Building a 3D Model of London

We started building our London model back in 2007. Our largest customer was a London building consultancy GIA who required an accurate 3D model for Rights of Light, Rights of View and Shadow/Glare analysis. That meant that we were forced to tailor our model to their strict requirements of high detail, high and consistent accuracy and high usability – by usability we mean the ability for them to prepare the model for analysis as well as the ability to print our models on our in house 3D printer. But these constraints also meant that the quality of capture and post-processing was far superior to everything on the market, realising that we had spotted a gap in this niche market we set out to create a wide area model of London. The first step involved connecting the smaller sites we were working on and then expanding the model on a project-by-project basis.

How do we utilise photogrammetry?

Stereo Photogrammetry is the practice of determining the geometric properties of objects from photographic images – in our case, the geometric properties of terrain, buildings and manmade features. Basically, an airborne vehicle, typically a plane, is equipped with a calibrated high-resolution camera and flies over a target area taking continuous photographs. Each of these photographs has metadata associated with it captured by an internal measurement unit (GPS coordinates + pitch, roll and tilt of the plane). We use this data to position the photographs in relation to their real-world location. Because these photographs include a 40% overlap, we are able to tie common points from adjoining photographs together – this process, together with adding control points and ground-based GPS is called Aerial Triangulation. We then view these stereo pairs using 3D stereoscopic displays (one photograph into one eye, second in the other eye) and trace the features as we see them in this 3D environment. To do this accurately you need photographs of ultra-high resolution – this is where the term GSD (Ground Sample Distance) comes to play a role. You can view on Google Maps the industry standard aerial imagery for mapping of 20cm GSD, meaning that each pixel represents a 20cm x 20cm tile in real life. Our 4cm GSD means that 1 pixel is 4cm x 4cm wide, allowing us to see and capture much more detail.

How is the model captured?

A small but state-of-the-art aerial-based photogrammetry flow line is at the heart of what we do here at Vertex Modelling. Although the costs associated with acquiring the hardware, software and source imagery needed for the set-up was considerable, there was no other way to meet all of the requirements set out by GIA. We were lucky that at the same time we were setting up our operation new digital ultra-high resolution aerial imagery with GSD of 4cm became available, which made it possible for us to be accurate to within few centimetres on all axis. Combining this imagery with a large number of ground survey points allowed us to create a central data set from which we are able to derive our 3D models from. Once the imagery is triangulated we manually trace each building as a set of features which we later post-process to create solid and clean 3D model of each building.

How does the final product look?

We invested a lot of time developing these processes. We created the production flow line with the aim of making the models as end user friendly as possible across a wide selection of 3D modelling packages. We have just taken delivery of the most recent high-resolution aerial imagery of London flown in May 2014 showing our commitment to creating the most up-to-date, accurate and detailed 3D model of London. We are immensely proud of what we have achieved so far however we have only just got started!

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