hi, g.r speeks of the ability to constract local lorentz frame. how can an observer construct such a frame if spacetime is curved? what are his rods and clocks? it seems that if one tried to construct a "hive" of coords using a ruler, then it will not cross as expected... he might even notice that tiangle angles do not sum to 180... i guess it all depends on accuracy of measurement but i find it hard to visualize and calculate that accuracy. it also is hard for me to understand how increased accuracy will cause tidal forces. increased accuracy means that can't construct flat space and therefore can't measure tidal forces... please help and elaborate an explanation if you have one... i am going crazy here (and out of hair).