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Tue Mar 28 06:50:12 2017

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Movement data in GIS #5: current research topics

In the 1st part of this series, I mentioned the Workshop on Analysis of Movement Data at the GIScience 2016 conference. Since the workshop took place in September 2016, 11 abstracts have been published (the website seems to be down currently, see the cached version) covering topics from general concepts for movement data analysis, to transport, health, and ecology specific articles. Here’s a quick overview of what researchers are currently working on:

  • General topics
    • Interpolating trajectories with gaps in the GPS signal while taking into account the context of the gap [Hwang et al., 2016]
    • Adding time and weather context to understand their impact on origin-destination flows [Sila-Nowicka and Fotheringham, 2016]
    • Finding optimal locations for multiple moving objects to meet and still arrive at their destination in time [Gao and Zeng, 2016]
    • Modeling checkpoint-based movement data as sequence of transitions [Tao, 2016]
  • Transport domain
    • Estimating junction locations and traffic regulations using extended floating car data [Kuntzsch et al., 2016]
  • Health domain
    • Clarifying physical activity domain semantics using ontology design patterns [Sinha and Howe, 2016]
    • Recognizing activities based on Pebble Watch sensors and context for eight gestures, including brushing one’s teeth and combing one’s hair [Cherian et al., 2016]
    • Comparing GPS-based indicators of spatial activity with reported data [Fillekes et al., 2016]
  • Ecology domain
    • Linking bird movement with environmental context [Bohrer et al., 2016]
    • Quantifying interaction probabilities for moving and stationary objects using probabilistic space-time prisms [Loraamm et al., 2016]
    • Generating probability density surfaces using time-geographic density estimation [Downs and Hyzer, 2016]

If you are interested in movement data in the context of ecological research, don’t miss the workshop on spatio-temporal analysis, modelling and data visualisation for movement ecology at the Lorentz Center in Leiden in the Netherlands. There’s currently a call for applications for young researchers who want to attend this workshop.

Since I’m mostly working with human and vehicle movement data in outdoor settings, it is interesting to see the bigger picture of movement data analysis in GIScience. It is worth noting that the published texts are only abstracts, therefore there is not much detail about algorithms and whether the code will be available as open source.

For more reading: full papers of the previous workshop in 2014 have been published in the Int. Journal of Geographical Information Science, vol 30(5). More special issues on “Computational Movement Analysis” and “Representation and Analytical Models for Location-based Social Media Data and Tracking Data” have been announced.

References

[Bohrer et al., 2016] Bohrer, G., Davidson, S. C., Mcclain, K. M., Friedemann, G., Weinzierl, R., and Wikelski, M. (2016). Contextual Movement Data of Bird Flight – Direct Observations and Annotation from Remote Sensing.
[Cherian et al., 2016] Cherian, J., Goldberg, D., and Hammond, T. (2016). Sensing Day-to-Day Activities through Wearable Sensors and AI.
[Downs and Hyzer, 2016] Downs, J. A. and Hyzer, G. (2016). Spatial Uncertainty in Animal Tracking Data: Are We Throwing Away Useful Information?
[Fillekes et al., 2016] Fillekes, M., Bereuter, P. S., and Weibel, R. (2016). Comparing GPS-based Indicators of Spatial Activity to the Life-Space Questionnaire (LSQ) in Research on Health and Aging.
[Gao and Zeng, 2016] Gao, S. and Zeng, Y. (2016). Where to Meet: A Context-Based Geoprocessing Framework to Find Optimal Spatiotemporal Interaction Corridor for Multiple Moving Objects.
[Hwang et al., 2016] Hwang, S., Yalla, S., and Crews, R. (2016). Conditional resampling for segmenting GPS trajectory towards exposure assessment.
[Kuntzsch et al., 2016] Kuntzsch, C., Zourlidou, S., and Feuerhake, U. (2016). Learning the Traffic Regulation Context of Intersections from Speed Profile Data.
[Loraamm et al., 2016] Loraamm, R. W., Downs, J. A., and Lamb, D. (2016). A Time-Geographic Approach to Wildlife-Road Interactions.
[Sila-Nowicka and Fotheringham, 2016] Sila-Nowicka, K. and Fotheringham, A. (2016). A route map to calibrate spatial interaction models from GPS movement data.
[Sinha and Howe, 2016] Sinha, G. and Howe, C. (2016). An Ontology Design Pattern for Semantic Modelling of Children’s Physical Activities in School Playgrounds.
[Tao, 2016] Tao, Y. (2016). Data Modeling for Checkpoint-based Movement Data.

