Flight planning altitude calculation?

I am trying to understand how does Dronedeploy finds the altitude value for a specific GSD.

For a Mavic Air camera sensor we have the following specifications:

  • sensor type: 1/2.3” CMOS

  • sensor size: 6,33 x 4,7 mm

  • image size:4056 x 3040 pixels

  • focal length (f): 35mm

To calculate the pixel size on image (lower case gsd) we can use the sensor size and the image size: gsd = 6,33/4056 = 1,56x10^(-6)m

Now, if i want a GSD = 1,3cm, I can use the following relation:

f/Altitude = gsd/GSD =>

0,035 / Altitude = 1,56x10^(-6) / 0,013

(0,035 * 0,013) / 1,56x10^(-6) = Altitude =>

Altitude = 291,666m

But Dronedeploy specifies an Altitude of 36m for a 1,3cm GSD! Why?

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Solved! I found out that it is necessary to use the real focal length of the camera as follows:

FR = (F35 * SwR) / 34.6 (mm)

where:

F35 = focal length that corresponds to 35 mm equivalent [mm]
FR = real focal length [mm]
SwR = the real sensor width [mm]

For the Mavic Air:

FR = (24mm * 6.17mm) / 34.6mm = 4.27977mm = 4.3mm (2 sig figs)


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