Page 2 of 116 (2307 posts)

  • talks about »

Tags

Last update:
Sat Aug 24 20:35:15 2019

A Django site.

QGIS Planet

Movement data in GIS #23: trajectories in context

Today’s post continues where “Why you should be using PostGIS trajectories” leaves off. It’s the result of a collaboration with Eva Westermeier. I had the pleasure to supervise her internship at AIT last year and also co-supervised her Master’s thesis [0] on the topic of enriching trajectories with information about their geographic context.

Context-aware analysis of movement data is crucial for different domains and applications, from transport to ecology. While there is a wealth of data, efficient and user-friendly contextual trajectory analysis is still hampered by a lack of appropriate conceptual approaches and practical methods. (Westermeier, 2018)

Part of the work was focused on evaluating different approaches to adding context information from vector datasets to trajectories in PostGIS. For example, adding land cover context to animal movement data or adding information on anchoring and harbor areas to vessel movement data.

Classic point-based model vs. line-based model

The obvious approach is to intersect the trajectory points with context data. This is the classic point data model of contextual trajectories. It’s straightforward to add context information in the point-based model but it also generates large numbers of repeating annotations. In contrast, the line data model using, for example, PostGIS trajectories (LinestringM) is more compact since trajectories can be split into segments at context borders. This creates one annotation per segment and the individual segments are convenient to analyze (as described in part #12).

Spatio-temporal interpolation as provided by the line data model offers additional advantages for the analysis of annotated segments. Contextual segments start and end at the intersection of the trajectory linestring with context polygon borders. This means that there are no gaps like in the point-based model. Consequently, while the point-based model systematically underestimates segment length and duration, the line-based approach offers more meaningful segment length and duration measurements.

Schematic illustration of a subset of an annotated trajectory in two context classes, a) systematic underestimation of length or duration in the point data model, b) full length or duration between context polygon borders in the line data model (source: Westermeier (2018))

Another issue of the point data model is that brief context changes may be missed or represented by just one point location. This makes it impossible to compute the length or duration of the respective context segment. (Of course, depending on the application, it can be desirable to ignore brief context changes and make the annotation process robust towards irrelevant changes.)

Schematic illustration of context annotation for brief context changes, a) and b)
two variants for the point data model, c) gapless annotation in the line data model (source: Westermeier (2018) based on Buchin et al. (2014))

Beyond annotations, context can also be considered directly in an analysis, for example, when computing distances between trajectories and contextual point objects. In this case, the point-based approach systematically overestimates the distances.

Schematic illustration of distance measurement from a trajectory to an external
object, a) point data model, b) line data model (source: Westermeier (2018))

The above examples show that there are some good reasons to dump the classic point-based model. However, the line-based model is not without its own issues.

Issues

Computing the context annotations for trajectory segments is tricky. The main issue is that ST_Intersection drops the M values. This effectively destroys our trajectories! There are ways to deal with this issue – and the corresponding SQL queries are published in the thesis (p. 38-40) – but it’s a real bummer. Basically, ST_Intersection only provides geometric output. Therefore, we need to reconstruct the temporal information in order to create usable trajectory segments.

Finally, while the line-based model is well suited to add context from other vector data, it is less useful for context data from continuous rasters but that was beyond the scope of this work.

Conclusion

After the promising results of my initial investigations into PostGIS trajectories, I was optimistic that context annotations would be a straightforward add-on. The line-based approach has multiple advantages when it comes to analyzing contextual segments. Unfortunately, generating these contextual segments is much less convenient and also slower than I had hoped. Originally, I had planned to turn this work into a plugin for the Processing toolbox but the results of this work motivated me to look into other solutions. You’ve already seen some of the outcomes in part #20 “Trajectools v1 released!”.

References

[0] Westermeier, E.M. (2018). Contextual Trajectory Modeling and Analysis. Master Thesis, Interfaculty Department of Geoinformatics, University of Salzburg.


