This quotation comes from Act I scene 2 and is uttered by Marlow in response to the rather curious and confusing instructions he is given by Tony as to how he is to reach Mr. Hardcastle's house. Of course, this is an example of dramatic irony, as Tony is deliberately pretending that Mr. Hardcastle's residence is much more difficult to reach than it actually is, so that he can pretend that Mr. Hardcastle's house is actually an inn and his father-in-law is a presumptuous member of the working class. Note the instructions that Tony gives Marlow and Hastings:
Coming to the farmer's barn, you are to turn to the right, and then to the left, and then to the right about again, till you find out the old mill.
This comes after some rather suggestive naming of the local environment, such as "Crack-skull common" which is of course designed to make the supposed journey Marlow and Hastings must venture on as unnattractive as possible. The comment in this quotation then is said in an exasperated and almost sarcastic tone by Marlow, as he realises how imprecise and rather vague the complex instructions Tony gives him are. The reference to the "longitude" is one that refers to a specific geographical description of the location. Of course, by using this term, Marlow is ironically observing how Tony's instructions are far from a "longitude."