Last time, I wrote about the little details that make a good flow map. The data in that post was made up and simpler than your typical flow map. That’s why I wanted to redo it with real-world data. In this post, I’m using domestic migration data of Austria.

Raw migration data, line width scaled to flow strength

With 9 states, that makes 72 potential flow arrows. Since that’s too much to map, I’ve decided in a first step to only show flows with more than 1,000 people.

Following the recommendations mentioned in the previous post, I first designed a basic flow map where each flow direction is rendered as a black arrow:

Basic flow map

Even with this very limited number of flows, the map gets pretty crowded, particularly around the north-eastern node, the Austrian capital Vienna.

To reduce the number of incoming and outgoing lines at each node, I therefore decided to change to colored one-sided arrows that share a common geometry:

Colored one-sided arrows

The arrow color is determined automatically based on the arrow direction using the following expression:

CASE WHEN
 "weight" < 1000 THEN color_rgba( 0,0,0,0)
WHEN
 x(start_point( $geometry)) - x(end_point($geometry)) < 0
THEN
 '#1f78b4'
ELSE
 '#ff7f00'
END

The same approach is used to control the side of the one-sided arrow head. The arrow symbol layer has two “arrow type” options for rendering the arrow head: on the inside of the curve or on the outside. This means that, if we wouldn’t use a data-defined approach, the arrow head would be on the same side – independent of the line geometry direction.

CASE WHEN
 x(start_point( $geometry)) - x(end_point($geometry)) < 0
THEN
 1
ELSE
 2
END

Obviously, this ignores the corner case of start and end points at the same x coordinate but, if necessary, this case can be added easily.

Of course the results are far from perfect and this approach still requires manual tweaking of the arrow geometries. Nonetheless, I think it’s very interesting to see how far we can push the limits of data-driven styling for flow maps.

Give it a try! You’ll find the symbol and accompanying sample data on the QGIS resource sharing plugin platform:

Details of good flow maps

By underdark

2016-12-18

QGIS

42 Comments

In my previous post, I shared a flow map style that was inspired by a hand drawn map. Today’s post is inspired by a recent academic paper recommended to me by Radoslaw Panczak @RPanczak and Thomas Gratier @ThomasG77:

Jenny, B., Stephen, D. M., Muehlenhaus, I., Marston, B. E., Sharma, R., Zhang, E., & Jenny, H. (2016). Design principles for origin-destination flow maps. Cartography and Geographic Information Science, 1-15.

Jenny et al. (2016) performed a study on how to best design flow maps. The resulting design principles are:

number of flow overlaps should be minimized;

sharp bends and excessively asymmetric flows should be avoided;

acute intersection angles should be avoided;

flows must not pass under unconnected nodes;

flows should be radially arranged around nodes;

quantity is best represented by scaled flow width;

flow direction is best indicated with arrowheads;

arrowheads should be scaled with flow width, but arrowheads for thin flows should be enlarged; and

overlaps between arrowheads and flows should be avoided.

Many of these points concern the arrangement of flow lines but I want to talk about those design principles that can be implemented in a QGIS line style. I’ve summarized the three core ideas:

use arrow heads and scale arrow width according to flow,
enlarge arrow heads for thin flows, and
use nodes to arrange flows and avoid overlaps of arrow heads and flows

This slideshow requires JavaScript.

To get started, we can use a standard QGIS arrow symbol layer. To represent the flow value (“weight”) according to the first design principle, all arrow parameters are data-defined:

scale_linear("weight",0,10,0.1,3)

To enlarge the arrow heads for thin flow lines, as required by the second design principle, we can add a fixed value to the data-defined head length and thickness:

scale_linear("weight",0,10,0.1,1.5)+1.5

The main issue with this flow map is that it gets messy as soon as multiple arrows end at the same location. The arrow heads are plotted on top of each other and at some point it is almost impossible to see which arrow starts where. This is where the third design principle comes into play!

