Corridor mapping includes project areas that are significantly larger in one dimension than another, e.g. railways, roads, rivers, etc. For more information about capturing a corridor: Step 1. Before Starting a Project > 1. Designing the Image Acquisition Plan > a. Selecting the Image Acquisition Plan Type.
We recommended that corridor projects include ground control points (GCPs) to ensure an accurate reconstruction. The minimum number of GCPs required for a corridor project depends on the same factors as many other types of projects, including the relative accuracy of the image geolocation, the amount of image overlap, and the length and width of the corridor.
We recommend that you allocate your GCPs so that they are offset from one another, regardless of the number of GCPs that you include in your project. You can consider including a pair of GCPs at each end of the corridor in addition to the set of offset GCPs that you collected along the length of the corridor.
|Figure 1. Recommended distribution of the GCPs in corridor mapping.|
Article feedback (for troubleshooting, post here instead)
please update or delete this page
a lot of confirmation has now been made that this is NOT the correct layout for corridor mapping...
GCPs should be set out in pairs, like ladder rungs as you go along the corridor
Thanks for the feedback. A heterogeneous distribution of GCPs is the general best practice that we recommended and worked for numerous users. If you experimented with more types of GCP arrangements that yielded better results, feel free to share your recommendations with us.
I have first hand experience of induced rolling cumulative errors in pix4D with the layout proposed here.
If you are going to use a chevron pattern like this, then you really need to close down the spacing to something like 1.5 to 2x your flight height.
My advice is to lay ground control in pairs, with the pairs positioned at direction changes in the alignment, and not greater than 2.5 to 3x the flight altitude apart.
Kind of like aeropoints recommend here
I am sorry to hear that this GCP distribution did not yield the best results for your projects, there is no "one-size-fits-all" when it comes to georeferencing. The suggestion provided by Propeller Aero is indeed very useful and I will consider it for our future recommendations. I would suggest posting on our community (https://community.pix4d.com/) and sharing your empirical insights to other users who have similar interests.
Hope to see you there :)
So what is the consensus on number of GCPs? Is there any rule of thumb for GCP's/ linear foot of corridor? I've got a corridor I'm wanting to fly that has airspace restrictions only allowing me to fly up to 100ft. This, along with restrictions of flight time due to battery life, shape of corridor, and length of corridor (approx. 1.8 mi), results in 12 flights needed to fully encompass the project extent. That's 60 GCPs if I go by the 5 GCP/flight standard, which seems really high. Could I cut down on the number of GCPs in this case? Thanks!
There is no rule of thumb for GCPs, but we do recommend using approximately 5 to 10 GCPs. I would also emphasize to take into consideration the homogeneous distribution and avoid placing GCPs on the edges (because they will be visible in less images). This article explains our findings regarding georeferencing quality and how more GCPs do not contribute significantly to increasing the accuracy, similar to the sigmoid curve (figure below): Do more GCPs equal more accurate drone maps?
So by 5-10 GCPs do you mean 5-10 per flight or per project? Thanks for the help!
If you want to merge more sub-projects, you should first add MTPs in your sub-projects, in order to use them for merging into one large project: https://support.pix4d.com/hc/en-us/articles/202558529-How-to-merge-projects. After merging, I recommend marking the GCPs (5-10) into this one large project, in order to georeference the entire study area.