The system is straightforward. We take digital photos at different angles of an object or place you want us to survey. We scale them by using a known distance in two photos or by transforming them onto known co-ordinates.

In the office, we load the pictures into our computer. Using a new photogrammetric system for 3D-coordinate location, we mark pairs of points on the photos. We keep marking pairs until we have enough to compile an image of your subject. The software then constructs your object digitally. With care and several photos, you can pick up 20 mm differentials across the survey. You can adjust each point at pixel level.

The system works with anything vou can measure, but is especially effective with building facades, landscaping projects, hilly terrain—anywhere vou can get a stereo pair of photos.

'This idea is not new, but the difficulty was calibrating cameras,' says Geocomp Systems' Jerry Cresp. 'The software developers have now done the full-blown mathematics behind the calibrations, so the method is no longer restricted to highly-accurate cameras.' It takes you through a short calibration sequence in which it works out the lens parameters for your camera. Do this once and it recognises your camera forever.

'Passing phenomena like floods and moving vehicles can be logistically difficult to survey. But two digital photos are all you'd need for this system,' says Jerry. 'And it's used in the US to map traffic accidents so the road can be cleared quickly.'

Data can be exported to any CAD package by means of DXF files or ASCII files.