# Image Measurement,GSD Calculation, Altitude and Fly Height

GSD Calculation
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Lets start with Figure 1 below. All values represented below is totally based on DJI Phantom 3 Professional drone data. Figure 1

Hence using formula;

GSD= (Sw x H x 100)/(image width x Fr) = (35mm x 33m x 100)/(4000 x 20)=1.44cm/pixel

The calculation print out from other website http://www.aerogis.de is shown below for validation purposes.

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***GSD CALCULATION REPORT***
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INPUT PARAMETERS:
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Sensor Width in [mm]:..35
Focal Length [mm]:…..20
Image Cols/width:…………4000
Image Rows/height:…………2250
Altitude aGr [m]:……33
CALCULATED PARAMETERS:
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Sensor Height [mm]:….19.687500000000003
Sensor Pixelsize [mm]:.0.00875
Footprint Width [m]:…57.75
Footprint Height [m]:..32.48437500000001
Footprint Area[sqm]:….1875.9726562500004
Footprint Area[ha]:….0.18759726562500004
Footprint Area[sqkm]:…0.0018759726562500003
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. .Figure 2
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Image Measurement
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The results are identical. We also carried out measurement direct on the image using IC Measure application from https://www.theimagingsource.com to validate that the width of image footprint on the ground which is 58m as calculated previously, see Figure 3. Ground Control Point (GCP) or On Ground Scale (OGS) with diameter of 1.5m was installed for measurement calibration purposes, see Figure 4.
. Figure 3 Figure 4
Altitude and Fly Height
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The altitude of where the image taken was derived from photo EXIF data, see Figure 5. The EXIF data shows that image was taken at the altitude of 68.717m (hereafter referred as ALP). The original level where drone take off was derived from Google Earth and our 3D reality modelling in Figure 6. The altitude where drone take off is 35.62m (hereafter referred as ALT). Substracting ALP and ALT will get the different of approximately 33m, where also known as “fly height” or “elevation” or H value keyed in Figures 1 and 2. The spatial reference system used to determine altitude in our 3D reality modeling is WGS84:EGM96.
. Figure 5 Figure 6
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