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QGIS Planet

Finding geospatial accounts on Mastodon

Besides following hashtags, such as #GISChat, #QGIS, #OpenStreetMap, #FOSS4G, and #OSGeo, curating good lists is probably the best way to stay up to date with geospatial developments.

To get you started (or to potentially enrich your existing lists), I thought I’d share my Geospatial and SpatialDataScience lists with you. And the best thing: you don’t need to go through all the >150 entries manually! Instead, go to your Mastodon account settings and under “Import and export” you’ll find a tool to import and merge my list.csv with your lists:

And if you are not following the geospatial hashtags yet, you can search or click on the hashtags you’re interested in and start following to get all tagged posts into your timeline:

Offline Vector Tile Package .vtpk in QGIS

Starting from 3.26, QGIS now supports .vtpk (Vector Tile Package) files out of the box! From the changelog:

ESRI vector tile packages (VTPK files) can now be opened directly as vector tile layers via drag and drop, including support for style translation.

This is great news, particularly for users from Austria, since this makes it possible to use the open government basemap.at vector tiles directly, without any fuss:

1. Download the 2GB offline vector basemap from https://www.data.gv.at/katalog/de/dataset/basemap-at-verwaltungsgrundkarte-vektor-offline-osterreich


2. Add the .vtpk as a layer using the Data Source Manager or via drag-and-drop from the file explorer


3. All done and ready, including the basemap styling and labeling — which we can customize as well:

Kudos to https://wien.rocks/@DieterKomendera/111568809248327077 for bringing this new feature to my attention.

PS: And interesting tidbit from the developer of this feature, Nyall Dawson:

Hi ‘Geocomputation with Python’

Today, I want to point out a blog post over at

https://geocompx.org/post/2023/geocompy-bp1/

In this post, Jakub Nowosad introduces our book “Geocomputation with Python”, also known as geocompy. It is an open-source book on geographic data analysis with Python, written by Michael Dorman, Jakub Nowosad, Robin Lovelace, and me with contributions from others. You can find it online at https://py.geocompx.org/

Mapping relationships between Neo4j spatial nodes with GeoPandas

Previously, we mapped neo4j spatial nodes. This time, we want to take it one step further and map relationships.

A prime example, are the relationships between GTFS StopTime and Trip nodes. For example, this is the Cypher query to get all StopTime nodes of Trip 17:

MATCH 
    (t:Trip  {id: "17"})
    <-[:BELONGS_TO]-
    (st:StopTime) 
RETURN st

To get the stop locations, we also need to get the stop nodes:

MATCH 
    (t:Trip {id: "17"})
    <-[:BELONGS_TO]-
    (st:StopTime)
    -[:STOPS_AT]->
    (s:Stop)
RETURN st ,s

Adapting our code from the previous post, we can plot the stops:

from shapely.geometry import Point

QUERY = """MATCH (
    t:Trip {id: "17"})
    <-[:BELONGS_TO]-
    (st:StopTime)
    -[:STOPS_AT]->
    (s:Stop)
RETURN st ,s
ORDER BY st.stopSequence
"""

with driver.session(database="neo4j") as session:
    tx = session.begin_transaction()
    results = tx.run(QUERY)
    df = results.to_df(expand=True)
    gdf = gpd.GeoDataFrame(
        df[['s().prop.name']], crs=4326,
        geometry=df["s().prop.location"].apply(Point)
    )

tx.close() 
m = gdf.explore()
m

Ordering by stop sequence is actually completely optional. Technically, we could use the sorted GeoDataFrame, and aggregate all the points into a linestring to plot the route. But I want to try something different: we’ll use the NEXT_STOP relationships to get a DataFrame of the start and end stops for each segment:

QUERY = """
MATCH (t:Trip {id: "17"})
   <-[:BELONGS_TO]-
   (st1:StopTime)
   -[:NEXT_STOP]->
   (st2:StopTime)
MATCH (st1)-[:STOPS_AT]->(s1:Stop)
MATCH (st2)-[:STOPS_AT]->(s2:Stop)
RETURN st1, st2, s1, s2
"""

from shapely.geometry import Point, LineString

def make_line(row):
    s1 = Point(row["s1().prop.location"])
    s2 = Point(row["s2().prop.location"])
    return LineString([s1,s2])

with driver.session(database="neo4j") as session:
    tx = session.begin_transaction()
    results = tx.run(QUERY)
    df = results.to_df(expand=True)
    gdf = gpd.GeoDataFrame(
        df[['s1().prop.name']], crs=4326,
        geometry=df.apply(make_line, axis=1)
    )

tx.close() 
gdf.explore(m=m)

Finally, we can also use Cypher to calculate the travel time between two stops:

MATCH (t:Trip {id: "17"})
   <-[:BELONGS_TO]-
   (st1:StopTime)
   -[:NEXT_STOP]->
   (st2:StopTime)
MATCH (st1)-[:STOPS_AT]->(s1:Stop)
MATCH (st2)-[:STOPS_AT]->(s2:Stop)
RETURN st1.departureTime AS time1, 
   st2.arrivalTime AS time2, 
   s1.location AS geom1, 
   s2.location AS geom2, 
   duration.inSeconds(
      time(st1.departureTime), 
      time(st2.arrivalTime)
   ).seconds AS traveltime

As always, here’s the notebook: https://github.com/anitagraser/QGIS-resources/blob/master/qgis3/notebooks/neo4j.ipynb

Mapping Neo4j spatial nodes with GeoPandas

In the recent post Setting up a graph db using GTFS data & Neo4J, we noted that — unfortunately — Neomap is not an option to visualize spatial nodes anymore.

