Guidelines for Best Practice and Quality Checking of Ortho Imagery

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Guidelines for Best Practice and Quality Checking of Ortho Imagery – Issue 3.0 Page 43 Image resolutionDefining the different types of image resolution (spatial, spectral, radiometric and temporal) is not inthe scope of this document (Poon et al, 2006). It is however important to make sure that the choicesbeing made when planning a project will be consistent and cost effective.3.1 GSDSince the introduction of digital technology the scale does not provide by itself a clear measure for thespatial resolution of the imagery as the size of the CCD element (respectively the scanning resolutionfor film imagery) has been introduced to the equation. The use of the Ground Sampling Distance(GSD) which represents the ground distance covered in a pixel has been established as the mostcommon measure of the spatial resolution of an image (although not a sufficient condition).[GSD = (H/f)*CCD]When orthoimage is to be produced it is the output pixel size that defines the GSD of the imagery. Incase of digital sensors the ratio of the final ortho resolution to the GSD is 1:1 whereas for filmcameras should be at least 1.2:1 (see 6.2.1).GSD size has great impact to the project cost for both analogue and digital airborne imagery;generally halving the GSD size will increase the cost of a project 2-4 times.3.2 Radiometric resolutionThe radiometric resolution of the acquired images should be at least 8bits/pixel but 11-12 bits ishighly recommended. The market seems to move towards even higher resolutions with most digitalairborne cameras operating already in 14-16 bits.3.3 Spectral resolutionThe spectral resolution of the imagery can be panchromatic, colour, NIR or IR. It is an importantdecision to make which can restrict the options of sensors or/and platforms for image acquisition andtherefore the final choice should be justified by the scope of the application.3.4 Temporal resolutionThis also a very important parameter to consider as it affects the cost and the time plan of the projects.The main questions to address concern with:• The use (or not) of archive imagery if available• Defining the window for the acquisition which can vary depending on the project’s scope(e.g. leaf-on imagery)• Defining the update cycle of the imagery
Guidelines for Best Practice and Quality Checking of Ortho Imagery – Issue 3.0 Page 54 Sensor calibration4.1 SensorsCamera calibration is a very important requirement at the photogrammetric process. Theintroduction of the digital airborne sensors has created a large variation of systems incorporatingdifferent imaging geometry (frame-line, single-multi heads) and can integrate auxiliary sensors(GPS/INS).Whereas digital airborne cameras are expected to operate under a similar workflow practice; andsuch systems are subject to the same QA requirements as standard film cameras, their internalgeometry can be very different than this of a film camera.4.2 Calibration of film camerasFor the analogue airborne cameras the calibration is a very well-established process taking place inspecialised laboratories which provide a camera calibration certificate normally valid for 2 years.Such certificates provide all the necessary information of the camera interior geometry (principaldistance, distortions etc) in a standard format and therefore it is not necessary to be described here indetail.4.3 Calibration of digital airborne camerasDue to the degree of complexity of the digital systems compared with the traditional analoguecameras the classic well established laboratory calibration process is changing and new calibrationapproaches are investigated. In so far the vendors provide camera calibration certificates based ontheir system-driven calibration processes either laboratory or in-situ.Appropriate geometric calibration, for example factory calibration or field calibration of theinstrument using an official test field (or validated by the instrument manufacturer), should becurrent (usually within past year).Radiometric calibration would normally be expected to be dependent upon factory certification andstate at least:• The level of live cells for each CCD array.• The radiometric resolution performance (at least 12-bit)• Metrics for the range of each spectral band (R,G,B, etc)Although such calibration methods should provide sufficient accuracy for most of the mappingapplications, the need for the establishment of generally accepted procedures for the certification ofdigital airborne cameras has been well-identified in the last years:• In Europe EuroSDR initiative on European Digital Airborne Camera Certification (EuroDAC 2 )has began since 2003 and for the coming years it is planned to test the geometric andradiometric aspects of the digital sensors in order to conclude on the strategy to be followed(Cramer, 2007).• In USA, USGS has decided to shift from individual sensor certification to the quality assuranceof the whole digital sensor’s product line. Following this strategy USGS has already beguncertifying commercial airborne digital cameras (Stensaas et al, 2007).
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Guidelines for Best Practice and Quality Checking of Ortho Imagery

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