Strictly speaking, an ortho image or an ortho imagery mosaic is a different product to that of a traditional aerial photograph. Ortho imagery is generated from regular aerial photography however it differs in that an ortho image has been corrected for tilt and ground relief. In effect a bird's eye view is displayed for all features within the ortho image, whereas in regular aerial photography vertical planes not directly under the aircraft when the frame is exposed will be visible to the user. Important elements of ortho imagery are the raw photo overlap, the scale of photography, the scanning resolution and most importantly the pixel size of the final ortho image.

Click to view an Aerial to Ortho clip (Apple Quicktime plugin required)

Aerial photography projects rely on maintaining a percentage of ground cover overlap between successive photos along a flight run, and between adjacent runs. Features towards the edge of each frame must be visible in the neighbouring frame. To produce ortho imagery, overlap figures of 30% forward and 30%side are normal in rural environments with overlaps of 60% and 30% being required if the photography is over urban or mountainous terrain. When ortho imagery projects are required to deliver a Digital Terrain Model (DTM) 30% / 60% overlap is essential.

Click to view a Photography Overlap Demonstration

Aerial photography projects are usually defined by the scale of the raw aerial photos. A photo scale of 1:10,000 is commonly selected by urban municipalities whilst rural shires often opt for photo scales of up to 1:40,000. From a photo scale of 1:10,000, ortho imagery at 0.125m Ground Spacing Distance (GSD) can be supplied to the client. By itself, the scale of photography is not usually sufficient to define a specification. Scale is closely related to flying height with 1:10,000 photography sourced from an altitude of approximately 1,500.

The ortho photo process starts by scanning each frame into a digital format. The resolution that the raw print is scanned at is directly related to the final "pixel size" or Ground Sample Resolution (GSD) that can be discerned in the ortho photo digital product. A typical scanning resolution is 12.5 microns (12.5 thousandths of a millimetre), otherwise expressed as 2000dpi. A scanning resolution of 12.5 microns (0.0000125m) applied to a raw aerial photo scale of 1:10,000 will give a pixel size on the digital product of 0.125m. Clearly, a number of different photo scales and scanning resolutions can be combined to deliver products with the same pixel size or GSD.

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re: the Imatizer 2305 Professional Image Scanner

Otherwise known as Pixel Size or Ground Sample Distance (GSD). The resolution of an ortho photo is probably it's most important characteristic and is the criteria around which the scale of photography and scanning resolution are determined. In recent years, improvements in digital scanning equipment have meant that while traditional scales of photography, for example 1:10,000, 1:25,000 have not changed, the resolution of the final digital ortho photo has been improved. Recent aerial photography projects have been completed to deliver resolutions of 0.05m and 0.08m over urban LGA's and resolutions of 0.5m and 1.0m over rural areas. Click + and - signs to zoom in/out

The output from scanning an aerial photograph is usually saved as a TIFF file. Scanning a single 23cm by 23cm aerial photo at 12.5 microns will generate a TIFF file of approximately 1000Mb. Obviously there is a need to reduce this file size for the end user. Ortho photos are delivered in a range of image formats such as JPEG, MrSID and ECW. These are all examples of different compression algorithms being applied to the image file. To our eye there is very little difference between these formats, most of the differences come about in areas of image re-fresh speeds.

Geo referencing a photograph really just means assigning a map coordinate to each pixel so that the ortho image can be draped over existing data such as property and road vector data. Land Victoria's Vicmap products are often used as the "control" framework to geo referencing an image. True ground coordinate data (such as that from a GPS survey) can also be used to geo reference photography.

Because aerial photography is flown with overlaps between successive photography frames, a 3 dimensional view can be constructed of the terrain. This allows digital processing software to generate a Digital Terrain Model (DTM) or Digital Elevation Model (DEM) over the site. The accuracy of the DTM is directly related to the scale of the photography. As an example; with the flying overlaps (60% / 30%) 1:4,500 scale photography can be used to generate contour data at a 0.2m interval.

Click to View DTM Flythrough

Dodging is a process that is run to even colours across an entire image or block or photography. Dodger will correct brightness and colour imbalances due to:

1. Hot Spots: Bright areas which are usually caused by the sun position with respect to the imaging lens.
2. Vignetting: Dark areas in the image corners due to light "fall-off" occurring radically as a result of the imaging system.
3. Differences in scanner quality or scanner settings.
4. Differing film types.
5. Images acquired at different times of day or year.

Click to View Dodger Example