Step 1. Before Starting a Project > 1. Designing the Image Acquisition Plan > c. Computing the Image Rate for a given Frontal Overlap

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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)

D = Dh = (imH * GSD) / 100     (4)

Where: 

Dh = 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 = Dh - overlap * Dh

x = Dh * (1 - overlap)

x =  ((imH* GSD) / 100) * (1 - overlap)     (5)

 
Note: x is given in [m], considering that the GSD is in [cm/pixel].

Combining the equations (3) and (5):

t = x / v = ((imH * GSD) / 100) * (1 - overlap) / v     (6)

 
Example: In order to achieve an overlap of 75% (overlap = 0.75) and a GSD of 5 [cm/pixel], supposing that the image height is 4000 [pixels] and the speed of the UAV/plane is 30 [km/h] = 8.33 [m/s], based on the equation (6), the image rate should be 6 seconds: 

t = ((imH * GSD) / 100) * (1 - overlap) / v = ((4000 * 5 ) / 100)  * (1 - 0.75) / 8.33 = 6 [s] 

 

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 = DW = (imW * GSD) / 100     (7)

Where: 

DWdistance 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 = DW - overlap * DW

x = DW * (1 - overlap)

x =  ((imW * GSD) / 100) * (1 - overlap)     (8)

 
Note: The result is given in [m], considering that the GSD is in [cm/pixel].

Combining equations (3) and (8):

t = x / v = ((imW * GSD) / 100) * (1 - overlap) / v     (9)

 

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