Focus length of P4p when on Infinity


#1

Hi people
I’m new to this site and to mapping.
Having done a few 3D maps now I have a site that is severely slanted and also has buildings on. I’d like to get as much detail as possible but want to set a single focus length for the grid flight (I will get the orbit shots manually or by POI). Is there an ideal height to fly in order to set the focus on Infinity and forget about it? That is to say if I have a 70ft drop how high do I need to be from the highest point before I can set the focus to Infinity?


#2

I would try and fly no closer than 200 feet over the top of the highest object.
Sometimes two separate flights at two different altitudes are necessary.


#3

Hi @Bazza,

Welcome to our community! I hope you’re able to learn a lot and get the help you need here.

I would also recommend taking a look at our blog post 4 Ways to Improve the Accuracy of Your Drone Models with 3D Mapping Software or our 3D Models Support page. There are a handful of useful tips in there from a pro. :slight_smile:

Cheers,
Christina


#4

The infinity point of a lens is generally 10x the focal length. The lenses for these cameras have incredibly short focal length ( small detector area ); normally less than 10 mm. So if you are beyond 100 mm ( 4" ) you are at infinity focus. No problem.

Spatial resolution ( how small an object can I see ) and elevation resolution ( precision of my elevation measurements) are competing figures of merit. Fly low for spatial resolution, fly high for elevation resolution. The quality of a 3D image is the elevation resolution. So what is good enough for your customer?

I could go through all the theory, but most people’s eyes cross. In the end I would agree with Gary, fly 200’. I fly an Inspire 1 with a zenmuse S-3 imager. At 200’ my GSD is 1 inche. At 270 feel, my GSD would be ~1.5 inches. My vertical elevation resolution will be about < 6 inches. This should be good enough for most people.