 


Gradient arrows

Today’s post was motivated by a question following up on my recent post “Details of good flow maps“: How to create arrows with gradients from transparent to opaque?

gradient_arrow

The key idea is to use a gradient fill to color the arrows:

gradient_arrow_settings

It all seems perfectly straightforward: determine the direction of the line and set the gradient rotation according to the line direction.

But wait! That doesn’t work!

The issue is that all default angle functions available in expressions return clockwise angles but the gradient rotation has to be set in counter-clockwise angles. So we need this expression:

360-angle_at_vertex($geometry,1)

Happy QGISing!


Small multiples for OD flow maps using virtual layers

In my previous posts, I discussed classical flow maps that use arrows of different width to encode flows between regions. This post presents an alternative take on visualizing flows, without any arrows. This style is inspired by Go with the Flow by Robert Radburn and Visualisation of origins, destinations and flows with OD maps by J. Wood et al.

The starting point of this visualization is a classic OD matrix.

migration_raw_data

For my previous flow maps, I already converted this data into a more GIS-friendly format: a Geopackage with lines and information about the origin, destination and strength of the flow:

migration_attribute_table

In addition, I grabbed state polygons from Natural Earth Data.

At this point, we have 72 flow features and 9 state polygon features. An ordinary join in the layer properties won’t do the trick. We’d still be stuck with only 9 polygons.

Virtual layers to the rescue!

The QGIS virtual layers feature (Layer menu | Add Layer | Add/Edit Virtual Layer) provides database capabilities without us having to actually set up a database … *win!*

Using a classic SQL query, we can join state polygons and migration flows into a new virtual layer:

virtual_layer

The resulting virtual layer contains 72 polygon features. There are 8 copies of each state.

Now that the data is ready, we can start designing the visualization in the Print Composer.

This is probably the most manual step in this whole process: We need 9 map items, one for each mini map in the small multiples visualization. Create one and configure it to your liking, then copy and paste to create 8 more copies.

I’ve decided to arrange the map items in a way that resembles the actual geographic location of the state that is represented by the respective map, from the state of Vorarlberg (a proud QGIS sponsor by the way) in the south-west to Lower Austria in the north-east.

To configure which map item will represent the flows from which origin state, we set the map item ID to the corresponding state ID. As you can see, the map items are numbered from 1 to 9:

small_multiples_print_composer_init

Once all map items are set up, we can use the map item IDs to filter the features in each map. This can be implemented using a rule based renderer:

small_multiples_style_rules

The first rule will ensure that the each map only shows flows originating from a specific state and the second rule will select the state itself.

We configure the symbol of the first rule to visualize the flow strength. The color represents the number number of people moving to the respective district. I’ve decided to use a smooth gradient instead of predefined classes for the polygon fill colors. The following expression maps the feature’s weight value to a shade on the Viridis color ramp:

ramp_color( 'Viridis',
  scale_linear("weight",0,2000,0,1)
)

You can use any color ramp you like. If you want to use the Viridis color ramp, save the following code into an .xml file and import it using the Style Manager. (This color ramp has been provided by Richard Styron on rocksandwater.net.)

