Guidelines for Best Practice and Quality Checking of Ortho Imagery
Guidelines for Best Practice and Quality Checking of Ortho Imagery
Guidelines for Best Practice and Quality Checking of Ortho Imagery
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<strong>Guidelines</strong> <strong>for</strong> <strong>Best</strong> <strong>Practice</strong> <strong>and</strong> <strong>Quality</strong> <strong>Checking</strong> <strong>of</strong> <strong>Ortho</strong> <strong>Imagery</strong> – Issue 3.0 Page 148 Airborne image orthorectification QA8.1 ScopeThis section outlines the process <strong>of</strong> creating orthophotos from airborne imagery, from the perspective<strong>of</strong> assuring final product quality. The guidelines detailed here are generally valid <strong>for</strong> medium scale(1:20 000 to 1:40 000) scale source air photos <strong>and</strong> the geometric tolerances are based on the ASPRSmap accuracy st<strong>and</strong>ard (ASPRS 1989, FGDC 1998) <strong>and</strong> it is known to be achievable if the datacapture <strong>and</strong> processing specification given in these guidelines is followed.Geometric correction tolerance is defined using one parameter: the maximum permissible RMSE <strong>of</strong>the check points. Tolerances are as stated in the relevant ITT.8.2 Geo-referenceGeo-referencing concerns with the determination <strong>of</strong> the exterior orientation elements <strong>of</strong> the sensors<strong>for</strong> the time <strong>of</strong> image acquisition. The number <strong>and</strong> pattern <strong>of</strong> GCPs recommended <strong>for</strong> possible flightconfigurations are listed into the table below:Purpose/MethodOrientation <strong>of</strong> a single modelBlock adjustment <strong>for</strong> aerial triangulation,without airborne DGPSDGPS controlled flight with cross strips(CBA-Method: Combined Block Adjustment)DGPS controlled flight (no cross strips)(OTF-Method: Ambiguity resolution “on thefly”.)DGPS/INS controlled flightNumber <strong>of</strong> GCPsMinimum four (allows <strong>for</strong> testing <strong>of</strong> residuals)One GCP every five base lengths (minimum) on the perimeter <strong>of</strong> theblock <strong>and</strong> one GCP across all flight strips, every four base lengths.One double GCP in each corner <strong>of</strong> a block <strong>and</strong> one double in themiddle. Possible additional requirement <strong>of</strong> cross strips <strong>and</strong> morecontrol within irregular blocks.Ambiguities which are not solved are removed as systematic errorsin the Block Adjustment at great distances possibleOne double GCP in each corner <strong>of</strong> a block <strong>and</strong> one double in themiddle.GPS Reference stations should be at a range <strong>of</strong> 50-100 km fromsurvey area, depending on possible interference.One double GCP in each corner <strong>of</strong> a block <strong>and</strong> one double in themiddle.Table 4. Flight configurations <strong>and</strong> recommended GCPs patterns8.3 <strong>Ortho</strong>rectification processTable 5 provides tolerances <strong>for</strong> each stage <strong>of</strong> the air photo orthorectification process. Themeasurements corresponding to each tolerance can be used to provide quantitative input to QCRs
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