To fix the overlap issue, we can add big round nodes at the flow start and end points. These node buffers are both used to render circles on the map, as well as to shorten the arrows by cutting off a short section at the beginning and end of the lines:

difference(
  difference(
    $geometry,
    buffer( start_point($geometry), 10000 )
  ),
  buffer( end_point( $geometry), 10000 )
)

Note that the buffer values in this expression only produce appropriate results for line datasets which use a CRS in meters and will have to be adjusted for other units.

It’s great to have some tried and evaluated design guidelines for our flow maps. As always: Know your cartography rules before you start breaking them!

PS: To draw a curved arrow, the line needs to have one intermediate point between start and end – so three points in total. Depending on the intermediate point’s position, the line is more or less curved.

New style: conveyor belt flows

By underdark

2016-12-08

Cartography, QGIS

1 Comment

The QGIS map style I want to share with you today was inspired by a hand-drawn map by Philippe Rekacewicz that I saw on Twitter:

Visionscarto met en ligne quelques unes des archives #Chine, dans le contexte régional et dans la mondialisation https://t.co/51nFLHUSXM pic.twitter.com/ZIlnEUxJK3

— Phil Reka de chez visionscarto (@visionscarto) November 25, 2016

The look reminds me of conveyor belts, thus the name choice.

You can download the symbol and a small sample dataset by adding my repo to the QGIS Resource Sharing plugin.

resourcesharing_conveyor

The conveyor belt is a line symbol that makes extensive use of Geometry generators. One generator for the circle at the flow line start and end point, respectively, another generator for the belt, and a final one for the small arrows around the colored circles. The color and size of the circle are data defined:

conveyor_details

The collection also contains a sample Geopackage dataset which you can use to test the symbol immediately. It is worth noting that the circle size has to be specified in layer CRS units.

It’s great fun playing with the power of Geometry generator symbol layers and QGIS geometry expressions. For example, this is the expression for the final geometry that is used to draw the small arrows around colored circles:

line_merge( 
  intersection(
    exterior_ring( 
      convex_hull( 
        union( 
          buffer( start_point($geometry), "start_size" ),
          buffer( end_point($geometry), 500000 )
        )
      )
    ),
    exterior_ring( 
      buffer( start_point( $geometry), "start_size" )
    )
  )
)

The expression constructs buffer circles, the belt geometry (convex_hull around buffers), and finally extracts the intersecting part from the start circle and the belt geometry.

Hope you enjoy it!

It’s holiday season, why not share one of your own symbols with the QGIS community?

Movement data in GIS #2: visualization

By underdark

2016-10-09

Movement data in GIS, QGIS, spatio-temporal data

1 Comment

In the first part of the Movement Data in GIS series, I discussed some of the common issues of modeling movement data in GIS, followed by a recommendation to model trajectories as LinestringM features in PostGIS to simplify analyses and improve query performance.

Of course, we don’t only want to analyse movement data within the database. We also want to visualize it to gain a better understanding of the data or communicate analysis results. For example, take one trajectory:

(data credits: GeoLife project)

Visualizing movement direction is easy: just slap an arrow head on the end of the line and done. What about movement speed? Sure! Mean speed, max speed, which should it be?

Speed along the trajectory, a value for each segment between consecutive positions.

With the usual GIS data model, we are back to square one. A line usually has one color and width. Of course we can create doted and dashed lines but that’s not getting us anywhere here. To visualize speed variations along the trajectory, we therefore split the original trajectory into its segments, 1429 in this case. Then we can calculate speed for each segment and use a graduated or data defined renderer to show the results:

Speed along trajectory: red = slow to blue = fast

Very unsatisfactory! We had to increase the number of features 1429 times just to show speed variations along the trajectory, even though the original single trajectory feature already contained all the necessary information and QGIS does support geometries with measurement values.

