Buidling Height Attribute in QGIS

Introduction

Background information on OS MasterMap® Topography Layer Building Height Attribute and details of the attributes supplied by Ordnance Survey can be found on the Building Height Attribute Overview page.

BHA data is available from Digimap in File Geodatabase format, which can be read by QGIS without requiring any processing. It is easy to visualise BHA in File Geodatabase format using a freely available QGIS plugin. The following steps describe the process in detail.

Getting started

Download the following datasets for your area of interest from Digimap using the OS download data tab in Digimap:

1. BHA data (BHA data is found in the OS MasterMap®category): select File Geodatabase as the format.
2. OS Terrain 5 DTM: this will be used as the base (surface) heights for the area.
3. Optionally download any backdrop data, such as OS VectorMap<&reg Local Raster, or 1:25 000 Scale Colour Raster.

Visualising the data in 3D

Visualising the data in 3D in QGIS is achieved using the Qgis2ThreeJS plugin, which can be installed using the QGIS plugin manager. The steps below describe how to use the plugin to create a 3D model:

Load data

1. Ensure you have all the data loaded in to your QGIS project that you wish to include in the 3D model, as a minimum you should have your DTM, the BHA dataset and a suitable map layer to drape over the DTM.
2. Turn off all layers in the Layers panel apart from the surface you wish to drape over the DTM; the buildings will be styled using the Qgis2ThreeJS plugin.
3. Launch the plugin, which can be found on the web toolbar.
4. Using the ‘DEM’ panel of the plugin select your DTM data as the ‘DEM Layer’, leave all other settings at their default values. Tip: by default the surface has a vertical exaggeration of 1.5, if you wish to reduce or increase this, the setting is configured on the ‘World’ panel of the plugin.

Settings and Processing

1. In the ‘Polygon’ panel of the plugin select your BHA dataset and complete the following settings:
   • Z coordinate: set to ‘Height from surface’ - this will ensure the buildings sit on the DTM surface.
   • Under ‘Style’: ensure the ‘Object type’ is set to ‘Extruded’ and select the height attribute you wish to use for the extrusion using the ‘Height’ drop down; as mentioned above, we’re using the RelH2 attribute from OS which is in the column ‘NT27_relh2? in our data.
   • Select suitable colours and transparency, we used a medium grey colour with 10% transparency to give a glasshouse effect.
2. Optionally specify an ‘Output HTML file path’ to save the resultant files. Whilst you’re experimenting we recommend you leave this blank and the plugin will save the data in a temporary location, when you’re happy with the result you can use this setting to save your final visualisation.
3. Press ‘Run’ to create the 3D model. Once it’s finished processing the model will open in your default web browser.
4. If you get an error when running the plugin, see the Tips section below.

The plugin outputs an HTML file, along with a small number of accompanying files. The HTML file requires a WebGL compatible browser (WebGL is a method of generating dynamic 3D graphics using JavaScript), most modern browsers are WebGL compatible including IE 11, FireFox, Chrome, Safari. The Can I Use site offers further information on browser compatibility.

As the files are output as a web page, you can share the results of your work with colleagues without them needing to have any specialist GIS software, however you are not permitted to make the website publicly available as the html and javaScript files contain map data rather than just images of maps. The Digimap licence does not permit the sharing of licensed data from Digimap with anyone other than registered users of the service: see licensing details.

Tips

• The plugin assumes there will be a height value for every feature in the dataset in the selected attribute column. If some of the building features do not have a height value for the selected attribute the plugin will generate an error similar to the below:

TypeError: float() argument must be a string or a number.

As it is not possible to edit ESRI File Geodatabases in QGIS without installing another driver, the easiest work around is to run a ‘selection by attribute’ to select only those features that have a value for the specified attribute then export the selection to a new shapefile dataset. In the example here the query would be:

“NT27_relh2” is not null

Having selected all the features that have a height value for this attribute we can then save the selected features to a new dataset and run the visualisation on this dataset.

• The plugin uses the extents of the current QGIS map canvas, so the bigger the area being displayed, the bigger the generated 3D scene and the slower it will display. We have found that areas of up to 10km² display okay, anything bigger tends to be a little slow to respond.
• If you wish to define specific extents for your 3D scene instead of using the map canvas extents this can be done on the ‘World’ panel of the plugin.
• Applying a vertical exaggeration to your buildings is achieved through the ‘Multiplier’ setting on the ‘Polygon’ panel of the plugin.
• You can create 3D models of multiple layers. For example in the screen shot below the trees were created by selecting the ‘Positioned Non Coniferous Trees’ from the OS MasterMap®Topographic Point layer. These were then added to the QGIS project twice. Using the plugin one of these layers was extruded as a brown cylinder with a radius of 0.75m and a height of 3m to form the trunk; the other was extruded as a green sphere with a radius of 4.5m and a z coordinate of 4.5m (i.e. the height above the ground surface of the centre of the sphere) to form the tree canopy:

The finished visualisation

The screen grab below shows the final visualisation centred on the south side of Edinburgh using OS 1:25,000 Colour Raster as the surface layer.