Take Abel Tasman’s stated longitude of Fredrick Henry Bay (where he planted the Princes Flag):

167 degrees 55 minutes East of Teneriffe.

Convert this to east of Greenwich using James Cooks stated longitude difference given in his journal on 24th September 1768 – Teneriffe lies “Longitude 16 degrees 32 minutes from Greenwich”:

151 degrees 23 minutes East of Greenwich.

Convert from east of Greenwich to west of Greenwich:

208 degrees 37 minutes West of Greenwich.

Now look at Cook’s journal. What is his given longitude given for Botany Bay? It’s the exact same value as above!

208 degrees 37 minutes West of Greenwich

But that isn’t where Botany Bay actually is. See map below at point labeled “Cook’s stated position of Botany Bay”:

I think the British were trying to ‘bait’ the Dutch into a trap. Here is how the trap should work (hypothetically).

1. Dutch read/hear about the British planning a settlement at Botany Bay(Stingray Bay). They look at Cook’s journal to find where it is.
2. They are provoked – because they note that the British are sticking a settlement at the EXACT same longitude as the Prince Flag (where their formal possession claim was made), therefore right smack-bang on top of “the border”.
3. First Fleet arrives in Botany Bay(Stingray Bay) but secretly relocates and settles in the real Botany Bay (in Sydney Cove).
4. A informant leaks (correct) information to the Dutch that Stingray Bay (landing place on map above) which the Dutch should be misled to believe is the settlement site, actually lies 10 minutes in longitude west of where Cook states it is.
5. Dutch call for a survey of the location of the settlement – as they believe (due to the informant) it lies on the Dutch side of the line. By calling for a survey on that basis, they inadvertently validate the entire premise of having a Anglo-Dutch border along a certain longitude (rather than geographical contiguity principle)
6. British agree to resurvey (of course, as was the plan).
7. Survey is completed and Stingray Bay and it’s opening into the sea is discovered to be on the Dutch side of the line.
8. British then pull the ace out of their sleeve and call for a resurvey of the Peak of Teneriffe/Greenwich (and therefore Tasman’s Prince Flag longitude).
9. Resurvey of Teneriffe shifts the Prince Flag location and derived border-line (reckoned from Teneriffe which is now moved) 6 minutes further to the west (to the red line on the map above). The true location of Teneriffe Peak is 16 degrees 38 minutes west of Greenwich – not 16d 32min (which the British know because Cook sailed past it and checked).
10. This new line (with 6 minute shift) is marked on map above in red. As Cook claimed “bays, harbours and rivers” – Sydney harbour is now the southernmost bay/harbour/river that opens east of the red line.
11. British then pull out the other ace card…. and reveal that the “Botany Bay” settlement is in fact – further north in Sydney Cove not in Stingray Bay. They reveal the true location of the settlement, and perhaps also reveal 1770 charts of Sydney Cove. Note also – that the Stingray Bay landing site and Sydney Cove lie at the exact same longitiude – also handy for trying to trick the Dutch.

But La Perouse would have ruined the whole plan by telling everyone in Europe where the settlement was really located. The Dutch would have been tipped off, and would have known the settlement was not at Stingray Bay at latitude 34. It’s no wonder he disappeared with no survivors.

And yes – Cook went into Sydney Cove.