Starting from QGIS 2.14, we have an alternative way to deal with this issue. We can stick to the original single trajectory feature and render it using the new geometry generator symbol layer. (This functionality is also used under the hood of the 2.5D renderer.) Using the segments_to_lines() function, the geometry generator basically creates individual segment lines on the fly:

Segments_to_lines( $geometry) returns a multi line geometry consisting of a line for every segment in the input geometry

Once this is set up, we can style the segments with a data-defined expression that determines the speed on the segment and returns the respective color along a color ramp:

Speed is calculated using the length of the segment and the time between segment start and end point. Then speed values from 0 to 50 km/h are mapped to the red-yellow-blue color ramp:

ramp_color(
  'RdYlBu',
  scale_linear(
    length( 
      transform(
	    geometry_n($geometry,@geometry_part_num),
		'EPSG:4326','ESRI:54027'
		)
    ) / (
      m(end_point(  geometry_n($geometry,@geometry_part_num))) -
      m(start_point(geometry_n($geometry,@geometry_part_num)))
    ) * 3.6,
    0,50,
    0,1
  )
)

Thanks a lot to @nyalldawson for all the help figuring out the details!

While the following map might look just like the previous one in the end, note that we now only deal with the original single line feature:

Similar approaches can be used to label segments or positions along the trajectory without having to break the original feature. Thanks to the geometry generator functionality, we can make direct use of the LinestringM data model for trajectory visualization.

This post is part of a series. Read more about movement data in GIS.

How to create round maps in Print Composer

By underdark

2016-05-01

Cartography, QGIS

If you follow me on Twitter, you’ve probably seen previews of my experiments with round maps. These experiments were motivated by a recent question on GIS.stackexchange whether this type of map can be created in QGIS and while it’s not very convenient right now, it is definitely possible:

All maps in this post are created using data from the Quantarctica project.

I’ve been planing to try the Quantarctica datasets for a long time and this use case is just perfect. When you download and open their project, you’ll see that they have already clipped all datasets to a circle around Antarctica:

Quantarctica project with some custom styling

Since the map of the full extent of the dataset is already clipped to a circle, the overview map is easy to deal with. The detail map on the other hand is rectangular by default:

Since we cannot change the shape of the map item, we have to use a mask instead. To create a circular mask, we can add an ellipse shape:

The main challenge when creating the mask is that there is no inverted polygon renderer for shapes in print composer. I’ve evaluated two workarounds: First, I created a style with a wide white outline that would cover all map parts outside the circle shape. But this solution slowed the print composer down a lot. An alternative, which doesn’t suffer from this slowdown is using draw effects:

In particular, I created a big outer glow effect:

Note that the effect only works if the symbol itself is not transparent. That’s why I set the symbol fill to black and used the Lighten blending mode:

Voilà! Both maps are nicely circular.

It is worth noting though that this workaround has a downside: it is not possible to create automatic grids/graticules for these maps. The graticule in the overview map only works because it is a layer in the main project that was already clipped to the circular shape.

Finally, you can add more depth to your map by adding shadows. To create the shadow effect, I added additional ellipse items which are styled with a drop shadow draw effect. If you only enable the drop shadow effect, you will notice that the shadow is cut off at the ellipse bounding box. To avoid this undesired effect, you can add a transform effect, which reduces the size of the drawn shape and it’s shadow so that the shadow fits into the bounding box:

It requires some manual adjustments to place the shadow at the optimal location on top of the mask:

Add another ellipse to create the shadow for the overview map.

For more cartography tips and tricks check my new book QGIS Map Design or join my QGIS training courses.

New demos: live labels & gradient editor

By underdark

2016-04-10

Cartography, QGIS

Following up on last week’s post, Nyall has continued his work on the QGIS gradient editor:

Latest version of the new QGIS interactive gradient edit. This now includes an interactive plot of the color hue/saturation/lightness/alpha, allowing a visual overview of these color components and easy editing.

Another equally awesome demo has been posted by Nathan, who is currently working on usability improvements for labeling and styling without blocking dialogs:

This is going to be great for map design work because it makes many complex styles much easier to create since you can interact with the map and attribute table at the same time.

These are definitely two developments to follow closely!

Towards better gradients

By underdark

2016-04-03

Cartography, QGIS, Visualization

Interesting developments going on if you like creating your own gradients. After all, that’s not as easy as it might initially seem, as Gregor Aisch describes in his post “Mastering Multi-hued Color Scales with Chroma.js”:

If you turn data into colors, this should be required reading: https://t.co/lOfpiFdpG3 via @driven_by_data #dataviz pic.twitter.com/RADLExzYxM
— Joshua Stevens (@jscarto) April 1, 2016

The issues with simple color interpolations, which include nonuniform changes in lightness between classes, also haunt us in cartography. Just have a look at the map and legend on the left-hand side, which has been created using a normal custom QGIS gradient with colors ranging from black to red, yellow and finally white. We end up with three classes in yellow which are nearly impossible to tell apart:

For comparison, on the right side, I’ve used Gregor’s corrected color ramp, which ensures that lightness changes evenly from one class to the next.

