Photoscale and ground cover

If you need to know what the ground cover for a given camera at a given height will be the photoscale formula will do this for a true nadir shot of a level surface.

Photoscale=f/h

where f= focal length of the lens  and  h= the height above the ground

Because we want to know the ground cover rather than the photoscale we can use the ratio upside down to ‘scale up’ the sensor size to match the flying height: Ground cover = Sensor size x (h/f) Some typical KAP cameras give these values:

Theoretical NADIR ground cover at 60 and 100m AGL

 

a

b

f

h

(h/f x a) x (h/f x b)

Camera

Sensor x

Sensor y

Focal length

At a height of

Area on ground

Sony RX 1

35.8mm

23.8mm

35mm

60m

61m x 40m

 

 

 

 

100m

102m x 68m

Ricoh GR

23.7

15.7

18.3

60m

77m x 51m

 

 

 

 

100m

129m x 85m

Canon S100

7.6

5.7

6*

60m

76m x 57m

 

 

 

 

100m

126m x 95m

HD GoPro 3

5.76

4.49

2.77

60m

124m x 97m

 

 

 

 

100m

207m x162m

Canon Eos M

22.3

14.9

22

60m

60m x 40m

 

 

 

 

100m

162m x 108m

*widest setting on ‘tele/wide’ control

Of course ‘real world’ results will vary as flying heights are rarely consistent and introducing camera tilts changes the aspect of photocover hugely. These values do not take into account variations in ground height or image resolution.

Although photoscale can be applied to scaling from photography it is not recommended as it is a purely theoretical value.  The ratio only applies to a truly orthoganal projection of an image free of lens distortion and tilts in the image plane (a particularly severe problem as KAP gives very small h and f  values compared to classic air survey values) as these have a significant effect on the accuracy of the scale ratio across the image area.

This is the classic diagram:

Focal Length

B

About billboyheritagesurvey

Heritage worker
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