Dearest Catalina - I do not think the treasure would be on another land mass in this case. As the game tends to pick the closest city and Gran Grenada if pretty far from any other city that is North of it. As an example of what I mean, if you look at the map and say the location is directly north of Gran Grenada, on the Northern most tip of the land mass the map should say N. East of Campeche, or possible S. West of Havanna.. That make since?
GIJoe, standard form response #1 (map locations)
<img src="http://www.piratesahoy.com/forum/style_emoticons/<#EMO_DIR#>/hi.gif" style="vertical-align:middle" emoid=":gday" border="0" alt="hi.gif" /> (fill in the blank)
I just wish I had a dollar for every thread and post about finding map locations. Every few days there is a new one. <img src="http://www.piratesahoy.com/forum/style_emoticons/<#EMO_DIR#>/rolleyes.gif" style="vertical-align:middle" emoid="

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Fact of the matter is, locations are different everytime you play the game. Where someone else found the lost city/treasure may not be where you will find it. Although to be fair, there are some landmarks that are repetitive.
The best bet is for you to simply decide to find that location and do so.. Do not get frustrated, or angry. Just start at the target city and walk in the general direction the map shows. If you come to the edge of the map and do not find a land mark that is on the map and that is not common, (totem pole or head statue), simply walk left or right for approx one screen and reverse course. Eventually you will find the location. Just search a "GRID" pattern up/down left/right. I have never not been able to find the correct location using the pattern.
There are more techniques that I use, but I do not feel like posting a more lengthy post at this time. Bottom line is, you cannot give up.
<span style='color:darkred'>Most people tend to forget that finding the map locations are PART OF THE GAME. It seems like most just consider it a "side" item to be dealt with as soon as possible so you can get back to playing the "game". IMHO</span>
EDIT: Adding this last bit above.