How to utilize Costmap in navigation process?

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Hello, I'm using ROS kinetic on ubuntu 16.04.

I'd like to use cost map of navigation precess, whose type is nav_msgs/OccupancyGrid.

When I tried checking how it looks like with rostopic echo ~, it seems the main information of cost map corresponds to costmap.data of nav_msgs/Occupancy

But it has 100 X 100 lengths, so I wonder how can I access it.

What I want to do is simple:

I want to convert the elements of costmap.data into Catesian-coordinate.

Anyone who knows how to do it?

Thanks in advance :)