TSA Client Guide - Digital Orthophotography

8/13/2019 TSA Client Guide - Digital Orthophotography

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DIGITAL ORTHOPHOTOGRAPHY V1. 0ndorsed by

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CONTENTS

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hat is an Orthophoto?

n orthophoto is a single or series of aerial photographic images in which displacements caused by scale, terrain relief and camera

rientation have been removed. Orthophotos have the high visual information content and familiarity of a photograph but contains theeometric qualities of a map and can be used in a multitude of GIS and mapping applications.

Ocinrthophotos can be presented as hard-copy or as digital image data. The hard-copy is presented as a photograph and can have

ontours, grid information or other map details overlaid and can be inserted into a format. Digital orthophotography can be used for

terpretation, measurement, quality assurance, or combined with vector map data as a backup to a GIS or CAD model.

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urrently, orthophotos are generated in a digital environment, the digital image being rectified to an orthographic projection by

rocessing of each individual pixel through photogrammetric computations derived from photo identified ground control points,

am and a digital elevation or terrain model. These functions are normally performed on a digital photogrammetric

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hat is required to produce a digital orthophoto?

order to generate a digital orthophoto from aerial photography acquired by an analogue camera, it is first necessary to translate the

erial imagery into a digital format. This is achieved by scanning the original film negatives or film diapositives with a geometrically and

adiometrically precise digital scanner. Some photogrammetric scanners have the facility to scan roll film, this leads to significant

provements in scan times for large projects with block photography where 500ft rolls of fi lm may have been used. Once scanned,

ach image pixel will have a radiometric value and an XY coordinate set unique to that image. Colour film is scanned to record the red,

reen and blue bands. Consequently, an image file from a colour photograph will be three times larger than a black and white one.

is important to assess whether the scans were generated on a photogrammetric or desktop scanner. A photogrammetric quality

canner will provide far superior levels of geometric accuracy and resolution, radiometric performance and colour functionality. The use

f a photogrammetric scanner will result in improved image and geometric quality, particularly when considering large scale, high-

esolution orthophotography.

he scanned images are subject to photogrammetric orientation to remove distortions associated with the aerial camera, and scale

nd relief displacement of the ground control points. Calibrated camera information is used to remove radial lens distortions and affine

lm shrinkage. The relative and absolute orientation process removes parallax between two adjacent, overlapping photographs and

hen mathematically scales and levels the stereo-model to the ground control points.

t this stage it is necessary to supply a Digital Elevation Model (DEM) or Digital Terrain Model (DTM) to orthorectify every pixel for

hanges in relief. This is required because an array of pixels that is positioned at the top of a hill will have a different scale to an array of

ixels further away from the camera in a valley. A DEM is normally associated with a regular grid of elevation data, whilst a DTM is

ollected using a more extensive layout where the regular elevation grid is supplemented by terrain breaklines that more accurately

efine significant changes in terrain data that relates to elevated features such as bridges and overpasses that require particular

ttention. The orthorectification process is usually undertaken in a digital photogrammetric workstation.

era calibration

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A typical orthophoto project will often consist of blocks of aerial photographs that have been rectified and mosaiced together.

Mosaicing involves balancing the radiometric values concerning colour, brightness and contrast so that a continuous tone, seamless

orthophoto is produced. The final orthophoto is often cut into manageable tiles that may correspond to National Map Agency map

sheets or to an existing vector map layout.

The imagery is sometimes delivered in a data pyramid, where lower resolution images of the base file are delivered together. This has

an advantage for the user in that the smaller lower resolution images can be used for review or coarse interpretation. The smaller files

are more easily managed and viewed rapidly. The higher resolution images are still available for more detailed analysis and

interpretation.

Accuracy versus Resolution

It is essential for the users of orthophotography, particularly those with a non-photogrammetric background to distinguish between

accuracy and resolution. A product that is quoted as 25cm orthophoto may refer to either the pixel resolution of the image or the

absolute spatial position to the ground coordinate system.

