Correcting Elevation of an entire set of images from Phantom 3 Pro

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Hi,

I’ve just been working on a geology mapping project in a mountainous region with a phantom 3 pro. Generally we’ve been very happy with results. We’ve been flying up to 6 grid missions per day, plus some free-flight and getting very nice results. For the final output we would like to combine the data from all the days (8 or so) of surveying.

The problem I’m finding is that there is often a significant change in recorded elevation from one day to the next (presumably due to errors in the GPS exif data recorded by the phantom). So in the overlapping regions between days there is a strange artefact of a large area of ‘floating land’ , around 20m or so above the terrain. 

I’d like to know the quickest method for correcting this. I’m not particularly concerned about the absolution elevation being exactly correct, but I’d like the overlapping regions between days to agree. Is there any way to subtract or add metres of elevation from an entire group of images or to raise or lower a part of a DEM? I hope this makes sense & I guess other people must have come across this issue before when surveying large areas of periods of several days.

Thanks

Fergus

2

Dear Fergus,

Have you added Manual Tie Points between projects? This is important as it gives extra information to the software and helps it merge the projects correctly. It sounds like your issue could be solved by adding a few MTPs in the region of overlap between the two projects. 

You can find the procedure to merge projects here: 202558529

And how to add MTPs here: 202560349

Best regards,

3

Try using the exit out and exit in commands on the photo import page before doing your initial processing. 

You will need to know your rough takeoff elevation and elevation of your mission lines. You can then to a manual adjustment in a csv or excel editor to the elevations to make them near to true altitude and bring them back into p4d using exit in. We do this all the time and get very accurate initial results.

 

best to have a hand held gps such as a Garmin or a barometer to record your takeoff altitude. Always take acouple of photos before takeoff and when landing so that you an do the height adjustment.

 

we have found that while the exit altitudes can be well off in the inspire the relative height differences of photos from each fought are usually pretty good. We all need to put pressure on DJI to bring back the functionality to have the barometric altitude written to the exit file.

4

Hey Tony, do you mind expanding on what you mean by exit out and exit in commands?

I’m in the midst of trying to make a multi-day mission level in processing. I have a few flights that are out in absolute altitude by up to 100m, but as you say, the relative altitude is still good data for the DSM. So far I’ve tried batch editing the exif data with exiftool but am having some difficulties.

Any help would be great.

5

Hi Bill the spell checker autio corrected my words in the above post. It should have read exif in and exif out. I use a simple csv format and edit the adjusted height’s in MS_Xcel then re-import them into P4D.

If you post your email address it would be better to communicate directly to explain as I don’t want to give a way too much intellectual property.

 

6

Thanks for the reply. You can reach me at bilswz(at)gmail.

Any advice would be super helpful. I’ve been trying to edit with exiftool in the commandline and its a nightmare.

 

cheers

7

https://community.pix4d.com/t/5220/comments/211540026

 

Tony, I have a similar question as Bill regarding editing the exif file to change the altitude. Can you please reach me at karyritter (at) gmail. 

 

 

8

I get exactly the same issue on the P4 Pro - every time I change batteries, the machine’s best estimate of its absolute take-off altitude is at best murky, but for certain, never the same, and different enough that it you use the photos from these various sorties in the same project, then you can generate a double / ghost surface… or worse even… introduce a mythical global tilt in order to get these two table-tops of information to align.

I agree that using GCPs would / should fix the problem, but a very simple fix, to effectively bring all the photos into a single sortie, would be to be able to batch-edit the exif data for elevation to make it all the same number.

this isn’t a pix problem, its a problem for ALL photogrammetry software… so I don’t really expect a pix solution… but does anyone else know a decent batch editor for exif?

9

@lee paterson

I use excel for that kind of batch processing. Export GPS-coordinates from Pix4d into .cvs file. Open .cvs in Excel use find/replace function (example: elevation in original 375.225, wanted elevation 395.225, find 375. and replace it with 395. make sure you put the dot at the right place so it won’t replace any other 375 sequence if it exists). Then import the modified .cvs back to Pix4D.This method has worked like a charm for me. Used it several times

10

Hi Kary and Lee. Are you still looking for answers. There are two very simple processes. If you are still interested  send me your email addresses and a quick note of Pix4D Altitude correction in the subject line.

my email is tony(at)nikkelsurv.co.nz

11

nnicoll80@gmail.com for anyone that has any guidance for us non python fluent speakers