Adding GCP's without known coordinate system

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

We have some data that is from a client, collected from a Phantom 4 using a 3rd party app other than Pix4dCapture(unfortunately) to collect the photos.  I was not totally happy with the path that the app took to collect the photos and I have survey grade GCP’s, but we do not know the coordinate system.  It really does not matter that much to us either.

Pix4d makes a nice ortho image by using the geolocation from the photos, but when I bring in my GCP’s and choose arbitrary for the coordinate system I get larger differences between the Initial Position and the Computed Position.  What I do is import my coordinates, then pick the GCP’s, click re-optimize and then run steps 2 and 3.  I have tried running step 1 again, with the GCP’s inserted, but it will scramble my images and not use all of the images. 

I think my issue is that after I insert the GCP’s and pick the points on the images I need to re-run step one, but there is not the best overlap with the images due to how the flight was run.

My question is whether or not there is a way to let the images georeference using the image location, lock this position and then drop it onto the ground control points?

Hopefully my rambling made sense.

2

Yes, your rambling makes sense. No Worries…

It sounds like your Phantom performed a different route than what you were expecting, and that is not a problem - As Long As - there is enough photo overlap to your Area of Interest, and enough photos of your Aerial Targets for your GCPs.

That said, in Pix4D there are 3 Coordinate Systems:

A. Photo (Input) Coords - (WGS84 - if photos are taken with a drone) (This is usually low-grade GPS - like 3-5m accuracy)

B. GCP Coords - This is the system your control points were taken in

C. Output Coords - This is the system you want your output to be in.

So, if you process the project with WGS84 as the input coord system, and WGS84 as the output coord system, it creates a nice orthomosaic.  Good.


Now, in order to tie that to ground using your GCPs, you will need to do the following: (You've probably done all of this)

1. Import or enter your Ground Control Points. (Be careful in your accuracy measure here. Were your GCPs shot with RTK Precision - (1.2cm accuracy), or did you use a Standalone Receiver (300cm accuracy) {Right side of the GCP Table})? 

2. Choose your Output Coordinate System (Top Line Menu-\> Project -\> Output Coordinate System)

       - Are your Northings and Eastings in a National (State Plane Coord System)? -\> Choose the Correct Projection from the Menu

       - Are your Northings and Eastings in a Local(Arbitrary) System [like 5000,5000,1000]? -\> Choose Arbitrary Coordinate System

3.  Choose your GCP Coordinate System (At the top right of the GCP/Manual Tie Point Manager Screen)

       - Press the [Edit] button and usually this will match your Output Coordinate System (see above Step 2).

4. Mark Targets - Be sure when you mark your Aerial Targets that you are marking the correct photo, with the correct target. 

       - Its best to mark the ground next to the target (before you fly) with the GCP number. This makes it easier for processing.

5. Re-run from Step 1. (You could do Process -\> Re-optimize, but sometimes that doesn't quite work, and the fix is usually - re-run Step 1).

A.) If you did everything as above, and then, when you run Step 1 - the project is ‘scrambled’, then you probably have one or more ‘bad GCPs’.

    - Try changing the GCP’s to Checkpoints, and then you will see where the “GCP’s” are relative to the project without shifting the point cloud. You may have to remove the GCPs one at a time until you find the ‘bad one’. (to speed this troubleshooting process - just process with the Rapid Check)

B.) If the Point cloud is flipped upside down, and a mirror image of what is true, then -  you have reversed your Northings and Eastings -   - Reverse your Northings and Eastings in the GCP/MTP Manager, and the project should be right side up and correct view.

C.) Larger Differences between the Initial and the Computed Position - remember that the Begin point (of the photo) is located with a low-grade GPS on the Phantom(Not RTK precise), and then your GCPs are located with a high-grade GPS (probably RTK precise).  This will lead to some shift in the photos after you tie the project to your GCPs (possibly a few meter shift to the computed position). 

Let me know if that helps.

Regards,

Sean Erickson.