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Visualizing 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 origindestination 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.

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<p><strong>Abstract.</strong> Origin-destination flow maps represent movement between locations using lines of optionally varying width connecting points or areas. When flow lines intersect or overlap, legibility can be improved by curving flows to avoid each other, which is a time-consuming process to do by hand when there are many flows.</p><p>We present a tool for ArcGIS Pro that implements a force-directed method developed by Jenny et al. (2017) for curving flows. This method reduces overlaps and intersections by curving flows away from nodes, other flows, and arrowheads, taking flow map design principles into account (Jenny et al., 2018). Users provide node coordinates and ids, and a table describing the volume of movement between the nodes. The tool produces two map layers, one containing curved flow lines, and another containing points that includes data about the total quantity of movement to and from each point. The new line and point layers are then added to a map in ArcGIS Pro. The width of each line represents the relative quantity of movement represented, and the diameter of each node represents the total movement in and out of that location. Users can opt to produce either net flows (the difference in movement between points) or total flows (the sum of movement between points in both directions). In the case of net flows, arrowheads are added to flows to indicate flow direction, and the ends of flows are clipped to prevent arrowheads from overlapping destination points. Arrowheads of smaller flows are made slightly larger to improve visibility. Users have several options to help reduce map clutter, including modifying the maximum width of flows and nodes, creating additional space between flows and their endpoints, and displaying only a selected number of the largest flows.</p><p>We also introduce more advanced experimental features that we implemented in standalone prototype software. Our prototype can create parallel flow lines for bidirectional flows. This avoids inconsistently spaced opposing flow lines between a pair of origin and destination nodes. The prototype can shorten flow lines to reduce overlaps with other flows, nodes and arrowheads, and keep a minimum distance between flow lines. We will also discuss placing flow lines between areas instead of nodes and demonstrate an algorithmic implementation for creating flows between areas. Our prototype provides an easy-to-use graphical user interface and will be released as free and open-source software.</p>