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Coordinate System of GCPs - GDA2020 (new datum for Australia)

Hi Pix4D Technical team,

We flied Phantom 4 RTK and obtained RGB images whose coordinate system is recognized by Pix4D as WGS84 / UTM Zone 56S (we are in NSW of Australia); The coordinate system of GCPs collected by our surveyor is MGA2020 (I think it is GDA2020, the new datum for Australia), zone 56s, Height is on AHD71.

Just wondering if I can import the GCPs with that specific coordinate system, and ensure they are compatible with the coordinate system of images at Pix4D for processing.

If not, how can I do to make them compatible?

Your help is much appreciated.

Cindy

Hi @cindy.wang,

First of all, you have to download the latest Pix4Dmapper preview 4.6.1 where GDA2020 is included and create a project from scratch. Afterwards, in my opinion, you should do the following:

  1. Add images and ensure WGS84 / UTM Zone 56S = EPSG:32756 is detected.
  2. In Select Output Coordinate System window, select Advanced Coordinate Options and look for EPSG:7856 in Known Coordinate System. In the Vertical Coordinate System select Arbitrary.
    Explanation: At the moment, Pix4Dmapper doesn’t support any geoid except for the global geoids EGM 84, EGM 96 and EGM 2008. That’s why AHD71 is not available. As a workaround, you should set your vertical output coordinate system to arbitrary and your vertical GCP coordinate system to arbitrary, but leave your collected GCP coordinates untouched. This will fit and export the vertical model according to your GCPs.

Let me know if it works.

Thanks! :slight_smile:

Hi Thank you for the reminder of updating the most recent version of Pix4D. Will contact Sales Pix4D about the payment.

Not sure how long the purchasing process will take, but I need to process the data and get the result ASAP. Therefore, I am transforming the GCPs from GDA2020 to GDA1994 which is included in Pix4D 4.4.12.

For vertical coordinate system set up, can I just follow your suggestions, and try the same steps???
Any suggestions, please feel free to let me know.

I am getting the GCPs with GDA1994 late this afternoon, and will try processing data after, and let you know how it goes.

Thanks !!

Hi @cindy.wang,

My apologies for the late reply. Yes, please follow the same steps and let me know how it goes.
In case something happens I’m here to support you :slight_smile:

Regards!

Hi Thank you for your reply. I followed your suggestions and ran the Pix4D Mapper 4.4.12, and got reasonable results.

However, I have another question about Volume calculation and Point cloud Classification.

I am doing Point cloud classification in order to get DTM and Contour.

If I do Point cloud Classification, Will this affect volume calculation? If so, How??

Many thanks!

Hi @cindy.wang,

Thank you for your message.

During the point cloud classification each point is automatically classified in one of the following predefined groups:

  • Ground
  • Road Surface
  • High Vegetation
  • Building
  • Human-Made Object

This result is used for the DTM generation, not for the DSM.

The volume calculation, however, is based on the DSM, not on the DTM.

But, if you, for instance, observe an unwanted object in the stockpile and you will remove it by performing a manual classification and assigning the points to Disabled category, rerun Step 3 it will indeed have a direct impact on the volume computation. Learn more in this article.

I hope that it helps!

Cheers!

Hi Good morning!

Thanks for your prompt reply. It is very helpful :slight_smile:

Can I ask one more question about Contour Line creation? It seems the default Elevation interval for Contour line is 10m, Any reason for this?
and I changed the Elevation Interval to 1m. Is it OK? How is the quality of 1m Contour Line?

Many thanks in anticipation.

Hi @cindy.wang,

Indeed, the default parameter is set to 10m. However, it isn’t a fixed number. You can freely change it to 1m and verify whether you’re satisfied with the results. Afterwards, you can adjust them accordingly to your expectations.

Cheers!

Hi Can I ask two more questions?

  1. How to adjust the contour line? Any reference documents or video?

  2. What is the quality of DTM and Contour Line over vegetated areas like bushes or woodlands?

Do you have any suggestions regarding how to improve the quality over vegetated areas?

Many thanks!

Hi @cindy.wang!

Please find the related material below:

If you won’t find the information you are looking for in those links. Please create a new topic in our Community. Thanks! :slight_smile: