Automated layout of origin–destination flow maps: U.S. county-to-county migration 2009–2013

Daniel M Stephen,Bernhard Jenny

doi:10.1080/17445647.2017.1313788

Abstract

ABSTRACTVisualizing large movement datasets with flow maps is difficult because overlapping flows create significant graphical conflicts that make accurate interpretation difficult or impossible. Interactive flow mapping applications allow users to explore large movement datasets by automatically generating flow maps from subsets of the data in response to queries by the user. However, even a small number of flows can overlap and cross each other in a way that impedes accurate interpretation. We introduce an interactive flow map of migration in the United States from 2009 to 2013 that uses a force-directed method to automatically lay out migration flows at the county-to-county and state levels. This map, available at http://usmigrationflowmapper.com/, aims at improving readability by automatically creating origin–destination flow layouts according to identified cartographic design principles. Map users explore high-level state-to-state migration patterns as well as detailed county-to-county movements through a custom user interface and interactive map features. We show migration flows between counties of different states by representing other states as nodes with a circular arrangement around the selected state, and connect county flows to those nodes. This constrains the map layout to a smaller area, reducing clutter and the amount of interaction required to view flows.

Highlights

Flow maps are a powerful tool for communicating information about geographic movement (Tufte, 1983)
The readability of flows in origin–destination maps can be improved by reducing graphical conflicts such as overlaps and intersections between flows, but even a small number of flows can overlap and cross each other in a way that impedes accurate interpretation
Automating the creation of origin–destination flow maps for large datasets like U.S county-to-county migration flows presents many challenges, most notably the graphical clutter resulting from overlap and intersections

Summary

Introduction

Flow maps are a powerful tool for communicating information about geographic movement (Tufte, 1983). Visualizing large datasets of hundreds or thousands of movements in a map creates significant graphical conflicts that make accurate interpretation difficult or impossible. The readability of flows in origin–destination maps can be improved by reducing graphical conflicts such as overlaps and intersections between flows, but even a small number of flows can overlap and cross each other in a way that impedes accurate interpretation. Large datasets must undergo a great deal of generalization, and/or be represented as a series of maps that show selected subsets of flows (Tobler, 1987). High-quality flow maps are created manually, applying cartographic design principles to ensure flows are arranged optimally for both interpretation and aesthetics (Jenny et al, 2016). It would be very time consuming to manually create enough flow maps to fully describe datasets with hundreds of thousands of movements

Objectives

Methods

Findings

Discussion

Conclusion