Coordinate conversion

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Dear,

can you please advise on the following coordinate conversion issue?

We would like to implement a 7 parameter conversion, however we are not able to get it working.

To implement the conversion we worked as follow :

we edited the *.prj file which was created by PIX4D for the selected coordinate system “TWD97 / TM2 zone 121”, and replaced the TOWGS84 [0,0,0,0,0,0,0] with the correct 7params for our area, as shown below

PROJCS[“TWD97 / TM2 zone 121”,GEOGCS[“TWD97”,DATUM[“Taiwan_Datum_1997”,SPHEROID[“GRS 1980”,6378137,298.257222101,AUTHORITY[“EPSG”,“7019”]],TOWGS84[-6.102270,10.161352,5.186370,0.05039095,-0.07010925,-0.05832248,-2.0293532674],AUTHORITY[“EPSG”,“1026”]],PRIMEM[“Greenwich”,0,AUTHORITY[“EPSG”,“8901”]],UNIT[“degree”,0.0174532925199433,AUTHORITY[“EPSG”,“9122”]],AUTHORITY[“EPSG”,“3824”]],PROJECTION[“Transverse_Mercator”],PARAMETER[“latitude_of_origin”,0],PARAMETER[“central_meridian”,121],PARAMETER[“scale_factor”,0.9999],PARAMETER[“false_easting”,250000],PARAMETER[“false_northing”,0],UNIT[“metre”,1,AUTHORITY[“EPSG”,“9001”]],AXIS[“X”,EAST],AXIS[“Y”,NORTH],AUTHORITY[“EPSG”,“3826”]]

 

Afterwards we checked the PIX4D coordinate conversion in the drone image/location preview window (bottom right), but we don’t see any difference

in the computed coordinates with or without the 7p values entered into the prj file.

In reality the 7p should apply a shift of circa 0.8m.

What is the correct way to implement a 7p coord conversion in Pix4Dmapper?

 

Thank you for your helping.

Best Regards,

Kevin Lu

 

2

Hey Kevin,

I would advise creating a totally new prj file and renaming it with a unique name, so that Pix4D does not use the old prj from its database. Do not forget to reprocess the project, so that the change will be taken into consideration. 

Let me know if this worked.

Best,

Teodora

3

Thanks Teodora,

Will try it out.

Thanks for the GREAT support provided by PIX4D.

Really appreciated!

 

Regards,

Kevin LU

 

 

 

4

Hi Teodora,

Thank you for your suggestion.
But it still doesn’t work with the new prj file…
The shift is still there.
Attached is our new prj file, is this file matches your meaning of “totally new prj file”?
Please tell us what is wrong with it.
Thank you!!

Regards,
Kevin LU
TPC_TWD97%207p|690x42

5

Hello again,

This might be related to a syntax problem in the Well-Known Text format. Please make sure you inserted the correct square brackets and the shift was also correctly computed. I am also not sure if the last EPSG should be left inside the .prj file, because the software might confuse it with the real, unshifted EPSG 3826:
PROJCS[“TWD97 / TM2 zone 121”,GEOGCS[“TWD97”,DATUM[“Taiwan_Datum_1997”,SPHEROID[“GRS 1980”,6378137,298.257222101,AUTHORITY[“EPSG”,“7019”]], TOWGS84[-6.102270,10.161352,5.186370,0.05039095,-0.07010925,-0.05832248,-2.0293532674] ,AUTHORITY[“EPSG”,“1026”]],PRIMEM[“Greenwich”,0,AUTHORITY[“EPSG”,“8901”]],UNIT[“degree”,0.0174532925199433,AUTHORITY[“EPSG”,“9122”]],AUTHORITY[“EPSG”,“3824”]],PROJECTION[“Transverse_Mercator”],PARAMETER[“latitude_of_origin”,0],PARAMETER[“central_meridian”,121],PARAMETER[“scale_factor”,0.9999],PARAMETER[“false_easting”,250000],PARAMETER[“false_northing”,0],UNIT[“metre”,1,AUTHORITY[“EPSG”,“9001”]],AXIS[“X”,EAST],AXIS[“Y”,NORTH],AUTHORITY[“EPSG”,“3826”]]
Could you try to change it to a unique number, which does not resemble to any of the already existent EPSG codes and also rename the .prj file accordingly?
Cheers,
Teodora

6

I have modified the .prj file a bit and changed its name, could you try using it in Pix4Dmapper and let me know if this worked?
3826777.prj (957 Bytes)

7

Hi Teodora,

We finally got our Pix4D mapper license!
But the conversion problem is still there…
The prj file you gave us is not recognized by Pix4D mapper, it couldn’t be applied.
Could you please check again if the prj file has some problem?

Second, we encountered a confusing situation for using GCP.
We have a flight with the drone phantom 4 RTK.
After the flight, we processed it 2 times,

  1. with 1GCP, no 7 parameters
  2. with 4GCP (one of them is the same as previous) , with 7 parameters

The results of two post processing have a shift at Horizontal positions and for some area Vertical as well.
Our questions are:

  1. Are the two processing should have same result? Although they use different 7 parameters, they both have a same GCP, right?
  2. Is the number of GCP (1 & 4) influencing the result of post processing? is this the reason they have a shift between each other? Also, if it is, what would be the difference between applying 1 & 4 GCP?
  3. If the number of GCP is not the reason, is the shift caused by the 7 parameters?

Third, we have a question about the Pix4D license.
Now we apply this license to a gmail account, but later when this project finish, we would like to transfer this license to another email account which is only for next project.
Is it possible to do this? I couldn’t find a way to change my email address in the account settings.

A lot of questions, but we are quite satisfying with using the Pix4D mapper!
Thank you for the helpings.
Best regards,
Kevin LU

8

Hi,

I don’t know why the .prj file is not working in your project, it worked in my project. Anyway, the idea is to create/modify a similar .prj file, but you do need some background knowledge regarding WKT synthax, this is not trivial. Usually, our users find and download the .prj files on websites such as https://spatialreference.org/ or http://epsg.io/.
I would contact a local surveyor for this kind of request, unfortunately we do not have a complete overview of all the coordinate systems out there (they must be thousands).

Concerning the georeferencing process, there should be a sufficient number of GCPs (we recommend 5-8) distributed in the project. The GCPs should be placed homogeneously in the area of interest. Imagine the area as a large table and the GCPs as the legs that will support it. If all the “legs” are placed at the same location of the “table”, then it will tilt. If the “legs” are homogeneously spread, then the “table” will be stable. Additionally, it is also recommended to place one GCP in the center of the area in order to further increase the quality of the reconstruction. To answer your question, no, the georeferencing should not be identical, because there was a different number of GCPs used.

Please ensure that the GCPs were correctly measured on the field or correctly marked on the image dataset (try marking the GCPs in all the images where they can be identified). Do not forget to insert the correct coordinate system in which the GCPs were measured.

To answer your license-related question, the users cannot change the email address themselves. Please contact our support team to request the change and we will get back to you as soon as possible: https://support.pix4d.com/hc/en-us

Cheers,
Teodora

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