Use PLATEAU Data in QGIS: Joins and 3D View

Introduction

This article shows how to join the disaster risk information in PLATEAU 3D city models, gives an example of spatial analysis, and explains how to display the data in 3D.

To learn about PLATEAU or how to add PLATEAU 3D city models to QGIS, see the following article.

Using the attached data with a table join

The disaster risk layers come with the building model, but they cannot be drawn on a map as they are. You need to attach them to the Building layer with a table join.

First, add the 3D city model to QGIS, as in “Add PLATEAU 3D City Models to QGIS.” The following steps also use this data, so adding the building model from the 3D City Model Kawasaki City (Fiscal Year 2022) Data is recommended.

Once the model is added, right-click the Building layer, open Properties, and go to the Joins tab. Click the green + button to open a dialog.

Set the following in the Add Vector Join dialog.

1. Join layer: select the disaster risk layer you want to join
2. Join field: select parent
3. Target field: select id
4. Custom field name prefix: select the checkbox and enter an easy-to-recognize name (ending the name with an underscore _ makes the field names easier to read in the attribute table)
5. Click OK

Close the layer properties and open the attribute table of the Building layer. The attributes have been added. Each new field name is the prefix you entered followed by the field name from the disaster risk layer.

Now that the attributes are added, visualize them on the map.

In the disaster risk layer RiverFloodingRisk 1 L2（最大想定規模） (maximum assumed scale), the rank attribute defines the scale of the assumed flood inundation.

The flood inundation assumption around Ikuta in Tama ward, Kawasaki, shows that areas along the Gotanda River are assumed to be flooded to a maximum of “5 m or more, less than 10 m.”

Spatial analysis: joining the information of the nearest point to each polygon

Next, run a spatial analysis with the data you added. This example uses the emergency water supply points data that Kawasaki City publishes as open data. It classifies and colors the building models by the emergency water supply point they are closest to.

The data is a CSV file. Its contents show that it has columns for location (latitude and longitude).

Adding a delimited text layer

CSV data with location information is easy to bring into QGIS.

From the menu bar, choose Layer → Add Layer → Add Delimited Text Layer to open the Data Source Manager. Set the following and add the layer.

1. File name: select the CSV file to add
2. Layer name: set a name as needed (here, water)
3. Encoding: set as needed (try Shift-JIS and UTF-8, and choose the one that does not garble the text)
4. X field: select 経度 (longitude)
5. Y field: select 緯度 (latitude)
6. Geometry CRS: set the coordinate reference system (CRS) as needed (here, the EPSG code given in the CSV file, EPSG:4612)
7. Click the Add button

Right-click the layer you added and choose Zoom to Layer(s). The point layer of emergency water supply points in Kawasaki City is now on the map.

Finding the nearest emergency water supply point for each building model

The data is ready, so run the analysis.

Click the gear icon in the toolbar to open the Processing Toolbox. Search for nearest, expand Vector general, and double-click Join attributes by nearest.

Join attributes by nearest joins the attributes of the spatially nearest feature to the input layer and creates a new layer. Here, specify Building (the PLATEAU building model) as the input layer, and join the attributes of the water (emergency water supply points) layer to it.

1. Input layer: Building (PLATEAU building model)
2. Input layer 2: water (emergency water supply points)
3. Click Run

QGIS adds the nearest-joined layer under the name Joined layer. Open its layer properties and set the symbology.

1. Drop-down: change Single Symbol to Categorized
2. Value: select 給水拠点 (water supply point)
3. Click Classify to color each water supply point with a random color
4. Click OK to apply the settings

On the map canvas, the buildings are now colored by their nearest emergency water supply point.

Displaying PLATEAU in 3D in QGIS

QGIS can visualize data that has a “height” attribute in three dimensions. It supports elevation data (digital elevation model, DEM), vector data, and 3D point cloud data.

3D display is built into QGIS as a standard feature, but this article uses a plugin to display the data in 3D.

Installing Qgis2threejs

Qgis2threejs is a plugin with 3D display functionality that is registered in the official QGIS plugin repository.

From the menu bar, choose Plugins → Manage and Install Plugins to open the dialog. Open the All tab and search for three. Qgis2threejs appears in the results. Select it and click Install Plugin. That completes the installation.

Displaying in 3D with the plugin

Create a new project and add the PLATEAU building model to QGIS with its 3D data retained. As before, add OpenStreetMap too.

To display data in 3D, the project CRS must be a coordinate system that uses meters.

Click EPSG:XXXX in the status bar at the bottom of the QGIS window to open the settings dialog.

Change the project CRS to a coordinate system that uses meters, such as EPSG:3857, as follows.

1. Filter: enter 3857
2. Expand Projected → Mercator and select WGS 84 / Pseudo-Mercator (EPSG:3857)
3. Click OK to apply the settings

Click the icon in the toolbar to launch the plugin.

A window with a blue-sky background opens. Select the Flat Plane and Building checkboxes to display OpenStreetMap and the building model.

Use the mouse wheel to zoom in and out. You can also change the viewing angle by dragging with the left mouse button.

In the 3D view, the building models float in the air, so change the settings.

In the Layers panel, right-click Building and choose Properties.

Change Type from Polygon to Extruded, and click OK to apply the settings.

On the map, the buildings no longer float in the air.

Conclusion

This article covered how to join disaster risk information layers, how to run a spatial analysis, and how to display data in 3D.

Bring PLATEAU 3D city models into QGIS and try out various analyses and display methods.