Orthophotography: now and then
An orthophoto is a photographic image which is either geometrically corrected so the scale is uniform (like a map), or is taken in such a way that the camera position is level and perpendicular to the object surface, resulting in minimal distortion.
Orthophotos are used to measure true distances, and are therefore valuable for producing accurate plans and elevations of buildings and landscape. The process is best applied to photographically accessible and less complex architectural surfaces, such as wall surfaces or flat ground surfaces. The rapidity and versatility of using a photographic image-based system, for flat surfaces, can make it more cost-effective than laser-scanning, LiDAR or full Total Station survey.
In the 1980s, the Director of our Northampton-based team, Steve Parry, painstakingly created an orthophoto of a cemetery site. He captured a series of photograph using a rather ingenious tripod system. An image-mosaic plan was then produced by cutting and sticking photographs together to remove the distortions that were present.
Back to the present day and MOLA surveyors and photographers are using orthophotography to record and produce elevations of Knole, as part of the National Trust’s flagship project to restore the property in Kent. The Brew House walls required stone by stone elevations to be produced in CAD for both heritage management and to assist the architectural design process.
Onsite, temporary survey targets were put on the vertical wall surfaces at regularly spaced intervals and our surveyors located these in 3D on OS coordinates using a Total Station. Precisely located overlapping orthophotographs were taken at a set distance from, and perpendicular to the wall, to minimise distortion. Back in the office, the 3D survey data was processed with CAD. The orthophotos were then correctly located by matching the targets in the photo images to the surveyed targets. Linework was then drawn over the orthophoto. In this case, because the photography had been taken from carefully located camera positions, there was no need to process most of the images in specialist software. A few images which did require rectification were processed and transformed using ArcGIS.
For another recent project in the City of London we laser-scanned St Alphage Tower, as part of the London Wall Place development project. The inaccessibility, height and complexity of the ecclesiastical architectural remains meant that laser-scanning was undoubtedly the best data-capture solution, in order to provide a disaster management dataset (point cloud), and enable detailed architectural elevations to be made, as part of the analysis by our built heritage team.
We have been creating accurately located plans and elevations for decades but our methods have definitely advanced and increased over the years, allowing us to select the most appropriate method for the project, work quickly, and be more precise than ever.
As part of the National Trust’s project to conserve and restore Knole in Kent, MOLA’s Geomatics team has been undertaking a measured survey...
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