<!DOCTYPE qgis_style>
<qgis_style version="0">
  <symbols/>
    <colorramp type="gradient" name="Viridis">
      <prop k="color1" v="68,1,84,255"/>
      <prop k="color2" v="253,231,36,255"/>
      <prop k="stops" v="0.04;71,15,98,255:0.08;72,29,111,255:0.12;71,42,121,255:0.16;69,54,129,255:0.20;65,66,134,255:0.23;60,77,138,255:0.27;55,88,140,255:0.31;50,98,141,255:0.35;46,108,142,255:0.39;42,118,142,255:0.43;38,127,142,255:0.47;35,137,141,255:0.51;31,146,140,255:0.55;30,155,137,255:0.59;32,165,133,255:0.62;40,174,127,255:0.66;53,183,120,255:0.70;69,191,111,255:0.74;89,199,100,255:0.78;112,206,86,255:0.82;136,213,71,255:0.86;162,218,55,255:0.90;189,222,38,255:0.94;215,226,25,255:0.98;241,229,28,255"/>
    </colorramp>
  </colorramps>
</qgis_style>

If we go back to the Print Composer and update the map item previews, we see it all come together:

small_multiples_print_composer

Finally, we set title, legend, explanatory texts, and background color:

migration

I think it is amazing that we are able to design a visualization like this without having to create any intermediate files or having to write custom code. Whenever a value is edited in the original migration dataset, the change is immediately reflected in the small multiples.


New style: flow map arrows

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

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:

migration_basic

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:

migration_twocolor

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:

resourcesharing_flowmap


Details of good flow maps

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  and Thomas Gratier :

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:

  1. use arrow heads and scale arrow width according to flow,
  2. enlarge arrow heads for thin flows, and
  3. use nodes to arrange flows and avoid overlaps of arrow heads and flows
Click to view slideshow.

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

arrow_head_thickness

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.

arrow_nodes

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

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:

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 #4: variations over time

In the previous post, I presented an approach to generalize big trajectory datasets by extracting flows between cells of a data-driven irregular grid. This generalization provides a much better overview of the flow and directionality than a simple plot of the original raw trajectory data can. The paper introducing this method also contains more advanced visualizations that show cell statistics, such as the overall count of trajectories or the generalization quality. Another bit of information that is often of interest when exploring movement data, is the time of the movement. For example, at LBS2016 last week, M. Jahnke presented an application that allows users to explore the number of taxi pickups and dropoffs at certain locations:

By adopting this approach for the generalized flow maps, we can, for example, explore which parts of the research area are busy at which time of the day. Here I have divided the day into four quarters: night from 0 to 6 (light blue), morning from 6 to 12 (orange), afternoon from 12 to 18 (red), and evening from 18 to 24 (dark blue).

 (data credits: GeoLife project,

Aggregated trajectories with time-of-day markers at flow network nodes (data credits: GeoLife project, map tiles: Carto, map data: OSM)

The resulting visualization shows that overall, there is less movement during the night hours from midnight to 6 in the morning (light blue quarter). Sounds reasonable!

One implementation detail worth considering is which timestamp should be used for counting the number of movements. Should it be the time of the first trajectory point entering a cell, or the time when the trajectory leaves the cell, or some average value? In the current implementation, I have opted for the entry time. This means that if the tracked person spends a long time within a cell (e.g. at the work location) the trip home only adds to the evening trip count of the neighboring cell along the trajectory.

Since the time information stored in a PostGIS LinestringM feature’s m-value does not contain any time zone information, we also have to pay attention to handle any necessary offsets. For example, the GeoLife documentation states that all timestamps are provided in GMT while Beijing is in the GMT+8 time zone. This offset has to be accounted for in the analysis script, otherwise the counts per time of day will be all over the place.

Using the same approach, we could also investigate other variations, e.g. over different days of the week, seasonal variations, or the development over multiple years.


Movement data in GIS #3: visualizing massive trajectory datasets

In the fist two parts of the Movement Data in GIS series, I discussed modeling trajectories as LinestringM features in PostGIS to overcome some common issues of movement data in GIS and presented a way to efficiently render speed changes along a trajectory in QGIS without having to split the trajectory into shorter segments.

