Plotting data with GPS coordinates

Hi

I have built a device that logs magnetic data at GPS locations.

The magnetic data is a value between 0 and 255. The GPS data is in the form latitude: 5050.1380,N longitude: 00007.3619,W

I am a Matlab novice and need help plotting the magnetic data on a visual grid where the GPS coordinates determine the position on the grid. Each Magnetic reading would refer to grey scale color or RGB color indicating the magnetic intensity. I have no idea where to start with this problem can someone point me in the right direction or give me some Matlab examples. I would like to create something like this ideally:

cheers Jim