Benefits of the XCAM Circle Trajectory for 3D City Models
The benefits of using the WaldoAir XCAM patent-pending circular path aerial mapping method.
As you hopefully already know, XCAM is a small aerial survey camera system which is designed to be highly cost-effective for survey areas larger than about 10 acres (the sweet spot for drones) and smaller than 2000 square kilometres (the sweet spot for larger cameras fitted to the inside of a dedicated survey aircraft).
A unique and impressive part of the XCAM system is the patent-pending Circular Trajectory flight pattern which provides XCAM with a significant lead over all other survey systems in the generation of high-resolution urban 3D models. Let's find out more.
What is a Circular Trajectory?
Most traditional aerial survey methods use a Track Pattern to systematically capture imagery. The camera points straight down and the aircraft flies in a straight line in one direction, turns around and flies back in the opposite direction, akin to mowing the lawn. This is how it's always been done. XCAM can capture using this method as well as circles.
The Circular Trajectory is very different, but for some very good reasons which we'll come onto further down. Instead of flying back and forth, the survey aircraft instead flies an advancing circular path over the Area of Interest. Each circle overlaps the previous circle by about 65% so the circles are actually more oval-shaped.
The image below shows a plan for a single set of circles over a town. The white line is the path of the aircraft and the red box is the project area. This survey would take approximately 8 minutes.
The circle plan isn't just for linear routes, it can also be used to capture larger areas, such as the plan below. This would take up to 16 minutes to fly.
What Data Does it Capture?
Our aim for a circular survey is to capture high and low obliques of the area of interest. It doesn't capture any nadir (straight down) images. Using modern processing methods we can use this oblique-only dataset to produce 3D mesh models, point clouds and even true-orthophotos with almost no manual input. Yes, true-orthos from obliques and no nadir images. So the data captured can be used for delivering traditional orthophoto and DEM products, as well as photo-realistic 3D mesh models that the market is beginning to demand. We've seen this demand from the planning, asset management, smart cities and 5G development market sectors, among others.
Below is an animation of the camera footprint taking into account both XCAM sensors. The XCAM takes approximately 48 images as it flies around each circle. That's six times more information than a conventional high cost oblique camera system (naming no names!).
What does the coverage look like?
The diagram below shows the photo coverage achieved by a typical Circular Trajectory as seen from a point on the ground at the centre of the diagram (say a church). This survey would have included three lines of circles tracking North to South.
The green dots show the position of the camera relative to the church and the concentric circles indicate the ‘obliqueness’ of the photo (the nearest ring is 15 deg from vertical, the next ring is 30 degrees from vertical and the third ring is 60 degrees from vertical). This pattern of photo positions is ideal for 3D city model construction because it means that the church has been viewed a very large number of times (over 100) from every possible direction and inclination. This pattern is typical for the coverage of every point in the survey Area of Interest and makes the difference between a good 3D model and a stunning 3D model!
What is the Advantage of a Circular Trajectory?
A high-quality 3D City survey requires every point in the Area-of-Interest to be ‘viewed’ by the survey camera from a wide range of directions and a from a wide range of ‘inclinations’ (from almost directly overhead to almost horizontal to the ground). This wide range of views ensures that every point of the building is included in the resulting 3D Model and there are no ‘occlusions’ (i.e. parts of the building that are hidden for instance behind a tree or the buttress of a church wall). Conventional 3D modelling cameras are usually restricted to 5 sensors (4 oblique and 1 vertical) and therefore only 4 oblique views around a position. The survey is then flown using a North/South ‘lawn-mowing’ pattern which results in the majority of the views coming from North, South, East and West. Some expensive 3D modelling cameras have 8 oblique cameras which give a better distribution of looks – but nothing gets close to the exceptional distribution of looks achieved by XCAM using a Circular Trajectory.
Modern processing software companies such as Skyline who produce Photomesh confirm that the XCAM (using the Circular Trajectory) results in exceptional 3D Models. As well as urban city applications we've also seen the circular path be very useful in the production of 3D models of quarries and mines. The multi-oblique viewpoints mean every part of the sheer quarry surfaces are covered in very high detail.
Experience XCAM and the Circle Trajectory
If you're a user of 3D data then you need to start demanding that survey companies use this method as it's the only way to get stunning 3D models of this quality and cost. Capturing and processing this data is actually cheaper than commissioning LOD models so there's no financial restrictions on getting access to this superior data.