While visualizing individual trajectories is important, the real challenge is trying to visualize massive trajectory datasets in a way that enables further analysis. The out-of-the-box functionality of GIS is painfully limited. Except for some transparency and heatmap approaches, there is not much that can be done to help interpret “hairballs” of trajectories. Luckily researchers in visual analytics have already put considerable effort into finding solutions for this visualization challenge. The approach I want to talk about today is by Andrienko, N., & Andrienko, G. (2011). Spatial generalization and aggregation of massive movement data. IEEE Transactions on visualization and computer graphics, 17(2), 205-219. and consists of the following main steps:

  1. Extracting characteristic points from the trajectories
  2. Grouping the extracted points by spatial proximity
  3. Computing group centroids and corresponding Voronoi cells
  4. Deviding trajectories into segments according to the Voronoi cells
  5. Counting transitions from one cell to another

The authors do a great job at describing the concepts and algorithms, which made it relatively straightforward to implement them in QGIS Processing. So far, I’ve implemented the basic logic but the paper contains further suggestions for improvements. This was also my first pyQGIS project that makes use of the measurement value support in the new geometry engine. The time information stored in the m-values is used to detect stop points, which – together with start, end, and turning points – make up the characteristic points of a trajectory.

The following animation illustrates the current state of the implementation: First the “hairball” of trajectories is rendered. Then we extract the characteristic points and group them by proximity. The big black dots are the resulting group centroids. From there, I skipped the Voronoi cells and directly counted transitions from “nearest to centroid A” to “nearest to centroid B”.

(data credits: GeoLife project)

From thousands of individual trajectories to a generalized representation of overall movement patterns (data credits: GeoLife project, map tiles: Stamen, map data: OSM)

The resulting visualization makes it possible to analyze flow strength as well as directionality. I have deliberately excluded all connections with a count below 10 transitions to reduce visual clutter. The cell size / distance between point groups – and therefore the level-of-detail – is one of the input parameters. In my example, I used a target cell size of approximately 2km. This setting results in connections which follow the major roads outside the city center very well. In the city center, where the road grid is tighter, trajectories on different roads mix and the connections are less clear.

Since trajectories in this dataset are not limited to car trips, it is expected to find additional movement that is not restricted to the road network. This is particularly noticeable in the dense area in the west where many slow trajectories – most likely from walking trips – are located. The paper also covers how to ensure that connections are limited to neighboring cells by densifying the trajectories before computing step 4.

trajectory_generalization

Running the scripts for over 18,000 trajectories requires patience. It would be worth evaluating if the first three steps can be run with only a subsample of the data without impacting the results in a negative way.

One thing I’m not satisfied with yet is the way to specify the target cell size. While it’s possible to measure ellipsoidal distances in meters using QgsDistanceArea (irrespective of the trajectory layer’s CRS), the initial regular grid used in step 2 in order to group the extracted points has to be specified in the trajectory layer’s CRS units – quite likely degrees. Instead, it may be best to transform everything into an equidistant projection before running any calculations.

It’s good to see that PyQGIS enables us to use the information encoded in PostGIS LinestringM features to perform spatio-temporal analysis. However, working with m or z values involves a lot of v2 geometry classes which work slightly differently than their v1 counterparts. It certainly takes some getting used to. This situation might get cleaned up as part of the QGIS 3 API refactoring effort. If you can, please support work on QGIS 3. Now is the time to shape the PyQGIS API for the following years!


More icons & symbols for QGIS

The possibility to easily share plugins with other users and discover plugins written by other community members has been a powerful feature of QGIS for many years.

The QGIS Resources Sharing plugin is meant to enable the same sharing for map design resources. It allows you to share collections of resources, including but not limited to SVGs, symbols, styles, color ramps, and processing scripts.