GeoPandas to the rescue!

But first we need the neo4j Python driver:

pip install neo4j

Then we can connect to our database. The default user name is neo4j and you get to pick the password when creating the database:

from neo4j import GraphDatabase

URI = "neo4j://localhost"
AUTH = ("neo4j", "password")

with GraphDatabase.driver(URI, auth=AUTH) as driver:
    driver.verify_connectivity()

Once we have confirmed that the connection works as expected, we can run a query:

QUERY = "MATCH (p:Stop) RETURN p.name AS name, p.location AS geom"

records, summary, keys = driver.execute_query(
    QUERY, database_="neo4j",
)

for rec in records:
    print(rec)

Nice. There we have our GTFS stops, their names and their locations. But how to put them on a map?

Conveniently, there is a to_db() function in the Neo4j driver:

import geopandas as gpd
import numpy as np

with driver.session(database="neo4j") as session:
    tx = session.begin_transaction()
    results = tx.run(QUERY)
    df = results.to_df(expand=True)
    df = df[df["geom[].0"]>0]
    gdf = gpd.GeoDataFrame(
        df['name'], crs=4326,
        geometry=gpd.points_from_xy(df['geom[].0'], df['geom[].1']))
    print(gdf)

tx.close() 

Since some of the nodes lack geometries, I added a quick and dirty hack to get rid of these nodes because — otherwise — gdf.explore() will complain about None geometries.

You can find this notebook at: https://github.com/anitagraser/QGIS-resources/blob/1e4ea435c9b1795ba5b170ddb176aa83689112eb/qgis3/notebooks/neo4j.ipynb

Next step will have to be the relationships. Stay posted.

Setting up a graph db using GTFS data & Neo4J

In a recent post, we looked into a graph-based model for maritime mobility data and how it may be represented in Neo4J. Today, I want to look into another type of mobility data: public transport schedules in GTFS format.

In this post, I’ll be using the public GTFS data for Riga since Riga is one of the demo sites for our current EMERALDS research project.

The workflow is heavily inspired by Bert Radke‘s post “Loading the UK GTFS data feed” from 2021 and his import Cypher script which I used as a template, adjusted to the requirements of the Riga dataset, and updated to recent Neo4J changes.

Here we go.

Since a GTFS export is basically a ZIP archive full of CSVs, we will be making good use of Neo4Js CSV loading capabilities. The basic script for importing the stops file and creating point geometries from lat and lon values would be:

LOAD CSV with headers 
FROM "file:///stops.txt" 
AS row 
CREATE (:Stop {
   stop_id: row["stop_id"],
   name: row["stop_name"], 
   location: point({
    longitude: toFloat(row["stop_lon"]),
    latitude: toFloat(row["stop_lat"])
    })
})

This requires that the stops.txt is located in the import directory of your Neo4J database. When we run the above script and the file is missing, Neo4J will tell us where it tried to look for it. In my case, the directory ended up being:

C:\Users\Anita\.Neo4jDesktop\relate-data\dbmss\dbms-72882d24-bf91-4031-84e9-abd24624b760\import

So, let’s put all GTFS CSVs into that directory and we should be good to go.

Let’s start with the agency file:

load csv with headers from
'file:///agency.txt' as row
create (a:Agency {
   id: row.agency_id, 
   name: row.agency_name, 
   url: row.agency_url, 
   timezone: row.agency_timezone, 
   lang: row.agency_lang
});

… Added 1 label, created 1 node, set 5 properties, completed after 31 ms.

The routes file does not include agency info but, luckily, there is only one agency, so we can hard-code it:

load csv with headers from
'file:///routes.txt' as row
match (a:Agency {id: "rigassatiksme"})
create (a)-[:OPERATES]->(r:Route {
   id: row.route_id, 
   shortName: row.route_short_name,
   longName: row.route_long_name, 
   type: toInteger(row.route_type)
});

… Added 81 labels, created 81 nodes, set 324 properties, created 81 relationships, completed after 28 ms.

From stops, I’m removing non-existent or empty columns:

load csv with headers from
'file:///stops.txt' as row
create (s:Stop {
   id: row.stop_id, 
   name: row.stop_name, 
   location: point({
      latitude: toFloat(row.stop_lat), 
      longitude: toFloat(row.stop_lon)
   }),
   code: row.stop_code
});

… Added 1671 labels, created 1671 nodes, set 5013 properties, completed after 71 ms.

From trips, I’m also removing non-existent or empty columns:

load csv with headers from
'file:///trips.txt' as row
match (r:Route {id: row.route_id})
create (r)<-[:USES]-(t:Trip {
   id: row.trip_id, 
   serviceId: row.service_id,
   headSign: row.trip_headsign, 
   direction_id: toInteger(row.direction_id),
   blockId: row.block_id,
   shapeId: row.shape_id
});

… Added 14427 labels, created 14427 nodes, set 86562 properties, created 14427 relationships, completed after 875 ms.

Slowly getting there. We now have around 16k nodes in our graph:

Finally, it’s stop times time. This is where the serious information is. This file is much larger than all previous ones with over 300k lines (i.e. times when an PT vehicle stops).

