During most of the time I’ve been mapping in OpenStreetMap I’ve concentrated mainly on mapping town and city centres in the UK in high levels of detail. I try and map all the shops and buildings on the streets I’ve surveyed by taking photos of the streets and buildings. In the last few years I extended this to start adding the ‘Simple 3D buildings’ tags (https://wiki.openstreetmap.org/wiki/Simple_3D_Buildings) to buildings, enabling additional details to be recorded about buildings and for them to be rendered in 3D.

Recently I finished mapping Hertford town centre in detail including 3D tags and this can be seen on OSM (in 2D) at https://www.openstreetmap.org/#map=18/51.79638/-0.07740, and in various 3D renderings such as F4 Map (https://demo.f4map.com/#lat=51.7962036&lon=-0.0795818&zoom=18) and OSM Buildings (https://osmbuildings.org/?lat=51.79617&lon=-0.07938&zoom=17.8&tilt=30). Many other renderings are available – see https://wiki.openstreetmap.org/wiki/3D.

Other places where I’ve done much 3D mapping in the UK have included Alton, Eastbourne, Dartford, Llandudno, Conwy, Leytonstone, Bridport, between Strood and Chatham, and bits of East London around Limehouse and Hackney. I also did a bit of 3D mapping in Santiago de Compostela and Madrid in Spain. The area though where I’ve done the most 3D mapping has been in central London. London I’m sure has the largest number of 3D mapped buildings in the UK with a few different mappers doing some incredible 3D mapping both in terms of the number of buildings mapped and the level of detail on several complex buildings. I’ve personally though tended to do mainly simpler buildings, and my mapping has been scattered around various different areas I’ve visited.

As I mentioned my usual method of mapping is by photographing the streets and buildings. With the Covid-19 pandemic this may not be allowed or advisable right now depending on the situation where you are in the world, but it’s not been a problem for me as I’ve got a big backlog of photos I took before the pandemic. In any case you don’t have to go out as there are lots of other resources you can use to survey buildings like Mapillary and Bing Streetside which have given permission for OSM to use their images in mapping (don’t use Google Streetview though, this isn’t allowed).

My preference is to use the JOSM editor for my detailed mapping as I find it works best for me doing this detailed work. I also open up my images in JOSM so everything is all in one screen.

I usually go through each picture in turn adding all the features I want to map. For 3D mapping the main thing you need to map is either the number of levels of the building (building:levels tag) or the height in metres (height tag) – or you can add both. For most buildings I use building:levels as that’s quicker to do, and it’s also a useful attribute of the building to record in any case. In addition a roof:levels tag can be added. Occasionally I do use height though, especially on more complex buildings where the building:levels tag on its own isn’t very helpful for 3D modelling of that building – for example a church which is one storey but significantly taller than a typical one storey building. The heights I enter are just rough estimates but a tip is to look for the doors to give you a sense of scale – a typical door in the UK is about 2 metres tall.

Often buildings will be more complex than this and the height will vary in different parts. This can be mapped using building parts – see the Simple 3D Buildings page on the Wiki for an explanation. It can take some time to map building parts so I’m selective in doing this. I’ll generally map building parts if I can see that a major part of the building has a different height from the rest of the building. It’s also nice to do this for some key landmark buildings in an area, to make the 3D rendering reflect some of the more interesting buildings. Though as I’ve got lots of photos to go through and want to map a reasonably significant area I don’t tend to spend too much time on mapping the more complex buildings.

There are a few other tags I usually add when doing 3D mapping. None of these tags are required, but they make a huge difference in making the 3D rendering of the map look much more realistic. Without them the 3D rendered map will typically just be a bunch of white blocks, which look little different from the non-3D tagged buildings. Again I’m not going to explain each tag fully – you’d need to read the Simple 3D Buildings page on the wiki for details. The first is building:colour, or alternatively building:material can be often used for a similar purpose in rendering the colour (eg building:material=brick, will give the building a brick like colour in some renderings like F4 Map and OSM Buildings). You can use both, the building:colour will probably override that renderings colour for that building:material. The colour doesn’t need to be exact, but for it to be rendered it needs to be in one of the formats detailed here: https://wiki.openstreetmap.org/wiki/Key:colour. For speed of mapping I often use colour names like ‘grey’ but if I need to I’ll look up the RGB hex code using a colour picker or on the internet.

Similarly roof:colour (and/or roof:material) makes a big difference to rendering. When I first started 3D mapping I didn’t bother with it, as you often can’t see the roof in a photo and I thought it could just be added from aerial imagery at a later date – whereas I wanted to concentrate just on the information recorded in my photos. I’ve found though that if you’ve got a building:colour (other than white) or building:material that it’s worth adding this as otherwise the 3D rendering looks a bit weird on some renderers like F4 Map which defaults to white if this tag isn’t set. So I get the roof colour from the aerial imagery if I can’t see the roof properly on my photo. Typically to speed things up I’ll just map this as grey or brown etc, rather than looking up the exact shade.

The next thing that is very good to map for 3D rendering is the shape of the roof using roof:shape and other roof tags. Once you have roof tags the buildings will generally look much more realistic. Unfortunately it can be very difficult to map this. This is especially the case with many older buildings in town centres which often have been extended and altered over the years. Frequently there are different roof shapes over different parts of the building. I therefore don’t bother with these tags for every building, usually just adding them where the roof shape is relatively simple or if it’s a landmark building I want to map in detail.

Using these tags and techniques to map buildings in 3D does take a little longer than mapping without them but perhaps not as long as you might think. It’s fairly simple to do once you’ve become familiar with the tags and if you aren’t doing anything too complex. In many areas a lot of buildings have similar attributes so you can do a lot of copy and pasting of tags enabling quick mapping with 3D tags. A whole terrace or a street of similar houses can be done quite quickly, just modifying the tags on the ones which differ from the norm. Though I usually find that less copy and pasting is possible in town and city centres as the buildings tend to vary more.