Starting with version 3.0, SplashTool now generates flow arrows. These are exported as point shapefiles with user-specified thinning levels. The flow direction is recorded in the “FlowDir” attribute column using geographic rotation, where 0 degrees indicates north, and the degrees are measured clockwise. This article guides you through visualizing flow arrows in QGIS and ArcGIS.
QGIS
After adding the flowvectors_XX.shp shapefile to QGIS, the thinned flow arrows are initially displayed as points:
Right-click on the file in the layer tree, select Properties, then go to Symbology. Change the symbology to an SVG marker and use the search function under “SVG Images” to find an arrow. Choose an arrow pointing north, like the first one offered, Arrow_01.svg. Adjust the width and height of the arrow symbol to a size that fits your layout. In the screenshot, 4 mm was selected:
The points have now been replaced by arrows, all pointing north:
In the next step, rotate the arrows according to their direction. Open the symbology settings again. Under “Rotation,” open the highlighted dropdown menu and select rotation by attribute field. Choose “FlowDir” as the field and click “Apply”:
The arrows are now displayed in the correct direction:
If you want, you can also adjust the arrow size based on direction, for example, using classification to show larger arrows for higher flow accumulation:
Additionally, you can define scale-dependent visibility for different thinning levels of flow directions, which makes zooming in and out more pleasant. You can find these and other visualization examples in the GIS sample dataset, available for download on the Download page.
ArcGIS
After adding the flowvectors_XX.shp shapefile to ArcGIS Pro, the thinned flow arrows are initially displayed as points:
Right-click on the file in the layer tree, select Symbology, and search the gallery for a suitable point symbol:
After applying the symbology, the points have been replaced by arrows:
The arrows point east. For correct display in geographic rotation, you first need to align the arrow symbol to the north. To do this, change the angle to 90 degrees in the properties:
Now all arrows point upward:
In the next step, rotate the arrows according to the attribute field. First, go back to the main symbology settings by clicking the back button:
Then, in the second tab, select “Vary symbology by attribute” and set the “Rotation” dropdown to the FlowDir field. Set the rotation style to Geographic:
After applying the symbology, the flow arrows are correctly rotated:
Additionally, you can define scale-dependent visibility for different thinning levels of flow directions, which makes zooming in and out more pleasant. You can find these and other visualization examples in the GIS sample dataset, available for download on the Download page.