In the end the reality is they map the manholes with an accurate GPS based device and get a solid XY&Z reading. It can be 6 inches off and not affect the reality of finding it again. They probed with a rod and found a solid surface. Used a shovel to clear off majority of the dirt, that found the hole used to remove the lid. Then one guy got his big sledge hammer and just whacked the lid hard. That makes the lid jump enough to loosen the edges. He hit it with the sledge 5 or 6 times with a full force swing. I asked if they break the lids and he said sometimes. The lid of course is cast iron and pretty stout. The drug the lid off, and did the measurements. I left then, I had seen the real action.
Of course once they have the XYZ of each lid they can plot them together and see the suystem, and add in the data from the camera and measurements.