Interactive Flow Mapping
Flow maps depict the movement of phenomena between pairs of locations (origins and destinations). In a flow map, nodes represent locations and links (flows) illustrate movements where the width and/or color of an edge are usually determined by the volume moved. Depending on the complexity of a flow map, it is a challenging task to visually compare flows and extract meaningful information such as spatial focusing and dispersion patterns (e.g., sinks/sources that receive/send a large number of flows from/to a variety of locations). Integrated and interactive environments offer a variety of user interactions to enhance knowledge discovery from flow maps.
Flow Map Design
Flow directions and volumes are the two variables that determine the layout of a directed flow map. Arrows are used to delineate the destination of flow lines and each flow line follows the right-hand traffic rule: a flow line is drawn on the right side while pointing to its destination. In addition to using arrows and the right-hand traffic rule, style of the line can also be used to differentiate flow directions. Instead of using straight lines, one can use Bezier curves to draw flow lines where each line is curvy at the origin and straight on the destination end.
Both color and width are redundantly used to differentiate the volume of flow lines. A sequential color scheme with classed flow symbols (straight or curved lines) are provided to the user for delineating the volume of flows (Figure 2). In order to reduce the visual clutter and maintain an acceptable performance for the online application, the flows with the top 100 volume are displayed. In order to help users associate flows with real world locations, administrative boundaries (e.g., countries, states, counties) and place names are provided with a light gray base map.
The back-end design of the flow mapping application is based on BlazeDS which is a server-based Java remoting technology that enables connecting to back-end distributed data and pushing data in real-time to Adobe Flex Applications on the client-side. The data needed for the application are stored in a Postgresql database and sent to the client-side by using Java remoting technology. On the other hand, the front-end design is based on ArcGIS Flex Viewer and ArcGIS Flex API although a large number of customized classes and functionality were added. Below are the example links that leverage the interactive flow mapping application:
Commodity Flows in the States
Commodity Flow Mapper is an interactive flow mapping application that depicts commodity flow data in the U.S. from 2007.Using the controls provided by the top panel, you may switch between commodity types and display commodity flows by either their value in million dollars or tons in thousands. For example, you may select all commodity flows or flows of a particular type of commodity such as cereal grains, tobacco products and coal, and display the volumes in terms of the cumulative values or tons. Using the controls, you may also customize the color, size, and symbols of the flow map. A legend is provided to convey the classification of the flow symbology.
The data for commodity flows is based on The Commodity Flow Survey (CFS), undertaken through a partnership between the Census Bureau and the Bureau of Transportation Statistics (BTS). The CFS is conducted every 5 years (years ending in "2" and "7") as part of the Economic Census and produces data on the movement of goods in the United States. It provides information on commodities shipped, their value, weight, and mode of transportation, as well as the origin and destination of shipments of commodities from manufacturing, mining, wholesale, and selected retail and services establishments. The data is freely available on the following link:
Flows of disaster aids in the world
Disaster aids between countries and agencies also form a spatial interaction network which could help us understand the distribution of funds for disaster reduction and recovery across the world. Disaster Aid Mapper depicts disaster aid flows by disaster categories, funding categories and years. You may display a flow map of funds by disaster type(e.g., drought, earthquake, flood) , funding source (e.g., government, ngos, United Nations), and by year (2000 - 2010). By displaying the dollar value of aids by different agencies, disaster types and across years, one can understand the spatial, temporal and multivariate patterns of disaster aids. Tools to customize the color, size, and symbols of the flow map is also provided.
The data is acquired from the Global Facility for Disaster Reduction and Recovery (GFDRR) which is a partnership of 41 countries and 8 international organizations committed to helping developing countries reduce their vulnerability to natural hazards and adapt to climate change. The data is freely available at: https://www.gfdrr.org/
Flow Map User Testing
You are invited to participate in a user test for flow mapping, where you will be presented a series of flow maps and answer a number of questions.