is it necessary to place GCP on top of a very large stockpile to get accurate volume calculations ?https://www.youtube.com/watch?v=z-KcxsU7lWs
Ground Control Points (GCPs) will increase the absolute accuracy of the model, i.e. the model will be correctly placed on a coordinate system. However, by adding GCPs you will also improve the relative accuracy of the model.
You can check about accuracy here: https://support.pix4d.com/hc/en-us/articles/202558889