[mturmon]

The OP makes a related mistake (although for their data, it worked out OK). It uses this grid:

  x_grid=np.arange(x_min-h,x_max+h,grid_size)
  y_grid=np.arange(y_min-h,y_max+h,grid_size)

where h is a small constant, and the range ends are the min/max of the data. In general, this will result in rectangular pixels, which you probably don't want.

In your example, even for a small geographic area, a degree of longitude will typically translate to a smaller distance than a degree of latitude, so as you say you can't treat them the same.