According to JDIC, both mean "observation". Judging from the Kanji used, I'm guessing the difference is 観測 has a nuance of observing something and measuring some aspect of it or taking data, whereas 観察 is just to observe and monitor what's happening.
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This might help too: hinative.com/ja/questions/943135 – rebuuilt Sep 10 '20 at 9:14