In case this news does not reach you via another forum…
I have just completed a whitepaper on groundplane measurements for loudspeakers. This reviews existing techniques and presents a new one that I recently developed. The document also describes how groundplane measurements can successfully be performed on non-traditional surfaces like grass or open ground.
I hope you will give it a read and think about trying these methods yourself.
You can find the whitepaper here in my GitHub repository: