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The image shooting rate to achieve a given frontal overlap depends on the speed of the UAV/plane, the GSD and the pixel resolution of the camera.

D = distance covered on the ground by one image in the flight direction [m] overlap = percentage of desired frontal overlap between two images od = overlap between two images in the flight direction [m] x = distance between two camera positions in the flight direction [m] v = flight speed [m/s] t = elapsed time between two images (image rate) [s] 
Figure 1. 
From Figure 1, we obtain the following equations:
od = overlap * D (1)
x = D  od (2)
t = x / v (3)
Two cases are possible:
 Camera oriented with the sensor width (long dimension) perpendicular to the flight direction (usual case)
 Camera oriented with the sensor width (long dimension) parallel to the flight direction
Camera oriented with the sensor width (long dimension) perpendicular to the flight direction (usual case)
D = D_{h} = (imH * GSD) / 100 (4) Where: D_{h} = distance covered on the ground by one image in the height direction (footprint height) [m] imH = image height [pixel] GSD = desired GSD [cm/pixel]


Figure 2. Sensor width placed perpendicular to the flight direction. 
Combining Equations (1) and (4) into Equation (2):
x = D_{h}  overlap * D_{h}
x = D_{h} * (1  overlap)
x = ((imH* GSD) / 100) * (1  overlap) (5)
Combining the equations (3) and (5):
t = x / v = ((imH * GSD) / 100) * (1  overlap) / v (6)
Camera oriented with the sensor width (long dimension) parallel to the flight direction
If the camera is placed on the plane / UAV having the sensor width (long dimension) parallel to the flight direction:
D = D_{W} = (imW * GSD) / 100 (7) Where: D_{W} = distance covered on the ground by one image in the width direction (footprint width) [m] imW = image width [pixel] GSD = desired GSD [cm/pixel] 

Figure 3. Sensor width placed parallel to the flight direction. 
Combining equations (1) and (7) into the equation (2):
x = D_{W}  overlap * D_{W}
x = D_{W} * (1  overlap)
x = ((imW * GSD) / 100) * (1  overlap) (8)
Combining equations (3) and (8):
t = x / v = ((imW * GSD) / 100) * (1  overlap) / v (9)
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