UK measurement institute the National Physical Laboratory (NPL) has developed a method of comparing very high-resolution photos taken at different times to highlight areas where a structure has changed and degraded.
The software has enabled engineers to more quickly identify the parts of Hastings Pier that need most restoration following a serious fire in October 2010, reducing the overall cost and time of the operation.
Despite large visual changes, the cast iron framework or ’sub-structure’ was largely unchanged by the fire with a few localised exceptions
‘We’re not trying to replace human inspection; we’re trying to make it more effective,’ principal research scientist Dr Nick McCormick told The Engineer.
‘Once an area has been defined as having substantial change, that can be examined in more detail manually, but engineers can’t generally afford that degree of examination for the whole structure.’
The digital image correlation system compares two high-resolution panoramic images up to 1.4 gigapixels in size and uses computer algorithms to identify differences between them at the sub-pixel level to show how a structure changes over time.
For the software’s first use outside the lab, the NPL analysed two images of Hastings Pier that each combined 45 photos. These were taken using a robotic camera stand that was able to replicate the exact position of the first picture to within a 1mm accuracy.
The final images were 80,000 pixels wide — 300–400 times more detailed than a standard basic digital photo.
The NPL originally wanted to monitor how the 140-year-old pier was deteriorating following storm damage that led to its closure in 2006 and took the first set of photos in July 2010.
After the fire, the NPL was able to take a second set of photos to provide detail of the severe damage caused to most of the pier’s wooden buildings.
‘We were more concerned with the ability to detect small changes but then there was an extremely large change and that accelerated the development we needed to do to the software and also gave us a whole load of extra challenges,’ said McCormick.
The software originally required a vast amount of computer processing power to track changes at a level of one 20th of a pixel, an operation that took several hours using a network of 30 desktop computers.
The NPL rewrote the software to use an imaging technique that made measurements at half the level of detail but around 10,000 times faster and could run on a single high-specification desktop.
The software also had to be able to distinguish structural changes from lighting changes, although McCormick admitted there were still issues with analysing the edges of shadows.
Using digital image correlation, the NPL was able to show that, despite large visual changes, the cast iron framework or ’sub-structure’ was largely unchanged by the fire with a few localised exceptions.
The software could also be of particular use to those surveying structures such as bridges and tunnels, which often have to be inspected at night in order to close them to the public.
As well as being able to detect and monitor very small changes such as cracks over time, it enables engineers to make their initial inspection away from the structure and in the day.
‘The additional benefit is if it’s easier and cheaper to do the inspection then you can do it more frequently and start to build up a better time history of how the structure is changing,’ said McCormick.