One of the limitations of many terrestrial laser scanners is acquiring onboard photography that is representative of reality. This can be quite challenging and/or time consuming to achieve well – the main reasons are low resolution, poor exposure due to difficulties in obtaining accurate settings for a 360 degree scan, and the high chance of external noise in the form of traffic, inqusitive bystanders or careless scanner operatives.
We have recently added an iSTAR Fusion camera to our scanning hardware. The iSTAR has 4 x pre-calibrated sensors delivering a 50MP full spherical accurate image with excellent high dynamic range. The iSTAR has several different modes, all of which are very straightforward to use and the one recommended for dealing with sites with variable lighting is HDR (high dynamic range).
Using the iSTAR requires a change to the normal workflow. After acquiring a scan as normal, the scanner is swapped for the iSTAR on the same tripod, and the imagery acquired. It is important to mount the camera so that its nodal point is coincident with that of the scanner mirror. Depending on the adapter you choose, it adds around 30 seconds to the time to swap the scanner for the iSTAR, but then only around 15 seconds to capture the full 360 degrees HDR image. There is a wireless facility to trigger the camera from a smartphone, or a countdown so that you can get out of shot in time. The short image capture time means that it’s possible to time taking the image with when the coast is clear of people or vehicle traffic.
The site presented in this post was chosen because it has a wide range of materials and surfaces, plus very challenging light conditions with the sun almost overhead, a dark building in shadow and bright paving in full sunlight.
FINAL RESULTS EXAMPLES: hover mouse over the images below to compare the Topcon RGB with that achieved with the iSTAR.
In all cases where we have used the iSTAR, the results have been excellent. The second of the examples above is slightly underexposed for the iSTAR and slightly overexposed in the Topcon onboard imagery. The manufacturers advise that we could have used a different exposure preset in this instance to get a better result.
Post processing is as easy as taking the pictures. The ColourCloud software comes free of charge with the iSTAR and simply requires an exported (non-registered) .e57 file of the pointcloud to be copied into the subdirectory containing the raw imagery from that scan. The result is another .e57 file that is renamed and has the added RGB information. This can then be re-imported into your chosen registration software for post processing.
It’s a minor hassle having to do the above, however the ColourCloud software is very fast in comparison to colouring a cloud using the onboard photos in Topcon Scanmaster – around 90 seconds versus over five minutes (depending on source file point density).
In our tests and subsequent live jobs, we were not able to discern any spurious colouring of the points (including matching up pictures and text on a billboard around 10m away).
Another advantage of this process is that the iSTAR software outputs an equirectangular panorama of the imagery which can be viewed in one of the the many commercial or open source panoramic viewers. NCTech have an cloud based panorama viewing platform, and the image from the above case study are shown below.
This brief writeup is not intended as a comprehensive review of the iSTAR and also does not aim to suggest that the onboard imaging of the Topcon GLS-2000 is of no use. Not all projects / scans require the level of photographic imaging that the iSTAR affords and the one minute acquisition using the wide angle onboard camera provides perfectly adequate images for field records and colouring in some instances. The Topcon scanner also has an internal telephoto camera which has been written about in another post and found to be excellent – and provide a higher pixel resolution at range than the iSTAR.