2017-2018,I had the pleasure to act as the academic host for Professor Nai YANG’s year-long visit in the Department of Geography at The Pennsylvania State University.That was a productive year for both of us,with opportunity to collaborate on research and discuss instructional strategies.During Professor YANG’s stay in the Department he was based in the GeoVISTA Center (for which I was Director).This provided an opportunity for him to take an active role in ongoing research (we collaborated particularly on developing,implementing,and assessing Tag Mapping methods).He also made use of the visit to be an observer in and have discussions about cartography/visualization courses (including the Dynamic Cartographic Representation Class I was teaching at the time).Thus,in addition to collaborating on mapping research,we were able to share ideas about instruction.　This book,which I was very happy to see come to fruition,reflects and builds upon those activities.It represents a much-needed compilation and explication of methods and technologies for constructing online mapping applications to meet a wide range of needs.The book provides a unique contribution to the field by detailing how to leverage open source tools to create effective online maps and by illustrating the potential with diverse examples of map-based visualization displays and applications that can be constructed today (relatively easily,by individuals with modest technology skills).

As detailed in the book,a wide range of open source technologies have been developed and are freely available.These technologies now make it possible for individuals from many applications domains to build custom mapping applications that meet the diverse needs of science and society.In Chapter 1,the book introduces the novice developer to the core concepts of map visualization.Then,Chapter 2 outlines the basics of web development.Building on this based,the the text focuses in on how to leverage one specific open source mapping technology,Leaflet,to build a wide range of map types.　In four chapters,Professor YANG leads the reader through the basics of Leaflet,core map operations it supports,thematic maps of multiple types,and map animation.The book is copiously illustrated and those illustrations provide a great sense of the range of mapping products possible using Leaflet.

We are at an exciting time in cartography,and in all fields for which geographic data and maps are relevant.Never before have so many people had access to so much geographic data,plus to the technologies that enable them to generate effective and dynamic maps from those data.I will look forward to the many maps that are sure to be generated by those who learn open source mapping skills from open web mapping with Leaflet.

Professor Alan M.MacEachren

Pennsylvania State University

April,2021