Using the Resource Sharing plugin is like using the Plugin Manager. Once installed, you are presented with a list of available resource collections for download. You will find that there are already some really nice collections, including nautical symbols, Mapbox Maki Icons, and my Google-like OSM road style.

resourcesharing_maki

By pressing Install, the resource collection is downloaded and you can have a look at the content using the Open folder button. In case of the Mapbox Maki Icon collection, it contains a folder of SVGs:

resourcesharing_folder

Using the new icons is as simple as opening the layer styling settings and selecting the Mapbox Maki Icons collection in the SVG group list:

resourcesharing_styling

Similarly, if you download the OSM Spatialite Googlemaps collection, its road line symbols are added to your existing list of available line symbols:

resourcesharing_roads2

By pressing the Open Library button, you get to the Style Manager where you can browse through all installed symbols and delete, rename, or categorize them.

resourcesharing_roads

The Resource Sharing plugin was developed by Akbar Gumbira during this year’s Google Summer of Code. The full documentation, including instructions for how to share your own symbols with the community, is available at www.akbargumbira.com/qgis_resources_sharing.


QGIS Atlas Tutorial – Material Design

This is a guest post by Mickael HOARAU @Oneil974

For people who are working on QGIS Atlas feature, I worked on an Atlas version of the last tutorial I have made. The difficulty level is a little bit more consequente then last tutorial but there are features that you could appreciate. So I’m happy to share with you and I hope this would be helpful.

Click to view slideshow.

You can download tutorial here:

Material Design – QGIS Atlas Tutorial

And sources here:

https://drive.google.com/file/d/0B37RnaYSMWAZUUJ2NUxhZC1TNmM/view?usp=sharing

 

PS : I’m looking for job offers, feel free to contact me on twitter @Oneil974


Movement data in GIS #2: visualization

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:

trajectory_segment_features

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:

geomgenerator

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:

segment_speed_color

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($geometry,@geometry_part_num),
		'EPSG:4326','EPSG:54027'
		)
    ) / (
      m(end_point(  geometry_n($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:

trajectory_geomgenerator

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.


How to visualize bird migration data with QGIS TimeManager

A common use case of the QGIS TimeManager plugin is visualizing tracking data such as animal migration data. This post illustrates the steps necessary to create an animation from bird migration data. I’m using a dataset published on Movebank:

Fraser KC, Shave A, Savage A, Ritchie A, Bell K, Siegrist J, Ray JD, Applegate K, Pearman M (2016) Data from: Determining fine-scale migratory connectivity and habitat selection for a migratory songbird by using new GPS technology. Movebank Data Repository. doi:10.5441/001/1.5q5gn84d.

It’s a CSV file which can be loaded into QGIS using the Add delimited text layer tool. Once loaded, we can get started:

1. Identify time and ID columns

Especially if you are new to the dataset, have a look at the attribute table and identify the attributes containing timestamps and ID of the moving object. In our sample dataset, time is stored in the aptly named timestamp attribute and uses ISO standard formatting %Y-%m-%d %H:%M:%S.%f. This format is ideal for TimeManager and we can use it without any changes. The object ID attribute is titled individual-local-identifier.

movebank_data

The dataset contains 128 positions of 14 different birds. This means that there are rather long gaps between consecutive observations. In our animation, we’ll want to fill these gaps with interpolated positions to get uninterrupted movement traces.

2. Configuring TimeManager

To set up the animation, go to the TimeManager panel and click Settings | Add Layer. In the following dialog we can specify the time and ID attributes which we identified in the previous step. We also enable linear interpolation. The interpolation option will create an additional point layer in the QGIS project, which contains the interpolated positions.

timemanager_settings

When using the interpolation option, please note that it currently only works if the point layer is styled with a Single symbol renderer. If a different renderer is configured, it will fail to create the interpolation layer.

Once the layer is configured, the minimum and maximum timestamps will be displayed in the TimeManager dock right bellow the time slider. For this dataset, it makes sense to set the Time frame size, that is the time between animation frames, to one day, so we will see one frame per day:

timemanager_dock

Now you can test the animation by pressing the TimeManager’s play button. Feel free to add more data, such as background maps or other layers, to your project. Besides exploring the animated data in QGIS, you can also create a video to share your results.

3. Creating a video

To export the animation, click the Export video button. If you are using Linux, you can export videos directly from QGIS. On Windows, you first need to export the animation frames as individual pictures, which you can then convert to a video (for example using the free Windows Movie Maker application).

