That would be the centroid of the simulation objects. Didn't mention where you wanted to get the centroids. Inside DOPs, each simulation object has a pivot point (p) defined from the origin along with the current position (t) in the Position sub-data. Just add the two gives you the pivot point world location. You can access this with the dopoption() or dopoptions() expression. For example in any DOP parameter processing the RBD objects, you could fetch "p" by: dopoption("/obj/AutoDopNetwork