About the photographs ...
University of Hull
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All of the aerial photography used on this site has been kindly provided by Getmapping plc.
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Getmapping’s educational department ‘Wildgoose’ is well known throughout both primary and secondary schools in the UK and offers the imagery at reduced rates in both hard and soft copy. Please contact Lynette Rowbottom or Jo Marriott on 01530 518568 if you would like to receive a catalogue or discuss any of the educational products. Alternatively visit the Getmapping Web Site and follow the links to education. |
Flying
A mixed fleet of twin engined aircraft are used to fly the photography. Each is equipped with forward motion compensated (FMC) large format mapping cameras. Two camera types are used, Zeiss LMK1015 and Leica RC30, both are FMC equipped and typically weigh about 500 lb. In most areas the planes have flown at a height of 5500 feet, with a focal length of 15 cm creating 1:10,000 scale photography. All of our survey work is carried out in stereo, meaning that the same point on the ground is photographed from two different vertical aspects with a 60% forward overlap on each of the negatives.
Large format colour negative film is used. Each film is 135 metres in length and compromises of approximately 500 negatives. All imagery is flown to Royal Institute of Chartered Surveyors (RICS ) specification.
Scanning
Once the films have been processed in the photo lab they are scanned at 1200 dots per inch (dpi 1200 ). Each frame is approximately 330mb in its uncompressed form. It is then compressed down to a JPEG (high quality) making each file approximately 30-40mb in size. Once the film has been scanned, each images is visually checked to make sure that the density and colours are satisfactory, rotated to make sure each of the images are north-facing and then saved onto DLT tape.
Two types of scanners may be used. A repro scanner is used for all Getmapping film but where required images may be processed on the photogrammetric scanner, which scans the negative to geometric accuracy of +/-2µ (microns).
Rectification
Once the photography has been scanned it is then rectified and mosaiced to create a seamless dataset. Initially the images are colour balanced through an automatic process which opens each image and ensures that it is tonally even.
Rectification means making the data ‘map-accurate’. The data are often referred to in terms of resolution, the majority of Getmapping's photography being 25cm resolution (1 pixel on the screen is 25cm on the ground). With the exception of the Northern Ireland photography, the imagery has been georectified to Ordnance Survey Land-line® data. In some areas the imagery has been fully ortho-corrected using the OS Profile® digital terrain model (DTM) as a source of height control.
- Georectification – ground control points (GCPs) are observed on both the OS Land-line® and the photograph, e.g. road junctions and field boundaries. Any distortion in the photograph caused by height variation is not taken into account. One image will require between 20 and 80 control points, the images are simple fitted to the mapping.
- Orthorectification – this method takes terrain height into account and is a more technical method. A DTM is used to drape the photography over. Control points are again taken from OS Land-line®.
Mosaicing
Once rectified and colour balanced, each image is then manually mosiaced to its neighbouring images to create a larger image (6x 6km). This may involve up to nine images. The cut-line is observed along the overlap between two images following linear features e.g. roads, rivers, hedges etc. to hide the join. When fully mosaiced the image is cut into 1km tiles which coincident with the OS grid.
Quality Assurance
Each tile is examined in Photoshop for errors or artefacts caused by scanning or mosaicing. Final colour checks are made to minimise any errors created in the final colour balancing software or due to any seasonal variations of the adjoining frames.