These are the basic steps to set up an animation for migration data. There are many potential extensions to this animation, including adding permanent traces of past movements. While this approach serves us well for visualizing bird migration routes, it is easy to imagine that other movement data would require different interpolation approaches. Vehicle data, for example, would profit from network-constrained interpolation between observed positions.

If you find the TimeManager plugin useful, please consider supporting its development or getting involved. Many features, such as interpolation, are weekend projects that are still in a proof-of-concept stage. In addition, we have the huge upcoming challenge of migrating the plugin to Python 3 and Qt5 to support QGIS3 ahead of us. Happy QGISing!


How to fix a broken Processing model with AttributeError: ‘NoneType’ object has no attribute ‘getCopy’

Broken Processing models are nasty and this error is particularly unpleasant:

...
File "/home/agraser/.qgis2/python/plugins/processing/modeler/
ModelerAlgorithm.py", line 110, in algorithm
self._algInstance = ModelerUtils.getAlgorithm(self.consoleName).getCopy()
AttributeError: 'NoneType' object has no attribute 'getCopy'

It shows up if you are trying to open a model in the model editor that contains an algorithm which Processing cannot find.

For example, when I upgraded to Ubuntu 16.04, installing a fresh QGIS version did not automatically install SAGA. Therefore, any model with a dependency on SAGA was broken with the above error message. Installing SAGA and restarting QGIS solves the issue.


Movement data in GIS: issues & ideas

Since I’ve started working, transport and movement data have been at the core of many of my projects. The spatial nature of movement data makes it interesting for GIScience but typical GIS tools are not a particularly good match.

Dealing with the temporal dynamics of geographic processes is one of the grand challenges for Geographic Information Science. Geographic Information Systems (GIS) and related spatial analysis methods are quite adept at handling spatial dimensions of patterns and processes, but the temporal and coupled space-time attributes of phenomena are difficult to represent and examine with contemporary GIS. (Dr. Paul M. Torrens, Center for Urban Science + Progress, New York University)

It’s still a hot topic right now, as the variety of related publications and events illustrates. For example, just this month, there is an Animove two-week professional training course (18–30 September 2016, Max-Planck Institute for Ornithology, Lake Konstanz) as well as the GIScience 2016 Workshop on Analysis of Movement Data (27 September 2016, Montreal, Canada).

Space-time cubes and animations are classics when it comes to visualizing movement data in GIS. They can be used for some visual analysis but have their limitations, particularly when it comes to working with and trying to understand lots of data. Visualization and analysis of spatio-temporal data in GIS is further complicated by the fact that the temporal information is not standardized in most GIS data formats. (Some notable exceptions of formats that do support time by design are GPX and NetCDF but those aren’t really first-class citizens in current desktop GIS.)

Most commonly, movement data is modeled as points (x,y, and optionally z) with a timestamp, object or tracker id, and potential additional info, such as speed, status, heading, and so on. With this data model, even simple questions like “Find all tracks that start in area A and end in area B” can become a real pain in “vanilla” desktop GIS. Even if the points come with a sequence number, which makes it easy to identify the start point, getting the end point is tricky without some custom code or queries. That’s why I have been storing the points in databases in order to at least have the powers of SQL to deal with the data. Even so, most queries were still painfully complex and performance unsatisfactory.

So I reached out to the Twitterverse asking for pointers towards moving objects database extensions for PostGIS and @bitnerd, @pwramsey, @hruske, and others replied. Amongst other useful tips, they pointed me towards the new temporal support, which ships with PostGIS 2.2. It includes the following neat functions:

  • ST_IsValidTrajectory — Returns true if the geometry is a valid trajectory.
  • ST_ClosestPointOfApproach — Returns the measure at which points interpolated along two lines are closest.
  • ST_DistanceCPA — Returns the distance between closest points of approach in two trajectories.
  • ST_CPAWithin — Returns true if the trajectories’ closest points of approach are within the specified distance.

