How can I get the coordinates from nav_msgs/OccupancyGrid message?

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Hello,

I am using Gmapping to extract a OccupancyGrid message to create a enviromment map. However, I want go in specific points of map after it is constructed, but I dont know how do that from OccupancyGrid message. I don't know the relationship between the vector field 'data' with respect with a map coordinate. If 'data' was a matrix it will make more sense for me, but the way it is, I don't understand. Someone can give me a explanation?

Thanks!