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

Presentations at FOSS4G 2015 in Seoul

Slides from our presentations at FOSS4G 2015 in Seoul: Keynote: The QGIS project and its evolution from a desktop GIS to a GIS platform - Slides New QGIS functions for power users - Slides QGIS Plugins - From Must-Haves to insider tips - Slides Building an OpenLayers 3 map viewer with React - Slides Thanks to the organizers of this great conference! It was a pleasure to get in contact with so many users from around the world.

Sourcepole at FOSS4G 2014 in Portland

In one week, the 2014 FOSS4G Conference will start in Portland/Oregon. Sourcepole supports this major event as a bronze sponsor. Our conference contributions: Workshop presented by Horst Düster (@moazagotl) Tuesday afternoon: QGIS Plugin Development with PyQt4 and PyQGIS Presentations by Pirmin Kalberer ((@implgeo)) Thursday, Session 2, Track 7, 13:00 - 13:25: State of QGIS Server Thursday, Session 2, Track 7, 13:30 - 13:55: From Nottingham to PDX: QGIS 2014 roundup Thursday, Session 3, Track 6, 16:25 - 13:25: Easy ETL with OGR Meet Pirmin and Horst at Sourcepole’s exhibition booth and have a look at our latest products.

NTv2 transformations with QGIS

Datum transformations with grid shift files are used in several countries to convert coordinates between different datums. In Switzerland, datum transformation using the NTv2 method is important because of the upcoming conversion between the LV03 system and the new LV95 system. Up to now, doing coordinate transformations with grid shift files was possible in QGIS, but unconvenient. To use an NTv2 transformation in QGIS, the grid shift file needs to be placed in a directory where proj4 can find it (usually /usr/share/proj on Linux and OSGeo4W\share\proj on Windows).

Publish Image Tooltips with QGIS Cloud

A lot of people are using QGIS Cloud as a service with ready to use QGIS webclient. It’s very easy to publish data and share maps in this way. Publishing of georeferenced images can be done with QGIS Cloud in a few steps as well. But the main problems are: how to upload the images into the cloud database? how to manage them? how to display the results? QGIS and QGIS Cloud are offering all tools for this task.

AGIT 2013: QGIS Cloud - Karten einfach ins Netz

Noch nie war es so einfach individuelle Web Map Services mit ansprechenden Karten, Geodatenbank und Web Client zu erstellen, wie mit QGIS Cloud. Der am 5. Juli an der AGIT 2013 in Salzburg präsentierte Vortrag kann hier herunter geladen werden. Präsentation

FOSSGIS 2019 in Dresden

Sourcepole war an der FOSSGIS 2019 in Dresden als Austeller, mit Vorträgen und einem Vektor Tile Workshop präsent.

QGIS Instant Print Plugin

As a side product of a customer project, we’re publishing a QGIS plugin for printing maps to a file with just two mouse clicks.

QGIS Cloud - Speed up the loading time of the web client

QGIS Cloud is your personal geo-data infrastructure in the internet. Publish maps and data. Share geo-information with others. And all of this very easily, without server, infrastructure and expert knowledge. If you know QGIS Desktop, then you know QGIS cloud just as well. Just install the QGIS cloud plugin from the official QGIS plugin repository and you’re good to go. You can publish as many maps as you want. But the default settings of QGIS projects you like to publish via QGIS Cloud are not the best with respect to the performance of the QGIS Webclient / WMS.

QGIS User Meeting Bern 21.4.10

Am 21. April 2010 findet das erste deutschsprachige QGIS User-Meeting in Bern statt. Das Programm und die detaillierten Angaben können auf der QGIS-Homepage nachgelesen werden. Sourcepole stellt bei dieser Gelegenheit ihr OpenLayers-Plugin vor, das Google Maps und Konsorten als QGIS-Layer ins Desktop-GIS integriert. Alle QGIS-Anwender und Interessierte sind herzlich zu diesem Anlass eingeladen!

Eigene GIS-Fachapplikationen mit QGIS entwickeln

Als Ergebnis der Zusammenarbeit zwischen der Sourcepole AG und der Hochschule Rapperswil (HSR) ist ein Video entstanden, dass die Entwicklung eigener Fachschalen mit QGIS beschreibt. Am Beispiel der Zonenplanung zeigt dieses Video, wie Sie mit Quantum GIS eine so genannte Fachapplikation erstellen könne. Sehen Sie Schritt für Schritt, wie sie Quantum GIS auf Ihre persönlichen Bedürfnisse anpassen können, passende Erweiterungen finden, eigene Eingabeformulare erstellen und eine Datenbank verwalten. Die Grundlage zu diesem Video war ein Workshop auf dem Geosummit 2012 Bern, den Pirmin Kalberer entwickelt hat.

Slides FOSS4G 2014

Slides from our presentations at FOSS4G 2014 in Portland/Oregon: From Nottingham to PDX: QGIS 2014 roundup State of QGIS Server Easy ETL with OGR Pirmin Kalberer (@implgeo)

The state of QGIS Globe

The Region of Umbria, Italy, sponsored 4 days of work to update QGIS Globe for current QGIS versions. Most of the functionality is working again and the globe is now compatible with osgEarth 1.0 up to 1.3. The bad news is, that the globe plugin is not working on Windows with OSGeo4W. It seems that one of the OSGeo4W libraries (GDAL?) is compiled with an incompatible MS compiler version. Christmas holidays are coming…

New configuration options in QGIS server

In QGIS server, it is now possible to selectively exclude layers from WMS publication. These layers will be available only on the desktop and hidden from WMS clients. Similarly, print layouts can be excluded from WMS publication. Of course, these settings are conveniently accessible from the project properties dialog of QGIS (but you need to have a nightly build or a recent compile): Additionally, attributes per layer can be excluded from WMS or WFS publication in the vector properties dialog:

Share and manage your Data with QGIS Cloud and WFS-T

A lot of people are using QGIS Cloud as a service with ready to use QGIS webclient. It’s very easy to publish data and share maps in this way. But QGIS Cloud has more power under the hood. A not so obvious feature of QGIS Cloud is the option to share your data via Web Feature Service (WFS) and manage them via Web Feature Service Transactional (WFS-T). “The basic Web Feature Service allows querying and retrieval of features.

FOSSGIS 2015

Sourcepole war an der FOSSGIS 2015 in Münster unter anderem mit zwei QGIS-Vorträgen präsent: Neues von QGIS QGIS Plugins - Must-Haves, Fachlösungen und Geheimtipps

„Geo For All“ - neue Technologien für eine Welt im Wandel

GEOSummit 2014, Bern „Geo for all“ ist nicht nur das Motto der weltumspannenden ICA-OSGeo Lab Initiative zur Förderung der GIS-Ausbildung an Hochschulen, sondern steht allgemein für den immer breiteren Zugang zu professionellen GIS-Werkzeugen. Im Kartenbereich haben Produkte wie TileMill oder D3.js, sowie Dienste wie CartoDB, GeoCommons, usw. den Anwenderkreis weit über das klassische GIS-Fachbebiet hinaus erweitert. Im Vortrag werden einige herausragende Beispiele vorgestellt und deren Relevanz für die Fachwelt erläutert.

Creating png8 images with QGIS server

For providing maps via WMS over the internet, it is important to generate image files with a small size. Because normally, most of the perceived WMS delay comes from transfering large images files over the internet (and not from map rendering itself). Therefore, QGIS server supports the conversion of png24 and png32 images into png8, therefore generating a file with only 1⁄3 resp. 1⁄4 of the original size (but with lower quality).

Slides FOSS4G 2017

Reporting back from FOSS4G 2017 in Boston, which started with the usual QGIS plugin programming workshop, this time at the Harvard University campus.

FOSS4G 2018 Dar es Salaam

This year’s FOSS4G edition took place in Dar es Salaam, Tanzania. As every year, Sourcepole was supporting this major event as a sponsor. We would like to thank for all the interesting discussions and feedback to our presentations!

  • <<
  • Page 2 of 116 ( 2307 posts )
  • >>

Back to Top

Sponsors