The grounds for all utilities should be the same point. This is required by the NEC (article 250, 800, 810, 820, and 830). The maximum distance allowed is 20' before supplemental grounding in required, which must be tied into the grounding system.
The reason for this is that the resistance of the earth varies, and by using the same grounding point, we can make sure that the electrical relationship between the grounded devices is maintained within the system.
Within any building, the goal is that (ground) = (ground) = (ground), and not (ground) = (ground +2 volts) = (ground +4 volts).
Voltage differential between grounds = current flow = electrical noise when grounds are connected together thru the connected equipment.
If the voltage differential is high enough, damage to equipment can occur, or injury to humans can occur if they are connected between grounds.
For this reason, all utilities (water, gas, electrical, telephone, catv), and all other metallic cabling (antennas, etc) should all be grounded to the main building ground, which may be multiple grounds tied together as a common ground system.
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