Many thanks for your advice. It will save a lot of thrashing around !
I can convert turtle graphics using standard geometry routines. (I did such things many years ago, but they are lost on microtapes, so I will need to do them again...).
As for reading data, I must study the Pascal Manuals again, to see how to grab huge amounts of data using nested FOR loops. This is new domain for me, and I hope to find some useful examples, with any luck.
All this is needed for a program which produces optical illusions, with very short code, a few turtle instructions, but masses of data...
If the holiday weather turns foul, I will start work on it earnestly before autumn...