Scanline format means that the data are stored in rows (y = constant) going from the ``top'' ( (north)) to the ``bottom'' ( (south)). Data within each row are ordered from ``left'' ( (west)) to ``right'' ( (east)). The node_offset signals how the nodes are laid out. The grid is always defined as the intersections of all x ( ) and y ( ) lines. The two scenarios differ in which area each data point represents. The default registration in GMT is grid line registration. Most programs can handle both types, and for some programs like grdimage a pixel registered file makes more sense. Utility programs like grdsample and grdproject will allow you to convert from one format to the other; grdedit can make changes to the grid header and convert a pixel- to a gridline-registred grid, or vice versa.