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The Ground Sampling Distance (GSD) is the distance between the center of two consecutive pixels on the ground. It influences the accuracy and the quality of the final results as well as the details that are visible in the final Orthomosaic.
The flight height H that is needed to obtain a given GSD can be computed and depends on the camera focal length, the camera sensor width [mm], and the image width [pixels].
Sw = real sensor width [mm] F_{R} = real focal length [mm] H = flight height [m] Dw = distance covered on the ground by one image in the width direction (footprint width) [m]

Some lens manufacturers give the focal length (F_{35}) in the 35 mm equivalent. It is not the 35 mm equivalent but the real focal length that should be used in Pix4Dmapper. In order to find the real focal length, some computations are needed. In the case of a 4:3 ratio, the formula for the real focal length F_{R} is given by:
F_{R} [mm] = (F_{35} * S_{W}) / 34.6 (1)
Where
F_{35 }= focal length that corresponds to the 35 mm equivalent
F_{R =} real focal length
Sw_{ }= the real sensor width
For more information about the 35mm equivalent focal length concept: Wikipedia article.
Using the fact that
H / F_{R} = D_{W} / S_{W},
the flight height H is given by:
H = (D_{W} * F_{R}) / S_{W} . (2)
The distance covered on the ground by one image in the width direction (footprint width) is given:
D_{W} = (imW * GSD) / 100. (3)
where
D_{W} = distance covered on the ground by one image [m] in the width direction (footprint width)
imW = image width [pixel]
GSD = desired GSD [cm/pixel]
Combining equation (2) and (3), the flight height is given by:
H [m] = (imW * GSD * F_{R}) / (S_{W} * 100) (4)
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