 # Calculate necessary change in frontlap/sidelap for a given altitude change?

The effective sidelap and frontlap decreases as the altitude above ground decreases (all other parameters being equal such as shutter frequency, etc). I’m flying a site that isn’t flat and at one point the ground elevation will be 100ft higher than the starting point of my mission.

The drone will be flying at one altitude AGL based on the starting point. How do I calculate the increase in frontlap and sidelap setting I need to get the same effective sidelap/frontlap at a certain lower altitude AGL?

Hi Jeff,

I will combine the information of these two articles of our Knowledge Base into a formula to compute the impact of height on the overlap:

1. Computing the image rate for a given frontal overlap: https://support.pix4d.com/hc/en-us/articles/202557479
2. GSD calculator: https://support.pix4d.com/hc/en-us/articles/202560249

Based on article 2. the Ground Sampling Distance (GSD) can be calculated in either of the two forms below:

( 1 ) GSD = (Sw*H*100)/(Fr*imW) = (Sh*H*100)/(Fr*imH)

with Sw = Sensor width (mm), H = flight height (m), imW = image width (pixels), Sh = sensor height (mm), Fr = focal length (mm), imH = image height (pixels)

Based on article 1. and assuming the camera is oriented with the sensor width (long dimension) perpendicular to the flight direction the frontal overlap can be expressed as:

( 2 ) x = ((imH*GSD)/100)*(1-overlap_f)

( 3 ) overlap_f = 1 - ((100*x)/(imH*GSD))

with overlap_f = frontal overlap (%), x = distance between two camera positions in the flight direction (m), GSD = Ground Sampling Distance (cm/pixel).

If we combine ( 1 ) and ( 3 ) we have:

( 4 ) overlap_f = 1 - ((100*x*Fr*imH)/(imH*Sh*H*100)) = 1 - ((x*Fr)/(Sh*H))

A similar calculation leads to the side overlap (still assuming the same orientation of the camera than above):

( 5 ) overlap_s = 1 - ((y*Fr)/(Sw*H))

with overlap_s = the side overlap (%), y = the distance between images of two flight lines.

The equations ( 4 ) and ( 5 ) give you the overlap in function of the flight height. You can plug in the lowest and highest altitude to see what overlap you would get with your current parameters and adjust if necessary.

In general if there are large variations in terrain height it is recommended to split the flight into smaller flights to keep a constant distance to the ground. More in this article about image acquisition plan for terrain with height variations: https://support.pix4d.com/hc/en-us/articles/204656209

Also make sure that the largest GSD is not be more than twice the lowest GSD in the project. This can lead to problems in the reconstruction of the project: https://support.pix4d.com/hc/en-us/articles/202558979

Here is a last article that can be useful on how to select camera focal length and flight altitude considering the desired GSD and terrain to map: https://support.pix4d.com/hc/en-us/articles/202558849