Wouldn’t it be great if the built-in gradient tool in QGIS could correct for lightness? Too bad the current dialog is not that great:

image from https://docs.qgis.org/2.8/en/docs/user_manual/working_with_vector/style_library.html

My first reaction therefore was to write a short script to import gradients from Gregor’s Chroma.js Color Scale Helper into QGIS:

https://twitter.com/underdarkGIS/status/716389969850404864

But we’ll probably have a much better solution in QGIS soon since Nyall Dawson has picked up the idea and is already working on a completely new version of the gradient tool. You can see a demo of the current work in progress here:

I’m really looking forward to trying this out once it hits master!

Creating dynamic icon series

By underdark

2016-03-19

Cartography, QGIS

6 Comments

Today’s post was motivated by a question on GIS.StackExchange, which is looking for an automated way to symbolize the amenities available at a location using a series of icons, like this:

Assuming the information is available in a format similar to this example attribute table

we can create a symbol, which adapts to the values in the icon columns using data-defined overrides:

The five potential symbol locations are aligned next to each other using offsets. We use the following expression to determine the correct SVG symbol:

CASE
WHEN "icon4" = 'dinner'
 THEN 'C:/OSGeo4W64/apps/qgis-dev/svg/entertainment/amenity=restaurant.svg'
WHEN "icon4" = 'sleep'
 THEN 'C:/OSGeo4W64/apps/qgis-dev/svg/accommodation/accommodation_hotel2.svg'
WHEN "icon4" = 'ship'
 THEN 'C:/OSGeo4W64/apps/qgis-dev/svg/transport/amenity=ferry_terminal.svg'
WHEN "icon4" = 'house'
 THEN 'C:/OSGeo4W64/apps/qgis-dev/svg/accommodation/accommodation_house.svg'
 ELSE  ''
END

To hide icons if the icon value is NULL, the marker size is set to 0 using, for example:

CASE
WHEN "icon4" is not NULL
 THEN 4
 ELSE 0
END

Finally, to ensure that the labels don’t cover the icons, we can use the cartographic label placement with the position priority set to ‘TR,TL,BL’, which restricts labels to the top right, top left, and bottom left position.

With these settings in place, we can zoom out and the labeling algorithm picks the most suitable position from the list of allowed positions:

For more cartography tips and tricks check my new book QGIS Map Design or join my QGIS training courses.

How to create a graduated renderer for polygon borders instead of fills

By underdark

2016-03-12

QGIS

Today’s post was motivated by a recent question on the #gistribe Twitter chat:

Is there a way in #qgis to have polygon outline colors changed the way the fill can be when you categorize data? #gistribe
— Michele M Tobias (@MicheleTobias) March 11, 2016

So what’s the issue?

Default polygon symbols come with a fill and a border color:

When they are used in a graduated renderer, the fill color is altered for each class:

What if you want to change the border color instead?

The simplest solution is to add an outline symbol layer to your polygon symbol:

The outline layer has only one color property and it will be altered by the graduated renderer.

If you now hit ok, the graduated renderer will alter both the simple fill’s fill color and the outline’s color. To stop the fill color from changing, select the simple fill and lock it using the small lock icon below the list of symbol layers:

Voilà:

For more cartography tips and tricks check my new book QGIS Map Design or join my QGIS training courses.

QGIS Map Design is out now!

By underdark

2016-02-28

Cartography, QGIS

8 Comments

My latest book “QGIS Map Design”, co-authored with well-known cartography expert Gretchen Peterson and with a foreword by the founder of QGIS, Gary Sherman himself, is now available as e-book.

In three parts, the book covers layer styling, labeling, and designing print maps. All recipes come with data and project files so you can reproduce the maps yourself.