It is unlikely that a 25cm size feature could be recognised from an image of the same resolution. Photogrammetrists generalise that the

ground resolution should be half the size of the object to be resolved. If a 50cm feature needs to be identified, then the ground

resolution should be no greater than 25cm.

An image that is scanned at a ground resolution of 25cm may have a significantly lower accuracy if poor quality planimetric control was

used to scale the product or if a low accuracy DEM was used to correct for relief.

The quality of the DEM/DTM used to orthorectify the imagery is an important factor in the resultant accuracy and aesthetic quality of

the orthophoto. If a 50m regular grid elevation model was used to rectify a 10cm resolution image of an urban area from 1:3,000 scale

photography, it is highly likely that the grid would fail to identify sufficient changes in relief to accurately correct the height

displacement caused by changes in relief. This would result in inaccuracies of scale and would be most noticeable if adjacent mages

contained a linear feature such as railway that ran along an embankment. The linear feature could be distorted badly and may evenresult in a step between images.

It is therefore essential that the following information is known:

> Scale of source aerial photography> Camera calibration data> What medium was scanned (negatives, diaposotives, contact prints)> Information regarding the scanner used and the resolution of the scanned image> Ground control information> Orientation and rectification solution> The source and quality of the DEM/DTM used to rectify the imagery

If in doubt, a TSA member company would be pleased to advise on this subject.


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Metric/Imperial Scan Resolutions

Microns

(0.001mm)

DPI (dots per inch)

7 3629

14 1814

21 1210

28 907

Size of Uncompressed Data File of a Single 230cm x 230cm Format Aerial Photograph at Different Scan Resolutions

7 Micron 14 Micron 21 Micron 28 Micron

Colour 3199Mb 800Mb 355Mb 200Mb

B/W 1066Mb 267Mb 118Mb 67Mb

Ground Resolution of Aerial Photography at Different Scan Resolutions

Photo Scale 7 Micron 14 Micron 21 Micron 28 Micron

1:2,500 0.018m 0.035m 0.05m 0.07m

1.5,000 0.04m 0.07m 0.11m 0.14m

1:10,000 0.07m 0.14m 0.21m 0.28m

1:25,000 0.18m 0.35m 0.5m 0.7m

1:40,000 0.28m 0.56m 0.84m 1.12m

1:50,000 0.4m 0.7m 1.1m 1.4m

Size of Uncompressed Data Files for Different Ortho Dimensions based on Different Ground Resolutions Colour

Ground Resolution 500 x 500m 1km x 1km 2km x 1km 5km x 5km 10km x 10km

0.05m 300Mb 1200Mb 2400Mb 30,000Mb 120,000Mb

0.1m 75Mb 300Mb 600Mb 7500Mb 30,000Mb

0.2m 18.75Mb 75Mb 150Mb 1875Mb 7500Mb

0.25m 12Mb 48Mb 96Mb 1200Mb 4800Mb

0.5m 3Mb 12Mb 24Mb 300Mb 1200Mb

1m 0.75Mb 3Mb 6Mb 75Mb 300Mb


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Black & White

Ground Resolution 500 x 500m 1km x 1km 2km x 1km 5km x 5km 10km x 10km

0.05m 100Mb 400Mb 800Mb 10,000Mb 40,000Mb0.1m 25Mb 100Mb 200Mb 2500Mb 10,000Mb

0.2m 6.25Mb 25Mb 50Mb 625Mb 2500Mb

0.25m 4Mb 16Mb 32Mb 400Mb 1600Mb

0.5m 1Mb 4Mb 8Mb 100Mb 400Mb

1m 0.25Mb 1Mb 2Mb 25Mb 100Mb

DIGITAL ORTHOPHOTOGRAPHY V1. 0

Documents

TSA Client Guide - Digital Orthophotography