Instead of  points, these functions expect trajectories that are stored as LinestringM (or LinestringZM) where M is the time dimension. This approach makes many analyses considerably easier to handle. For example, clustering trajectory start and end locations and identifying the most common connections:

animation_clusters

(data credits: GeoLife project)

Overall, it’s an interesting and promising approach but there are still some open questions I’ll have to look into, such as: Is there an efficient way to store additional info for each location along the trajectory (e.g. instantaneous speed or other status)? How well do desktop GIS play with LinestringM data and what’s the overhead of dealing with it?


How to use Print Composer templates

In the previous post, Mickael shared a great map design. The download includes a print composer template, that you can use to recreate the design in a few simple steps:

1. Create a new composition based on a template

Open the Composer manager and configure it to use a specific template. Then you can select the .qpt template file and press the Add button to create a new composition based on the template.

2. Update image item paths

If the template uses images, the paths to the images most likely need to be fixed since the .qpt file stores absolute file paths instead of relative ones.

update_image_paths

With these steps, you’re now ready to use the design for your own maps. Happy QGISing!


Material design map tutorial for QGIS Composer

This is a guest post by Mickael HOARAU @Oneil974

For those wishing to get a stylized map on QGIS composer, I’ve been working on a tutorial to share with you a project I’m working on. Fan of web design and GIS user since few years, I wanted to merge Material Design Style with Map composer. Here is a tutorial to show you how to make simply a Material Design Map style on QGIS.

Click to view slideshow.

You can download tutorial here:

Tutorial Material Design Map

And sources here:

Sources Material Design Map

An Atlas Powered version is coming soon!


Reporting back from Bonn & Oslo

Over the last two weeks, I had the pleasure to attend both the international FOSS4G conference in Bonn, Germany, as well as the regional FOSS4G-NOR in Oslo, Norway. Both events were superbly organized and provided tons of possibilities to share experiences and find new inspiration.

Talks at both conferences have been recorded and can be watched online: Bonn / Oslo

I enjoyed having the opportunity to give two very different talks. In Bonn, I presented work on pedestrian routing and navigation, which was developed within the PERRON project:

It was particularly nice that we had plenty of time for Q&A after this presentation since only two talks were scheduled for this session rather than the usual three. I’d also like to thank everyone for the great feedback – both in person and on Twitter!

In Oslo, I had the honor to give the opening keynote on OpenSource in general and the QGIS project in particular:

2 – Anita Graser – QGIS – A Community-powered GIS Project from krokskogstrollet on Vimeo.

Both conferences were packed with great sessions and talks. If I had to pick favorites from last week’s presentations, I would have to opt for Iván Sánchez presenting his latest projects, including what3fucks and geohaiku:

6 – Iván Sánchez Ortega, Mazemap – Addressing NSFW Geodesic Grids from krokskogstrollet on Vimeo.

Followed closely by the impressive project presentations of the student organizers of FOSS4G-NOR:

10 – Program Committee – What are the results when students use Open Source? from krokskogstrollet on Vimeo.

All three projects: OPPTUR, GISTYLE, and the flexible traffic web viewer were great demos of what can be achieved with open source tools. Mathilde’s GISTYLE project is also available on Github.

An inspiring GISummer comes to an end, but with so many videos to watch and workshop materials to explore, I’m convinced that the autumn will be no less exciting.


OSGeo Code Sprint in Bonn

It’s been a great week in Bonn! I joined the other members of the QGIS project at the pre-FOSS4G code sprint at the Basecamp, the weirdest location we’ve had for a developer meeting so far. We used this opportunity to have a face-to-face meeting of the QGIS PSC  with special guests Matthias Kuhn (on QGIS 3.0 and bug tracker discussions) and Lene Fischer (on community team issues)  – notes here.

picture by Tim Sutton

QGIS PSC meeting in action (from left to right: Otto Dassau, Paolo Cavallini, Anita Graser, Andreas Neumann, Jürgen E. Fischer), picture by Tim Sutton

I also finally took the time to compile a blog post on the results of the QGIS user survey 2015.

