Link maps are used in link analysis to show the relationships between locations. Link maps can show the relationships either without direction (spider lines) or with directions (flow maps). The connections can also show the magnitude of the relationship, such as the number of phone calls between locations.
Link maps help answer questions about your data, such as: How is it related?
A nongovernmental health organization is studying the spread of an infectious disease during a past outbreak. A directed link map (also called a flow map) is used to visualize the spread from two of the countries hit the hardest by the disease to other countries.
Create a link map
To create a link map, complete the following steps:
- Expand a dataset in the data pane so that the fields are visible.
- Select one of the following data options:
- Two location fields
- Two location fields plus a number or rate/ratio field
One or more location fields can be added to your dataset using Enable Location or by creating a relationship with a dataset that has a location field. When enabling location for a link map by coordinates or addresses, it is best practice to keep the Repeat identical features parameter unchecked.
You can search for fields using the search bar in the data pane.
- Drag the fields to the page and drop them on the Map drop zone.
Click the Flip card button to turn the map card over. The back of the card includes a place to write a description of the map.
The Layer options pane is accessible from the layer legend and can be used to change the style and statistics of the map.
Nodes can be sized using the following centrality methods:
- Degree—The number of direct neighbors of the node. If the map is directed (a flow map), the degree can be measured as either indegree (the number of direct neighbors with connections directed toward the node) or outdegree (the number of direct neighbors with connections directed away from the node).
- Betweenness—The extent to which a node lies on the shortest path between other nodes in the network. The normalization parameter is disabled for this centrality method because the betweenness calculation always applies normalization.
- Closeness—The average of the shortest distance paths to all other nodes. The normalization parameter is disabled for this centrality method.
- Eigenvector—The measure of the influence of a node in a network based on its proximity to other important nodes.
The Symbology tab and Appearance tab display different options based on the selections you make in the Layer options pane. The following options are available for link maps:
The Directional flow parameter can be used to change the links to arrows from one node to the other.
The centrality method can be set from the Size node using parameter.
The Edge weight parameter is used to calculate weighted centrality values. By default, the Edge weight parameter is set to Uniform, meaning the centrality calculation is unweighted. A field can be chosen to apply weights to the calculation. Edge weight is available for betweenness, closeness, and eigenvector centralities.
The Normalized parameter can be used to normalize the node centralities by dividing by another field to create a ratio or proportion. The Normalized parameter is enabled by default but can be disabled for nodes using betweenness and closeness centrality.
The Natural Breaks, Equal Interval, and Unclassed classifications can be chosen in the Classification type parameter. If Natural Breaks or Equal Interval is chosen, the number of classes can also be edited.
Click View centralities to create a reference table showing the centrality values for each node. The table includes a column for entity (field name), node (feature), and centrality.
Change the layer transparency of the link map.
Drag a string field to the Layer options pane and drop it on the link to style the links by unique values.
Use the Choose node field parameter to switch the selected node to a different location field.
Change the Node style options, including the following options:
Use the Add button and Delete button to add new node fields or delete existing node fields. New node fields will be connected to the selected node field. You must have three or more node fields to delete a node.
Drag a location field to the Layer options pane and drop it on the Add button or on an existing node to add node fields.
Use Ctrl+click to select multiple nodes. The following options are available:
The Weight parameter can be used to change or remove the number or rate/ratio field being used to apply weight to the links.
The Type parameter can be used to change or remove the string field being used to style the links by unique category.
Change the Link style options parameter, including the following options:
The Legend tab is enabled if a Weight field or Type field is added. The Legend can be used to view the classification values or unique categories for the links and to make selections on the chart.
If the arrows are pointing in the wrong direction, use the Flip button to change the direction of the flow.
If the map includes three or more node fields, the Delete button can be used to remove a link from the map. Deleting a link also removes a node field that has become disconnected from the rest of the map.
Drag a number or rate/ratio field to the Layer options pane and drop it on the selected link to change the Weight parameter. Use a string field to change the Type parameter.
The distance between nodes can be viewed by hovering over the links. The distance is reported in the default units for your account.
The Visualization type button can be used to switch a link map to another visualization, such as a bar chart. To change the visualization type, the location fields must have a Display field set.
A limit to the number of connections that can be displayed is based on the maximum query limit for the dataset. The error message There's too much data to complete this operation will be displayed if the number of connections is greater than the limit. The maximum query limit for point features is 16 thousand. The maximum query limit for line and area features is 8 thousand.
For example, a dataset of flights throughout Europe contains hundreds of thousands of flight numbers for 126 airports. Every airport has at least one direct flight to every other airport. Therefore, the number of connections is:
126 origins * 126 destinations = 15876 connections
The number of flights does not affect the query limit, but the number of airports does. If one extra airport is added to the dataset with direct flights to all other airports, the number of connections increases to 16,129, which is over the query limit. However, if there is not a connection between every unique value, the number of unique values can be higher. If some of the airports do not have direct flights between each other then the number of airports that can be displayed could increase until the number of connections surpasses the query limit.