Guidelines for Best Practice and Quality Checking of Ortho Imagery

Guidelines for Best Practice and Quality Checking of Ortho Imagery – Issue 3.0 Page 65 Airborne image acquisition5.1 ScopeThe recent advances of the digital airborne cameras provide different options for getting to thedesired product but they can also create difficulty for making the right choices. The scope of thischapter is to outline the main issues to be addressed for data acquisition planning and execution.5.2 Flight plan and executionThe flight planning for airborne image acquisition should ensure that issues related to height aboveground, overlaps, sun angle etc. issues are adequately addressed.5.2.1 Flying heightThe flying height is derived using the equation of 3.1 and it should be defined so that the images willbe acquired at the predefined GSD.Aquisition of 0.5m GSD airborne imagery800070006000h (m)5000400030006.5 µm9 µm12 µm15 µm20 µm20001000060 80 100 120 150f (mm)Figure 1. Flying height for different camera configuration (f, CCD size).5.2.2 Image overlapsForward overlap should be at least 60% when stereo imagery is collected whereas for mosaicking 20-30% could be used but with the compromise of using off-nadir part of the images during theorthorectification (not recommended).Side overlap must ensure that all the surveyed area will be covered with imagery and generally it willbe in between 15 – 45% depending on the terrain relief. When fully automatic triangulation isforeseen the overlap should be up to 60%.5.2.3 Scale variationScale variation is caused mostly due to variation of relief and will result in varying GSD. Usuallyshould not exceed 10%-15% (depending on flying height) but in mountainous areas could be difficultto achieve.5.2.4 Roll and driftIn average roll should not exceed +/- 1 degree across the flight with a maximum of 3 degrees for asingle image. The displacement due to drift should not be more than 10% of the image width for anysuccessive 3 images.