Aerial photographs - such as those commissioned by the Ordnance Survey - are typically taken by aircraft flying at between 1,500m and 3,000m. Those taken by the world's air forces can be snapped from anything up to 9,000m. Photographs are either vertical (looking straight down) or oblique (taken at an angle to the ground).
Vertical photographs are taken for survey purposes and usually overlap by around 60%. When viewed through a stereoscope, the overlap appears as a three dimensional view.
This 3D photography gives significant advantages over two dimensional shots, says Henry: 'For example, we can see the depth of quarries, fill going back into quarries and see where the water table was when the quarry was open'.
Over the last 25 years Henry has worked on projects across the globe while watching his discipline develop in both scope and sophistication.
The biggest advance says Henry has been the switch - about 10 years ago - to the use of colour photography: 'A colour photograph, for example, will allow you to distinguish, for example, between different types of soil and vegetation more easily.'
The type of work Henry is asked to do, both by Arup colleagues and outside clients, has also changed.
'In the 1970s we had more motorway work or other types of long linear jobs which lent themselves very clearly to rapid interpretation of geology over big distances. Now, the balance of work is environmental - in that we spend a lot of time looking at contaminated land. It's not geomorphology any more, it's history. We're looking at what man has done in the past 50 or 60 years.'
The drive to exploit brownfield sites in particular has led to an increase in clients coming to Henry looking for guidance on the liabilities and costs they might face should they develop the site. Photo-interpretation of the site will not give all the answers, but it provides a highly useful guide which focuses the field inspection and site-investigation design and provides a rational framework for extrapolating the SI results.
And, of course, even the greenest of sites in such a highly developed country as the UK, can be brown.
'I looked at one scheme', remembers Henry, 'which the engineer swore was a greenfield site to the extent that he hadn't even bothered to do a desk study. But I was able to find a photograph from the 1950s, which showed there was a deep hole in the middle of the site. It had been so well restored and the fill had had enough time to mature that it looked like it had always been there.'
Every now and then, Henry is called in to provide evidence in legal disputes. This happened on the Holbeck Hall case, where the Scarborough hotel's insurers sued, with Arup's help, the local council for not taking the necessary measures to prevent the building slipping into the sea.
'We wanted to get as much recent photography as possible,' says Henry, 'and because it was a news event there was a lot of coverage. Even the coastguard took a video camera with them when they overflew the area. We were able to track the cliff failure at intervals of hours and days, see it go down and from that work out a view of the mechanism that was operating and the failure planes that were in the ground.'
The case is currently under appeal.
Henry says that the future of his discipline lies, to an extent, in digitised imagery - although that cannot give you 3D yet. One particularly exciting development is the TerraServer website (http://terraserver.microsoft.com) which contains digital images from around the globe. The source is a mix of US digitised aerial photography and Russian satellite photography. The latter has much higher resolution than conventional satellite imagery (such as SPOT and LANDSAT), although the fact that
it exists at all considering that the film had to be dropped from an orbiting satellite and then recovered is even more startling.
TerraServer allows users to call up the images required and, once a payment has been made, to receive them by post or, where the technology allows, simply download them to their own computer.