I need help with contour plots in Matlab please?

I have three columns of data: one represents longitude, one represents latitude, and the other represents concentration of some chemical at a site defined by a long/lat point. How do I contour this data? When I use contour(x,y,z) - with x representing longitude, y representing latitude, and z representing concentration - I get a blank 3D graph. I have also tried creating a meshgrid (although I'm not entirely sure what this is, if you can tell me that would be great) and that didn't work. Can anyone please help?