This requires another tweak to Bert’s script since using periodic commit is not supported anymore: The PERIODIC COMMIT query hint is no longer supported. Please use CALL { … } IN TRANSACTIONS instead. So I ended up using the following, based on https://community.neo4j.com/t/best-practice-for-replacement-of-using-periodic-commit-to-call-in-transactions/48636/2:

:auto
load csv with headers from
'file:///stop_times.txt' as row
CALL { with row
match (t:Trip {id: row.trip_id}), (s:Stop {id: row.stop_id})
create (t)<-[:BELONGS_TO]-(st:StopTime {
   arrivalTime: row.arrival_time, 
   departureTime: row.departure_time,
   stopSequence: toInteger(row.stop_sequence)})-[:STOPS_AT]->(s)
} IN TRANSACTIONS OF 10 ROWS;

… Added 351388 labels, created 351388 nodes, set 1054164 properties, created 702776 relationships, completed after 1364220 ms.

As you can see, this took a while. But now we have all nodes in place:

The final statement adds additional relationships between consecutive stop times:

call apoc.periodic.iterate('match (t:Trip) return t',
'match (t)<-[:BELONGS_TO]-(st) with st order by st.stopSequence asc
with collect(st) as stops
unwind range(0, size(stops)-2) as i
with stops[i] as curr, stops[i+1] as next
merge (curr)-[:NEXT_STOP]->(next)', {batchmode: "BATCH", parallel:true, parallel:true, batchSize:1});

This fails with: There is no procedure with the name apoc.periodic.iterate registered for this database instance. Please ensure you've spelled the procedure name correctly and that the procedure is properly deployed.

So, let’s install APOC. That’s a plugin which we can install into our database from within Neo4J Desktop:

After restarting the db, we can run the query:

No errors. Sounds good.

Let’s have a look at what we ended up with. Here are 25 random Trips. I expanded one of them to show its associated StopTimes. We can see the relations between consecutive StopTimes and I’ve expanded the final five StopTimes to show their linked Stops:

I also wanted to visualize the stops on a map. And there used to be a neat app called Neomap which can be installed easily:

However, Neomap does not seem to be compatible with the latest Neo4J:

So this final step will have to wait for another time.

Bringing QGIS maps into Jupyter notebooks

Earlier this year, we explored how to use PyQGIS in Juypter notebooks to run QGIS Processing tools from a notebook and visualize the Processing results using GeoPandas plots.

Today, we’ll go a step further and replace the GeoPandas plots with maps rendered by QGIS.

The following script presents a minimum solution to this challenge: initializing a QGIS application, canvas, and project; then loading a GeoJSON and displaying it:

from IPython.display import Image

from PyQt5.QtGui import QColor
from PyQt5.QtWidgets import QApplication

from qgis.core import QgsApplication, QgsVectorLayer, QgsProject, QgsSymbol, \
    QgsRendererRange, QgsGraduatedSymbolRenderer, \
    QgsArrowSymbolLayer, QgsLineSymbol, QgsSingleSymbolRenderer, \
    QgsSymbolLayer, QgsProperty
from qgis.gui import QgsMapCanvas

app = QApplication([])
qgs = QgsApplication([], False)
canvas = QgsMapCanvas()
project = QgsProject.instance()

vlayer = QgsVectorLayer("./data/traj.geojson", "My trajectory")
if not vlayer.isValid():
    print("Layer failed to load!")

def saveImage(path, show=True): 
    canvas.saveAsImage(path)
    if show: return Image(path)

project.addMapLayer(vlayer)
canvas.setExtent(vlayer.extent())
canvas.setLayers([vlayer])
canvas.show()
app.exec_()

saveImage("my-traj.png")

When this code is executed, it opens a separate window that displays the map canvas. And in this window, we can even pan and zoom to adjust the map. The line color, however, is assigned randomly (like when we open a new layer in QGIS):

To specify a specific color, we can use:

vlayer.renderer().symbol().setColor(QColor("red"))

vlayer.triggerRepaint()
canvas.show()
app.exec_()
saveImage("my-traj.png")

But regular lines are boring. We could easily create those with GeoPandas plots.

Things get way more interesting when we use QGIS’ custom symbols and renderers. For example, to draw arrows using a QgsArrowSymbolLayer, we can write:

vlayer.renderer().symbol().appendSymbolLayer(QgsArrowSymbolLayer())

vlayer.triggerRepaint()
canvas.show()
app.exec_()
saveImage("my-traj.png")

We can also create a QgsGraduatedSymbolRenderer:

geom_type = vlayer.geometryType()
myRangeList = []

symbol = QgsSymbol.defaultSymbol(geom_type)
symbol.setColor(QColor("#3333ff"))
myRange = QgsRendererRange(0, 1, symbol, 'Group 1')
myRangeList.append(myRange)

symbol = QgsSymbol.defaultSymbol(geom_type)
symbol.setColor(QColor("#33ff33"))
myRange = QgsRendererRange(1, 3, symbol, 'Group 2')
myRangeList.append(myRange)

myRenderer = QgsGraduatedSymbolRenderer('speed', myRangeList)
vlayer.setRenderer(myRenderer)

vlayer.triggerRepaint()
canvas.show()
app.exec_()
saveImage("my-traj.png")

And we can combine both QgsGraduatedSymbolRenderer and QgsArrowSymbolLayer:

geom_type = vlayer.geometryType()
myRangeList = []

symbol = QgsSymbol.defaultSymbol(geom_type)
symbol.appendSymbolLayer(QgsArrowSymbolLayer())
symbol.setColor(QColor("#3333ff"))
myRange = QgsRendererRange(0, 1, symbol, 'Group 1')
myRangeList.append(myRange)

symbol = QgsSymbol.defaultSymbol(geom_type)
symbol.appendSymbolLayer(QgsArrowSymbolLayer())
symbol.setColor(QColor("#33ff33"))
myRange = QgsRendererRange(1, 3, symbol, 'Group 2')
myRangeList.append(myRange)

myRenderer = QgsGraduatedSymbolRenderer('speed', myRangeList)
vlayer.setRenderer(myRenderer)

vlayer.triggerRepaint()
canvas.show()
app.exec_()
saveImage("my-traj.png")

