Ca.150 m needed for the Grand Slam (of 4) [ANSWERED]

I hope this is not too easy but I'm afraid for some hard core geo fans it is. Others may learn some trivia...

I have no idea what that is?

I will wait for another clue, to find out what it is 

I had many ideas, but unfortunately they were all dead ends

I mean... en.wikipedia.org/wiki/Grand_Slam

I need some time 

CLUE
Sorry if the title is misleading, it has nothing to do with tennis or with any sports whatsoever. So it is meant figuratively.
This time it refers to a geographical feat, achieving 4 of something which it almost did with an error of ca. 150 m.
Had it achieved that Grand Slam, it would have been the only one of its kind on the planet Earth.

CLUE 2

SPOILER: Click to show

{Spoiler}

I'm missing this by at least 150 metres too. 

CLUE 3 (final)

SPOILER: Click to show

thanks for the clueS

great puzzle anyway! i've been misled by the word "error" (my error, haha)!

Apr 30, 2015 17:09:32 GMT hhgygy said:

CLUE 3 (final)

SPOILER: Click to show

Could you add in a few more yellow lines, please, I still don't get it. 

Apr 30, 2015 21:59:21 GMT johnluke said:

Apr 30, 2015 17:09:32 GMT hhgygy said:

CLUE 3 (final)

SPOILER: Click to show

Could you add in a few more yellow lines, please, I still don't get it. 

You don't need any more yellow lines. If these yellow lines would meet in one point that would be the Grand Slam.

So that everybody else knows this is not the sea, like I've always believed since the beginning: (I mean, the sea is BIG).

ANSWER

SPOILER: Click to show

All this Grand Slam thing was about the word quadripoint.
I'm quite lazy so please read the relevant Wikipedia article.