# Step 1. Before Starting a Project > 1. Designing the Image Acquisition Plan > b. Computing the Flight Height for a given GSD

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] FR = 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 (F35) 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 FR is given by:

FR [mm] = (F35 * SW) / 34.6     (1)

Where

F35 = focal length that corresponds to the 35 mm equivalent

FR = real focal length

Sw = the real sensor width

Using the fact that

H / FR = DW / SW,

the flight height H is given by:

H = (DW * FR) / SW .     (2)

The distance covered on the ground by one image in the width direction (footprint width) is given:

DW = (imW * GSD) / 100.     (3)

where

DW = 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 * FR) / (SW * 100)     (4)

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

Example: Computation of the flight height to get a GSD of 5 [cm/pixel] using a camera with a real focal length of 5 [mm] and a real sensor width of 6.17 [mm]. Assuming that the image width is 4000 [pixels] and using the equation (4), the flight height should be 162 [m].

H = (imW * GSD*  F) / (Sw * 100) = (4000 * 5 * 5) / (6.17 * 100) = 162.07 [m]

Tools: The GSD Calculator can be found here: TOOLS - GSD calculator.

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### Article feedback (for troubleshooting, post here)

• Lim Eontaek
`What is the formula for the real focal length F(R) for a 3: 2 ratio?`
• Ina (Pix4D)

Hi Lim,

In your case in the formula FR [mm] = (F35 * SW) / 34.6  you will have to substitute the f35 = 34.6 f /w mm with f35 = 36.0 f /w mm. The conversion you can find it here: https://en.wikipedia.org/wiki/35_mm_equivalent_focal_length .

Hence the formula will become FR [mm] = (F35 * SW) / 36 .0

Best,

Ina

Edited by Ina (Pix4D)
• Lim Eontaek

thanks for Ina ^^