Maybe the most powerful option is to use data-defined symbology. For example, to control line width and color:

renderer = QgsSingleSymbolRenderer(QgsSymbol.defaultSymbol(geom_type))

exp_width = 'scale_linear("speed", 0, 3, 0, 7)'
exp_color = "coalesce(ramp_color('Viridis',scale_linear(\"speed\", 0, 3, 0, 1)), '#000000')"

# https://qgis.org/pyqgis/3.0/core/Symbol/QgsSymbolLayer.html?highlight=property#qgis.core.QgsSymbolLayer.PropertySize
renderer.symbol().symbolLayer(0).setDataDefinedProperty(
    QgsSymbolLayer.PropertyStrokeWidth, QgsProperty.fromExpression(exp_width))
renderer.symbol().symbolLayer(0).setDataDefinedProperty(
    QgsSymbolLayer.PropertyStrokeColor, QgsProperty.fromExpression(exp_color))
renderer.symbol().symbolLayer(0).setDataDefinedProperty(
    QgsSymbolLayer.PropertyCapStyle, QgsProperty.fromExpression("'round'"))

vlayer.setRenderer(renderer)

vlayer.triggerRepaint()
canvas.show()
app.exec_()
saveImage("my-traj.png")

Find the full notebook at: https://github.com/anitagraser/QGIS-resources/blob/master/qgis3/notebooks/layer-styling.ipynb

Exploring a hierarchical graph-based model for mobility data representation and analysis

Today’s post is a first quick dive into Neo4J (really just getting my toes wet). It’s based on a publicly available Neo4J dump containing mobility data, ship trajectories to be specific. You can find this data and the setup instructions at:

Maryam Maslek ELayam, Cyril Ray, & Christophe Claramunt. (2022). A hierarchical graph-based model for mobility data representation and analysis [Data set]. Zenodo. https://doi.org/10.5281/zenodo.6405212

I was made aware of this work since they cited MovingPandas in their paper in Data & Knowledge Engineering“The implementation combines several open source tools such as Python, MovingPandas library, Uber H3 index, Neo4j graph database management system”

Once set up, this gives us a database with three hierarchical levels:

Neo4j comes with a nice graphical browser that lets us explore the data. We can switch between levels and click on individual node labels to get a quick preview:

Level 2 is a generalization / aggregation of level 1. Expanding the graph of one of the level 2 nodes shows its connection to level 1. For example, the level 2 port node “Audierne” actually refers to two level 1 nodes:

Every “road” level 1 relationship between ports provide information about the ship, its arrival, departure, travel time, and speed. We can see that this two level 1 ports must be pretty close since travel times are only 5 minutes:

Further expanding one of the port level 1 nodes shows its connection to waypoints of level1:

Switching to level 2, we gain access to nodes of type Traj(ectory). Additionally, the road level 2 relationships represent aggregations of the trajectories, for example, here’s a relationship with only one associated trajectory:

There are also some odd relationships, for example, trajectory 43 has two ends and begins relationships and there are also two road relationships referencing this trajectory (with identical information, only differing in their automatic <id>). I’m not yet sure if that is a feature or a bug:

On level 1, we also have access to ship nodes. They are connected to ports and waypoints. However, exploring them visually is challenging. Things look fine at first:

But after a while, once all relationships have loaded, we have it: the MIGHTY BALL OF YARN ™:

I guess this is the point where it becomes necessary to get accustomed to the query language. And no, it’s not SQL, it is Cypher. For example, selecting a specific trajectory with id 0, looks like this:

 MATCH (t1 {traj_id: 0}) RETURN t1

But more on this another time.


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

Comparing geographic data analysis in R and Python

Today, I want to point out a blog post over at

https://geocompx.org/post/2023/ogh23/

written together with my fellow “Geocomputation with Python” co-authors Robin Lovelace, Michael Dorman, and Jakub Nowosad.

In this blog post, we talk about our experience teaching R and Python for geocomputation. The context of this blog post is the OpenGeoHub Summer School 2023 which has courses on R, Python and Julia. The focus of the blog post is on geographic vector data, meaning points, lines, polygons (and their ‘multi’ variants) and the attributes associated with them. We plan to cover raster data in a future post.

I’ve archived my Tweets: Goodbye Twitter, Hello Mastodon

Today, Jeff Sikes @[email protected], alerted me to the fact that “Twitter has removed all media attachments from 2014 and prior” (source: https://firefish.social/notes/9imgvtckzqffboxt). So far, it seems unclear whether this was intentional or a system failure (source: https://mas.to/@carnage4life/110922114407553901).

Since I’ve been on Twitter since 2011, this means that some media files are now lost. While the loss of a few low-res images is probably not a major loss for humanity, I would prefer to have some control over when and how content I created vanishes. So, to avoid losing more content, I have followed Jeff’s recommendation to create a proper archival page:

https://anitagraser.github.io/twitter-archive/

It is based on an export I pulled in October 2022 when I started to use Mastodon as my primary social media account. Unfortunately, this export did not include media files.

To follow me in the future, find me on:

https://fosstodon.org/@underdarkGIS

Btw, a recent study published on Nature News shows that Mastodon is the top-ranking Twitter replacement for scientists.

To find other interesting people on Mastodon, there are many useful tools and lists, including, for example:


Update 2023-11-04: I’ve completely deleted my X / Twitter account. If you find any account pretending they are me on that platform, it’s an impostor.

