Guidelines for Best Practice and Quality Checking of Ortho Imagery

Guidelines for Best Practice and Quality Checking of Ortho Imagery – Issue 3.0 Page 20ResamplingVisual accuracycheckAccuracy of themaster imageFor imagery unlikely to be quantitatively analysed/classified bilinear interpolation or Cubic convolution isappropriate; output pixel size ≅ input pixel size.Nearest neighbour may be used if justified (e.g.classification), but output pixel size should be 0.5x inputpixel size.Overlay digital map data on the image and inspectsystematicallyMeasure the accuracy of the master image using checkpoints which were not used as GCPs during geometriccorrection.Record summary results in metadata/QCR.Record resampling method and output pixelsize.Independent check by supervisor.Log Pass/Fail and inspection date for thisimage in QCR.Minimum of 20 check points distributed on aregular grid.Accuracy: 3 x tolerable RMSE.File dated record of the check results.Record result in metadata and identify asmaster image.Table 9. Specification for Satellite Image Rectification9.5 QCRs and quality audits for satellite image rectificationA file naming convention should be introduced and a meta-database developed which allows thefollowing information to be associated with each image product and any supplementary files (e.g.GCPs, checkpoint results):• Image ID, Master Image ID, Project site ID, Sensor, Acquisition date, View angle or beamnumber, Cloud cover, Product level, Initial QC (OK/Problem), Pre-processing (e.g. filtering),DEM grid size or average distance, DEM accuracy, Result of DEM QC.• Software Used, Blunder check completed, Number of GCPs, Residual RMSE(metres),√PRESS(metres), Correction method (poly, ortho), Order of Polynomial, Resampling method,Output pixel Size, Number of checkpoints, Checkpoint RMSE, Maximum CheckpointDiscrepancy, Production Date, Comments, Operator name.Further information should include:• input and output file names, sources of ground control, projection details, detailed results ofthe DEM checks, corner co-ordinates and result of visual QC signed and dated by asupervisor.It is strongly recommended that a paper pro-forma designed to record all the information listed aboveis devised by the contractor, there should be one form for each output image and the relevant datafrom these can then be entered into the metadata database.A procedure should be applied to ensure that the final product is clearly labelled as such and that theinformation retained in the QCRs is that which applies to this final productContractors will generate the QCRs identified above for their Internal QA. They should be madeavailable for inspection during a quality audit. The type of quality audit is shown in Table 10 as“Normal” or “Tightened” (see 2.3).