In the Star Trek:TNG episode "The Defector" Data States:
RIKER: A cloaking device of some sort, to hide the entire base?
DATA: A cloaking device operating on the surface would be given away by visible distortion effects.
In an earlier season 1 episode of TNG "When the Bough Breaks" Data States:
DATA: Sensors indicate that the shield is electromagnetic, a complicated light refracting mechanism.
PICARD: A cloaking device?
DATA: Aye, sir.
LAFORGE: It's got to be pretty sophisticated to hide an entire planet.
A planetary cloaking device would have to mask it's enormous energy output as well as its gravitational pull.
Why would a cloaking device on a planets surface have a visible distortion effect as Data stated?
If an entire planet were cloaked in an earlier episode that Data knew about, why would he state that a smaller portion of a planet's surface makes distortions more visible?