How to use Kaggle’s Taxi Trajectory Data in MovingPandas

Kaggle’s “Taxi Trajectory Data from ECML/PKDD 15: Taxi Trip Time Prediction (II) Competition” is one of the most used mobility / vehicle trajectory datasets in computer science. However, in contrast to other similar datasets, Kaggle’s taxi trajectories are provided in a format that is not readily usable in MovingPandas since the spatiotemporal information is provided as:

  • TIMESTAMP: (integer) Unix Timestamp (in seconds). It identifies the trip’s start;
  • POLYLINE: (String): It contains a list of GPS coordinates (i.e. WGS84 format) mapped as a string. The beginning and the end of the string are identified with brackets (i.e. [ and ], respectively). Each pair of coordinates is also identified by the same brackets as [LONGITUDE, LATITUDE]. This list contains one pair of coordinates for each 15 seconds of trip. The last list item corresponds to the trip’s destination while the first one represents its start;

Therefore, we need to create a DataFrame with one point + timestamp per row before we can use MovingPandas to create Trajectories and analyze them.

But first things first. Let’s download the dataset:

import datetime
import pandas as pd
import geopandas as gpd
import movingpandas as mpd
import opendatasets as od
from os.path import exists
from shapely.geometry import Point

input_file_path = 'taxi-trajectory/train.csv'

def get_porto_taxi_from_kaggle():
    if not exists(input_file_path):
        od.download("https://www.kaggle.com/datasets/crailtap/taxi-trajectory")

get_porto_taxi_from_kaggle()
df = pd.read_csv(input_file_path, nrows=10, usecols=['TRIP_ID', 'TAXI_ID', 'TIMESTAMP', 'MISSING_DATA', 'POLYLINE'])
df.POLYLINE = df.POLYLINE.apply(eval)  # string to list
df

And now for the remodelling:

def unixtime_to_datetime(unix_time):
    return datetime.datetime.fromtimestamp(unix_time)

def compute_datetime(row):
    unix_time = row['TIMESTAMP']
    offset = row['running_number'] * datetime.timedelta(seconds=15)
    return unixtime_to_datetime(unix_time) + offset

def create_point(xy):
    try: 
        return Point(xy)
    except TypeError:  # when there are nan values in the input data
        return None
 
new_df = df.explode('POLYLINE')
new_df['geometry'] = new_df['POLYLINE'].apply(create_point)
new_df['running_number'] = new_df.groupby('TRIP_ID').cumcount()
new_df['datetime'] = new_df.apply(compute_datetime, axis=1)
new_df.drop(columns=['POLYLINE', 'TIMESTAMP', 'running_number'], inplace=True)
new_df

And that’s it. Now we can create the trajectories:

trajs = mpd.TrajectoryCollection(
    gpd.GeoDataFrame(new_df, crs=4326), 
    traj_id_col='TRIP_ID', obj_id_col='TAXI_ID', t='datetime')
trajs.hvplot(title='Kaggle Taxi Trajectory Data', tiles='CartoLight')

That’s it. Now our MovingPandas.TrajectoryCollection is ready for further analysis.

By the way, the plot above illustrates a new feature in the recent MovingPandas 0.16 release which, among other features, introduced plots with arrow markers that show the movement direction. Other new features include a completely new custom distance, speed, and acceleration unit support. This means that, for example, instead of always getting speed in meters per second, you can now specify your desired output units, including km/h, mph, or nm/h (knots).


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

Deep learning from trajectory data

I’ve previously written about Movement data in GIS and the AI hype and today’s post is a follow-up in which I want to share with you a new review of the state of the art in deep learning from trajectory data.

Our review covers 8 use cases:

  1. Location classification
  2. Arrival time prediction
  3. Traffic flow / activity prediction
  4. Trajectory prediction
  5. Trajectory classification
  6. Next location prediction
  7. Anomaly detection
  8. Synthetic data generation

We particularly looked into the trajectory data preprocessing steps and the specific movement data representation used as input to train the neutral networks:

On a completely subjective note: the price for most surprising approach goes to natural language processing (NLP) Transfomers for traffic volume prediction.

The paper was presented at BMDA2023 and you can watch the full talk recording here:

References

Graser, A., Jalali, A., Lampert, J., Weißenfeld, A., & Janowicz, K. (2023). Deep Learning From Trajectory Data: a Review of Neural Networks and the Trajectory Data Representations to Train Them. Workshop on Big Mobility Data Analysis BMDA2023 in conjunction with EDBT/ICDT 2023.


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

Tracking geoprocessing workflows with QGIS & DVC

Today’s post is a geeky deep dive into how to leverage DVC (not just) data version control to track QGIS geoprocessing workflows.

“Why is this great?” you may ask.

DVC tracks data, parameters, and code. If anything changes, we simply rerun the process and DVC will figure out which stages need to be recomputed and which can be skipped by re-using cached results.

This can lead to huge time savings compared to re-running the whole model

You can find the source code used in this post on my repo https://github.com/anitagraser/QGIS-resources/tree/dvc

I’m using DVC with the DVC plugin for VSCode but DVC can be used completely from the command line, if you prefer this appraoch.

Basically, what follows is a proof of concept: converting a QGIS Processing model to a DVC workflow. In the following screenshot, you can see the main stages

  1. The QGIS model in the upper left corner
  2. The Python script exported from the QGIS model builder in the lower left corner
  3. The DVC stages in my dvc.yaml file in the upper right corner (And please ignore the hello world stage. It’s a left over from my first experiment)
  4. The DVC DAG visualizing the sequence of stages. Looks similar to the QGIS model, doesn’t it ;-)

Besides the stage definitions in dvc.yaml, there’s a parameters file:

random-points:
  n: 10
buffer-points:
  size: 0.5

And, of course, the two stages, each as it’s own Python script.

