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Increasing Relative Altitude Accuracy

Hello, I am still learning so please forgive any ignorance on the subject.

 

Long story short, I am attempting to obtain fill volume measurements empty pits to be later filled with water.

 

We have taken scans of several of these pits and I continue to run into the issue of there appearing to be a several foot elevation difference between opposite corners of the pits. The images I am attaching show only a ~2ft difference but more often than not I get results showing an 8-10ft difference between corners (which makes it appear that one corner of the pit would overflow before the rest), so this one of the better results that I have gotten.

In actuality these pits are level and not built on a downhill slope of any kind so I know this elevation difference is an error. 

How can I increase the accuracy of the elevations/altitudes across a wide area? I understand that GCPs or MTPs are probably going to be my answer, but I am unsure of the best way to go about using this feature. Do I need to just paint some boards to place in the project area for me to later mark on the GCP/MTP Basic Editor, or do I need to invest in a survey-grade GPS tool to be able to obtain accurate xyz coordinates to place into pix4d? Or are there some other settings I can tweak in the mission planner app (such as increasing my overlap) to obtain more accurate results?

Hi Chad. GCPs are definitely the way to go. You may not need to buy GPS equipment if you aren’t collecting GCPs that often. You can contact a surveyor in your area that will come out and collect the coordinates for you and that will be the best option if you don’t need to collect that often. You can do the cost-benefit of the cost of having the surveyor collect the points vs how often you will be collecting points to decide if you need to buy GPS equipment. Keep in mind that operating the GPS takes a level of expertise to ensure accurate results. 

For this project, you would do something like the following.

  • Layout GCP targets
  • Fly the area and collect images for processing
  • Have the surveyor provide you with the precise GPS coordinates of the targets. (you need to know what coordinate system was used to collect the GCPs)

Ideally, you will collect the GCPs using whatever ellipsoid corresponds to the horizontal coordinate system used. You can also tell the surveyor to collect in one of the supported geoid models (EGM96, EGM2008). This is a simpler workflow than if they are collected in another geoid or something like NAVD88.

  • Process step one 
  • Standard GCP workflow. 

If the targets are able to remain fixed you can continually refly and as long as the targets have not moved you can use the same coordinates if you need to do multiple flights over time.  After a while, the GCPs will start to be less accurate due to continental drift and plate tectonics depending on the area but this is not super relevant in the short/mid-term. Review the materials in the support article below to help clarify the GCP workflow. 

https://support.pix4d.com/hc/en-us/articles/202558699-Using-GCPs

 

Thanks for the reply Holden, 

 

That’s pretty much what everything I’ve learned the past few days seems to be pointing towards. I’ve done some experimenting with scale constraints and MTPs with sub-optimal results. My next move will probably be looking into having a surveyor take some GCPs for me then.

 

 

Sounds good. Make sure you review the GCP material as there are things to consider in terms of what CS is used to capture and where to place the GCPs. Happy Mapping.

I have another question for you, do you have any recommendations of low-cost GNSS Receiver solutions for taking GCPs?

 

So far I’ve came across the reach RS+ by Emlid which seems to be a great option, as well as a really cool product specifically designed for drone mapping called AeroPoints by AeroPropeller. 

I have not had any experience with the RS+ so I cannot say. The propeller aero points are nice but there have been some issues with vertical coordinate systems as the orthometric height models used are not natively supported in Pix4D and require a bit more advanced knowledge to get processing successfully. You can consider creating a new post in the community asking for some advice on a good GPS given your use case, budget and experience level.