The code sprint was also a great opportunity to present the results of Akbar Gumbira’s Google Summer of Code project: the QGIS Resource Sharing plugin. This plugin makes it possible to easily share resources (such as SVG icons, symbol definitions, QGIS styles, and Processing scripts) with other QGIS users through an interface that closely resembles the well-known QGIS Plugin Manager. Akbar has also prepared great presentation with background info and screencasts showcasing his project.

QGIS Resource Sharing presentation, picture by @foss4g

QGIS Resource Sharing presentation, picture by @foss4g

The plugin is now available in the Plugin Repository and we have created the official QGIS Resources repository on Github. If you have symbols or styles that you want to share with the community, please create a resource collection and send us a pull request to add it to the official list.

Thanks to all the organizers who worked hard to make this one of the most well-organized and enjoyable code sprints I’ve ever been to. You are awesome!


City flows unfolding with the other Processing

A previous version of this post has been published in German on Die bemerkenswerte Karte.

Visualizations of mobility data such as taxi or bike sharing trips have become very popular. One of the best most recent examples is cf. city flows developed by Till Nagel and Christopher Pietsch at the FH Potsdam. cf. city flows visualizes the rides in bike sharing systems in New York, Berlin and London at different levels of detail, from overviews of the whole city to detailed comparisons of individual stations:

The visualizations were developed using Unfolding, a library to create interactive maps and geovisualizations in Processing (the other Processing … not the QGIS Processing toolbox) and Java. (I tinkered with the Python port of Processing in 2012, but this is certainly on a completely different level.)

The insights into the design process, which are granted in the methodology section section of the project website are particularly interesting. Various approaches for presenting traffic flows between the stations were tested. Building on initial simple maps, where stations were connected by straight lines, consecutive design decisions are described in detail:

The results are impressive. Particularly the animated trips convey the dynamics of urban mobility very well:

However, a weak point of this (and many similar projects) is the underlying data. This is also addressed directly by the project website:

Lacking actual GPS tracks, the trip trajectories are rendered as smooth paths of the calculated optimal bike routes

This means that the actual route between start and drop off location is not known. The authors therefore estimated the routes using HERE’s routing service. The visualization therefore only shows one of many possible routes. However, cyclists don’t necessarily choose the “best” route as determined by an algorithm – be it the most direct or otherwise preferred. The visualization does not account for this uncertainty in the route selection. Rather, it gives the impression that the cyclist actually traveled on a certain route. It would therefore be undue to use this visualization to derive information about the popularity of certain routes (for example, for urban planning). Moreover, the data only contains information about the fulfilled demand, since only trips that were really performed are recorded. Demand for trips which could not take place due to lack of bicycles or stations, is therefore missing.

As always: exercise some caution when interpreting statistics or visualizations and then sit back and enjoy the animations.

If you want to read more about GIS and transportation modelling, check out
Loidl, M.; Wallentin, G.; Cyganski, R.; Graser, A.; Scholz, J.; Haslauer, E. GIS and Transport Modeling—Strengthening the Spatial Perspective. ISPRS Int. J. Geo-Inf. 2016, 5, 84. (It’s open access.)


Special FOSS4G offer: 25% off QGIS Map Design

FOSS4G2016 is drawing closer quickly. To get in the mood for a week full of of geogeekery, Locate Press is offering a special FOSS4G discount for QGIS Map Design.

Use the code foss4gbonn to get 25% off your copy.

QGIS Map Design is the reference book to get if you want to bring your mapping skills up to speed. The book comes with a download for all our example map projects:

qmd114 qmd132 qmd140 qmd128 qmd174 qmd58 qmd152 qmd158 qmd64 qmd46 qmd146 qmd188 qmd164 qmd20 qmd90 qmd78 qmd84 qmd108 qmd184 qmd34 qmd180 qmd120 qmd26 qmd104 qmd168 qmd100 qmd52

Looking forward to meeting you in Bonn!


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