First, random-points.py which reads the random-points.n parameter to create the desired number of points within the polygon defined in qgis3/data/test.geojson:

import dvc.api

from qgis.core import QgsVectorLayer
from processing.core.Processing import Processing
import processing

Processing.initialize()

params = dvc.api.params_show()
pts_n = params['random-points']['n']

input_vector = QgsVectorLayer("qgis3/data/test.geojson")
output_filename = "qgis3/output/random-points.geojson"

alg_params = {
    'INCLUDE_POLYGON_ATTRIBUTES': True,
    'INPUT': input_vector,
    'MAX_TRIES_PER_POINT': 10,
    'MIN_DISTANCE': 0,
    'MIN_DISTANCE_GLOBAL': 0,
    'POINTS_NUMBER': pts_n,
    'SEED': None,
    'OUTPUT': output_filename
}
processing.run('native:randompointsinpolygons', alg_params)

And second, buffer-points.py which reads the buffer-points.size parameter to buffer the previously generated points:

import dvc.api
import geopandas as gpd
import matplotlib.pyplot as plt

from qgis.core import QgsVectorLayer
from processing.core.Processing import Processing
import processing

Processing.initialize()

params = dvc.api.params_show()
buffer_size = params['buffer-points']['size']

input_vector = QgsVectorLayer("qgis3/output/random-points.geojson")
output_filename = "qgis3/output/buffered-points.geojson"

alg_params = {
    'DISSOLVE': False,
    'DISTANCE': buffer_size,
    'END_CAP_STYLE': 0,  # Round
    'INPUT': input_vector,
    'JOIN_STYLE': 0,  # Round
    'MITER_LIMIT': 2,
    'SEGMENTS': 5,
    'OUTPUT': output_filename
}
processing.run('native:buffer', alg_params)

gdf = gpd.read_file(output_filename)
gdf.plot()

plt.savefig('qgis3/output/buffered-points.png')

With these things in place, we can use dvc to run the workflow, either from within VSCode or from the command line. Here, you can see the workflow (and how dvc skips stages and fetches results from cache) in action:

If you try it out yourself, let me know what you think.

Visualizing trajectories with QGIS & MobilityDB

In the previous post, we — creatively ;-) — used MobilityDB to visualize stationary IOT sensor measurements.

This post covers the more obvious use case of visualizing trajectories. Thus bringing together the MobilityDB trajectories created in Detecting close encounters using MobilityDB 1.0 and visualization using Temporal Controller.

Like in the previous post, the valueAtTimestamp function does the heavy lifting. This time, we also apply it to the geometry time series column called trip:

SELECT mmsi,
    valueAtTimestamp(trip, '2017-05-07 08:55:40') geom,
    valueAtTimestamp(SOG, '2017-05-07 08:55:40') SOG
FROM "public"."ships"

Using this SQL query, we again set up a — not yet Temporal Controller-controlled — QueryLayer.

To configure Temporal Controller to update the timestamp in our SQL query, we again need to run the Python script from the previous post.

With this done, we are all set up to animate and explore the movement patterns in our dataset:


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

Visualizing IOT time series with QGIS & MobilityDB

Today’s post presents an experiment in modelling a common scenario in many IOT setups: time series of measurements at stationary sensors. The key idea I want to explore is to use MobilityDB’s temporal data types, in particular the tfloat_inst and tfloat_seq for instances and sequences of temporal float values, respectively.

For info on how to set up MobilityDB, please check my previous post.

Setting up our DB tables

As a toy example, let’s create two IOT devices (in table iot_devices) with three measurements each (in table iot_measurements) and join them to create the tfloat_seq (in table iot_joined):

CREATE TABLE iot_devices (
    id integer,
    geom geometry(Point, 4326)
);

INSERT INTO iot_devices (id, geom) VALUES
(1, ST_SetSRID(ST_MakePoint(1,1), 4326)),
(2, ST_SetSRID(ST_MakePoint(2,3), 4326));

CREATE TABLE iot_measurements (
    device_id integer,
    t timestamp,
    measurement float
);

INSERT INTO iot_measurements (device_id, t, measurement) VALUES
(1, '2022-10-01 12:00:00', 5.0),
(1, '2022-10-01 12:01:00', 6.0),
(1, '2022-10-01 12:02:00', 10.0),
(2, '2022-10-01 12:00:00', 9.0),
(2, '2022-10-01 12:01:00', 6.0),
(2, '2022-10-01 12:02:00', 1.5);

CREATE TABLE iot_joined AS
SELECT 
    dev.id, 
    dev.geom, 
    tfloat_seq(array_agg(
        tfloat_inst(m.measurement, m.t) ORDER BY t
    )) measurements
FROM iot_devices dev 
JOIN iot_measurements m
  ON dev.id = m.device_id
GROUP BY dev.id, dev.geom;

We can load the resulting layer in QGIS but QGIS won’t be happy about the measurements column because it does not recognize its data type:

Query layer with valueAtTimestamp

Instead, what we can do is create a query layer that fetches the measurement value at a specific timestamp:

SELECT id, geom, 
    valueAtTimestamp(measurements, '2022-10-01 12:02:00') 
FROM iot_joined

Which gives us a layer that QGIS is happy with:

Time for TemporalController

Now the tricky question is: how can we wire our query layer to the Temporal Controller so that we can control the timestamp and animate the layer?

I don’t have a GUI solution yet but here’s a way to do it with PyQGIS: whenever the Temporal Controller signal updateTemporalRange is emitted, our update_query_layer function gets the current time frame start time and replaces the datetime in the query layer’s data source with the current time:

l = iface.activeLayer()
tc = iface.mapCanvas().temporalController()

def update_query_layer():
    tct = tc.dateTimeRangeForFrameNumber(tc.currentFrameNumber()).begin().toPyDateTime()
    s = l.source()
    new = re.sub(r"(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})", str(tct), s)
    l.setDataSource(new, l.sourceName(), l.dataProvider().name())

tc.updateTemporalRange.connect(update_query_layer)

Future experiments will have to show how this approach performs on lager datasets but it’s exciting to see how MobilityDB’s temporal types may be visualized in QGIS without having to create tables/views that join a geometry to each and every individual measurement.

Detecting close encounters using MobilityDB 1.0

It’s been a while since we last talked about MobilityDB in 2019 and 2020. Since then, the project has come a long way. It joined OSGeo as a community project and formed a first PSC, including the project founders Mahmoud Sakr and Esteban Zimányi as well as Vicky Vergara (of pgRouting fame) and yours truly.

This post is a quick teaser tutorial from zero to computing closest points of approach (CPAs) between trajectories using MobilityDB.

Setting up MobilityDB with Docker

The easiest way to get started with MobilityDB is to use the ready-made Docker container provided by the project. I’m using Docker and WSL (Windows Subsystem Linux on Windows 10) here. Installing WLS/Docker is out of scope of this post. Please refer to the official documentation for your operating system.

Once Docker is ready, we can pull the official container and fire it up:

docker pull mobilitydb/mobilitydb
docker volume create mobilitydb_data
docker run --name "mobilitydb" -d -p 25432:5432 -v mobilitydb_data:/var/lib/postgresql mobilitydb/mobilitydb
psql -h localhost -p 25432 -d mobilitydb -U docker

Currently, the container provides PostGIS 3.2 and MobilityDB 1.0:

Loading movement data into MobilityDB

Once the container is running, we can already connect to it from QGIS. This is my preferred way to load data into MobilityDB because we can simply drag-and-drop any timestamped point layer into the database:

For this post, I’m using an AIS data sample in the region of Gothenburg, Sweden.

After loading this data into a new table called ais, it is necessary to remove duplicate and convert timestamps:

CREATE TABLE AISInputFiltered AS
SELECT DISTINCT ON("MMSI","Timestamp") *
FROM ais;

ALTER TABLE AISInputFiltered ADD COLUMN t timestamp;
UPDATE AISInputFiltered SET t = "Timestamp"::timestamp;

Afterwards, we can create the MobilityDB trajectories:

CREATE TABLE Ships AS
SELECT "MMSI" mmsi,
tgeompoint_seq(array_agg(tgeompoint_inst(Geom, t) ORDER BY t)) AS Trip,
tfloat_seq(array_agg(tfloat_inst("SOG", t) ORDER BY t) FILTER (WHERE "SOG" IS NOT NULL) ) AS SOG,
tfloat_seq(array_agg(tfloat_inst("COG", t) ORDER BY t) FILTER (WHERE "COG" IS NOT NULL) ) AS COG
FROM AISInputFiltered
GROUP BY "MMSI";

ALTER TABLE Ships ADD COLUMN Traj geometry;
UPDATE Ships SET Traj = trajectory(Trip);

Once this is done, we can load the resulting Ships layer and the trajectories will be loaded as lines:

Computing closest points of approach

To compute the closest point of approach between two moving objects, MobilityDB provides a shortestLine function. To be correct, this function computes the line connecting the nearest approach point between the two tgeompoint_seq. In addition, we can use the time-weighted average function twavg to compute representative average movement speeds and eliminate stationary or very slowly moving objects:

SELECT S1.MMSI mmsi1, S2.MMSI mmsi2, 
       shortestLine(S1.trip, S2.trip) Approach,
       ST_Length(shortestLine(S1.trip, S2.trip)) distance
FROM Ships S1, Ships S2
WHERE S1.MMSI > S2.MMSI AND
twavg(S1.SOG) > 1 AND twavg(S2.SOG) > 1 AND
dwithin(S1.trip, S2.trip, 0.003)

In the QGIS Browser panel, we can right-click the MobilityDB connection to bring up an SQL input using Execute SQL:

The resulting query layer shows where moving objects get close to each other:

To better see what’s going on, we’ll look at individual CPAs:

Having a closer look with the Temporal Controller

Since our filtered AIS layer has proper timestamps, we can animate it using the Temporal Controller. This enables us to replay the movement and see what was going on in a certain time frame.

I let the animation run and stopped it once I spotted a close encounter. Looking at the AIS points and the shortest line, we can see that MobilityDB computed the CPAs along the trajectories:

A more targeted way to investigate a specific CPA is to use the Temporal Controllers’ fixed temporal range mode to jump to a specific time frame. This is helpful if we already know the time frame we are interested in. For the CPA use case, this means that we can look up the timestamp of a nearby AIS position and set up the Temporal Controller accordingly:

More

I hope you enjoyed this quick dive into MobilityDB. For more details, including talks by the project founders, check out the project website.


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

Forget label buffers! Better maps with selective label masks in QGIS

Cartographers use all kind of tricks to make their maps look deceptively simple. Yet, anyone who has ever tried to reproduce a cartographer’s design using only automatic GIS styling and labeling knows that the devil is in the details.

This post was motivated by Mika Hall’s retro map style.

