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question about vertical (z) accuracies

I have a question for you folks.  What kind of vertical accuracy can you achieve with say a Phantom 4 when using precise (1-2 cm) GCP taken with the right equipment?  I know the initial GPS tagged image can have significant errors but can processing with hard and accurate GCP’s eliminate this large error?  The use case I am looking at is evaluating volume changes on a construction site (estimating volume of dirt moved in a 2 week period) and I’m on the fence if I really need to get a drone with RTK corrected location or just get high quality control points.  I know that the RTK drone will in fact give a better result, but what can I expect without it.  Some of the areas of interest on the site have fill volumes down to 2", so I am not sure I would be able to effectively measure this with my current setup.



Hi Gabe, 

I will share two articles I think are relevant to answer this question: 

The first article describes relative and absolute accuracy and what you can expect from your outputs. The accuracy you can expect is generally expressed in multiples of the average Ground Sampling Distance (GSD) of the project. On the Z axis, this is expected to be between 1 to 3 times the GSD. If the GSD is 2cm, the expected accuracy is 2 to 6cm. This can change depending on the parameters of the project, but it’s a fair enough estimate.

For volume measurements, you will mostly need relative accuracy. GCPs can help for that (although their first usage is generally to have a higher absolute accuracy). Another option than GCPs would be to add a scale constraint to your project. The second article describes the errors you can expect on your volumes.

Thank you

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 Same question on the two options: which one suits better and what are their pros&cons?

RTK GPS on board vs. high quality GCPs