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Fri Nov 15 01:35:23 2019

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Five QGIS network analysis toolboxes for routing and isochrones

In the past, network analysis capabilities in QGIS were rather limited or not straight-forward to use. This has changed! In QGIS 3.x, we now have a wide range of network analysis tools, both for use case where you want to use your own network data, as well as use cases where you don’t have access to appropriate data or just prefer to use an existing service.

This blog post aims to provide an overview of the options:

  1. Based on local network data
    1. Default QGIS Processing network analysis tools
    2. QNEAT3 plugin
  2. Based on web services
    1. Hqgis plugin (HERE)
    2. ORS Tools plugin (openrouteservice.org)
    3. TravelTime platform plugin (TravelTime platform)

All five options provide Processing toolbox integration but not at the same level.

If you are a regular reader of this blog, you’re probably also aware of the pgRoutingLayer plugin. However, I’m not including it in this list due to its dependency on PostGIS and its pgRouting extension.

Processing network analysis tools

The default Processing network analysis tools are provided out of the box. They provide functionality to compute least cost paths and service areas (distance or time) based on your own network data. Inputs can be individual points or layers of points:

The service area tools return reachable edges and / or nodes rather than a service area polygon:

QNEAT3 plugin

The QNEAT3 (short for Qgis Network Analysis Toolbox 3) Plugin aims to provide sophisticated QGIS Processing-Toolbox algorithms in the field of network analysis. QNEAT3 is integrated in the QGIS3 Processing Framework. It offers algorithms that range from simple shortest path solving to more complex tasks like Iso-Area (aka service areas, accessibility polygons) and OD-Matrix (Origin-Destination-Matrix) computation.

QNEAT3 is an alternative for use case where you want to use your own network data.

For more details see the QNEAT3 documentation at: https://root676.github.io/index.html

Hqgis plugin

Access the HERE API from inside QGIS using your own HERE-API key. Currently supports Geocoding, Routing, POI-search and isochrone analysis.

Hqgis currently does not expose all its functionality to the Processing toolbox:

Instead, the full set of functionality is provided through the plugin GUI:

This plugin requires a HERE API key.

ORS Tools plugin

ORS Tools provides access to most of the functions of openrouteservice.org, based on OpenStreetMap. The tool set includes routing, isochrones and matrix calculations, either interactive in the map canvas or from point files within the processing framework. Extensive attributes are set for output files, incl. duration, length and start/end locations.

ORS Tools is based on OSM data. However, using this plugin still requires an openrouteservice.org API key.

TravelTime platform plugin

This plugin adds a toolbar and processing algorithms allowing to query the TravelTime platform API directly from QGIS. The TravelTime platform API allows to obtain polygons based on actual travel time using several transport modes rather, allowing for much more accurate results than simple distance calculations.

The TravelTime platform plugin requires a TravelTime platform API key.

For more details see: https://blog.traveltimeplatform.com/isochrone-qgis-plugin-traveltime

Topology in QGIS

Introduction

Topology rules define the permissible relationships of features within a given GIS layer or between features in two different GIS layers. An example is that features in a road dataset must be connected to other roads at both ends, unless the road is specified as a dead end street.

Advantage of topology over queries

A lot of the checks that topology rules carry out could be achieved using spatial queries. You may have to use queries if the GIS software you’re using doesn’t have a topology feature.

Topology rules have the advantage that they only need be created once and then they can check your work as you go.

Queries would need to be re-created each time they are run. They can be saved, depending on the GIS being used, but this is still more time consuming and it is a task that must be carried out separately at the end of a work session.

Rules

QGIS 2.2 topology tool has the following rules pre-defined:-

  • End points must be covered by (e.g. a railway line usually begins and ends at a station)
  • Must contain (e.g. a building polygon must contain at least one address point seed)
  • Must not have dangles (a line must begin and end at another line)
  • Must not have duplicates (each feature should be unique, e.g. postcode areas)
  • Must not have gaps (e.g. administrative area polygons cannot have gaps)
  • Must not have invalid geometries
  • Must not have multi-part geometries (each feature should be a separate entry)
  • Must not overlap (e.g. administrative area polygons cannot overlap each other)
  • Must not overlap with (a feature from layer must not overlap with another layer)

Example 1 – Roads must not have dangles

The following example uses the “Must not have dangles” rule to identify polylines from a roads dataset that are not snapped to other lines. Roads usually begin and end at a junction with another road, so this is a useful rule to identify where lines were not correctly snapped together.

To create and validate a Topology Rule

  • Open the Topology Panel, by selecting Vector menu, Topology Checker, Topology Checker
  • The Topology Panel appears in the lower right corner of the QGIS desktop window

Image

  • Press the Configure button to open the Topology Rule Settings dialog
  • The top of the box will have 2 or 3 pull down boxes depending on the layer and rule that is chosen. Use these to build the rule and then press the Add Rule button.

Image

  • Press OK when done, the dialog box closes and the window returns to the QGIS Desktop.
  • Press either the Validate All or Validate extent, depending on whether you wish to validate the entire dataset or just the current view extent.
  • The errors will be listed. Double click on a row will make the map window zoom and pan to the error.

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