There are a lot of things going on in this design but I want to draw your attention to the labels – and particularly their background:

Detail of Mike’s map (c) Mike Hall. You can see that the rail lines stop right before they would touch the A in Valencia (or any other letters in the surrounding labels).

This kind of effect cannot be achieved by good old label buffers because no matter which color we choose for the buffer, there will always be cases when the chosen color is not ideal, for example, when some labels are on land and some over water:

Ordinary label buffers are not always ideal.

Label masks to the rescue!

Selective label masks enable more advanced designs.

Here’s how it’s done:

Selective masking has actually been around since QGIS 3.12. There are two things we need to take care of when setting up label masks:

1. First we need to enable masks in the label settings for all labels we want to mask (for example the city labels). The mask tab is conveniently located right next to the label buffer tab:

2. Then we can go to the layers we want to apply the masks to (for example the railroads layer). Here we can configure which symbol layers should be affected by which mask:

Note: The order of steps is important here since the “Mask sources” list will be empty as long as we don’t have any label masks enabled and there is currently no help text explaining this fact.

I’m also using label masks to keep the inside of the large city markers (the ones with a star inside a circle) clear of visual clutter. In short, I’m putting a circle-shaped character, such as ◍, over the city location:

In the text tab, we can specify our one-character label and – later on – set the label opacity to zero.
To ensure that the label stays in place, pick the center placement in “Offset from Point” mode.

Once we are happy with the size and placement of this label, we can then reduce the label’s opacity to 0, enable masks, and configure the railroads layer to use this mask.

As a general rule of thumb, it makes sense to apply the masks to dark background features such as the railways, rivers, and lake outlines in our map design:

Resulting map with label masks applied to multiple labels including city and marine area labels masking out railway lines and ferry connections as well as rivers and lake outlines.

If you have never used label masks before, I strongly encourage you to give them a try next time you work on a map for public consumption because they provide this little extra touch that is often missing from GIS maps.

Happy QGISing! Make maps not war.

Official Austrian basemap and cadastre vector tiles

The BEV (Austrian Bundesamt für Eich- und Vermessungswesen) has recently published the Austrian cadastre as open data:

The URLs for vector tiles and styles can be found on https://kataster.bev.gv.at under Guide – External

The vector tile URL is:

https://kataster.bev.gv.at/tiles/{kataster | symbole}/{z}/{x}/{y}.pbf

There are 4 different style variations:

https://kataster.bev.gv.at/styles/{kataster | symbole}/style_{vermv | ortho | basic | gis}.json

When configuring the vector tiles in QGIS, we specify the desired tile and style URLs, for example:

For example, this is the “gis” style:

And this is the “basic” style:

The second vector tile source I want to mention is basemap.at. It has been around for a while, however, early versions suffered from a couple of issues that have now been resolved.

The basemap.at project provides extensive documentation on how to use the dataset in QGIS and other GIS, including manuals and sample projects:

Here’s the basic configuration: make sure to set the max zoom level to 16, otherwise, the map will not be rendered when you zoom in too far.

The level of detail is pretty impressive, even if it cannot quite keep up with the basemap raster tiles:

Vector tile details at Resselpark, Vienna
Raster basemap details at Resselpark, Vienna

Building an interactive app with geocoding in Jupyter Lab

This post aims to show you how to create quick interactive apps for prototyping and data exploration using Panel.

Specifically, the following example demos how to add geocoding functionality based on Geopy and Nominatim. As such, this example brings together tools we’ve previously touched on in Super-quick interactive data & parameter exploration and Geocoding with Geopy.

Here’s a quick preview of the resulting app in action:

To create this app, I defined a single function called my_plot which takes the address and desired buffer size as input parameters. Using Panel’s interact and servable methods, I’m then turning this function into the interactive app you’ve seen above:

import panel as pn
from geopy.geocoders import Nominatim
from utils.converting import location_to_gdf
from utils.plotting import hvplot_with_buffer

locator = Nominatim(user_agent="OGD.AT-Lab")

def my_plot(user_input="Giefinggasse 2, 1210 Wien", buffer_meters=1000):
    location = locator.geocode(user_input)
    geocoded_gdf = location_to_gdf(location, user_input)
    map_plot = hvplot_with_buffer(geocoded_gdf, buffer_meters, 
                                  title=f'Geocoded address with {buffer_meters}m buffer')
    return map_plot.opts(active_tools=['wheel_zoom']) 

kw = dict(user_input="Giefinggasse 2, 1210 Wien", buffer_meters=(0,10000))

pn.template.FastListTemplate(
    site="Panel", title="Geocoding Demo", 
    main=[pn.interact(my_plot, **kw)]
).servable();

You can find the full notebook in the OGD.AT Lab repository or run this notebook directly on MyBinder:

To open the Panel preview, press the green Panel button in the Jupyter Lab toolbar:

I really enjoy building spatial data exploration apps this way, because I can start off with a Jupyter notebook and – once I’m happy with the functionality – turn it into a pretty app that provides a user-friendly exterior and hides the underlying complexity that might scare away stakeholders.

Give it a try and share your own adventures. I’d love to see what you come up with.

Dynamic Infographic Map Tutorial

This is a guest post by Mickael HOARAU @Oneil974

As an update of the tutorial from previous years, I created a tutorial showing how to make a simple and dynamic color map with charts in QGIS.

In this tutorial you can see some of interesting features of QGIS and its community plugins. Here you’ll see variables, expressions, filters, QuickOSM and DataPlotly plugins and much more. You just need to use QGIS 3.24 Tisler version.

Here is the tutorial.

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