# Number and distribution of GCPs in Corridor Mapping .

I have to survey a 3 mile (5 KM) road using an UAV. I have gone through following links

https://support.pix4d.com/hc/en-us/articles/202559299

https://support.pix4d.com/hc/en-us/articles/202557459#label6

and understood that there should be around 8-10 GCPs, but the distance of length of flight is not clear for 8-10 GCPs requirement. What should be the distribution of GCPs, whether it should be in zig zag ( kind of random) or it should be linear in pairs ?

The road is surrounded by the vegetation, and the UAV will be flying at the height of 100 m with 5.2-26 mm camera.

the software will make a more accurate surface if you have equally spaced points, but also, points that reflect the ‘highs’ and ‘lows’ of the road. I would suggest putting points on either side of the road, but also on the middle of the road (if the road drops or rises relative to the natural surface.) I.e like a 5 points pattern.

Thanks for the reply ! Can you also clarify that, for what corridor length 8-10 GCPs are required ? And how many GCPs will be sufficient for 3 miles corridor, if we take 5 point pattern ?

Thanks !

I never really went on any training for this so I cannot give you definitive answers on what configuration will work in your site, trial and error will lead to experience. why are you limited to 8-10 GCP’s? that seems fine though. 100m height flight/26mm lens. what frequency do you take pictures at? how much overlap?

in any case,  if it is possible, experiment with using 3, 5, 10 GCP’s (or take 10 GCP’s and then re-run the process by systematically eliminating one from the calculation and look at the quality report, if your mean error on the gcp’s are ‘low’ enough, then you know you have a good accuracy with that GCP configuration.

Actually I was going through the links -

https://support.pix4d.com/hc/en-us/articles/202559299

https://support.pix4d.com/hc/en-us/articles/202557459#label6

and they mentioned that 8-10 GCPs will be enough. But they didn’t mention about the corridor length. So I was confuse that for how much distance they mentioned to take 8-10 GCPs.

At every 2-3 seconds I will be taking pictures and I will be keeping >85% frontal overlap and around 75% side overlap.

I am also curious about how to distribute GCPs, and how many to include, in corridor mapping after looking at the referenced support articles.

My corridor is 3 miles (5km) by 200 feet (60 meters).

I also saw that someone got an answer of having 3 GCPs/per flight (https://community.pix4d.com/t/4950-How-many-GCP-s-#gsc.tab=0). I am curious if that only pertains to varied terrain or should be utilized in all cases?

Bikram,

Have you learned anything about GCP distribution since your posting? I am doing a 1.8 mi corridor flight and would appreciate any advice.