At the out set I doubt if this system is still current.learnt it about 20 yrs ago,
never the less here goes
T/A:is 1000' above the highest obst within 25nm radius of the stn rounded off to the next higher 1000.
T/L: is 1500' above t/a when stn QNH is <1013.2
is 1000' above t/a when stn QNH is>1013.2
Some countries have standardised TA to be a common figure that covers all arpts.