Question: When referring to "ridges" and "troughs," are you speaking of systems in a 2-dimensional sense on a map based on latitude variations? Or variations based on altitude?

Answer: Typically, we are looking at ridges and troughs as visualized on a flat piece of paper or computer screen. In order to visualize them in that manner, one usually uses contour lines or color-filled contours. There are two main ways used to accomplish this for pressure. First, one can pick a certain altitude and plot contours of equal atmospheric pressure at that altitude. This is commonly done for "surface" pressure, but showing contours of pressure at an altitude of mean sea level. For upper air charts, however, it is common to choose a pressure level, say 500 millibars or 300 millibars, and plot contours showing the height of that pressure "surface." Because of the near-hydrostatic relationship between pressure and altitude, this makes maps that appear very similar to equivalent pressure-contour maps centered on the altitude ranges appropriate to those pressures. When done in this way, you can imagine each pressure level as a 3-dimensional "sheet" in the atmosphere, in which the 500 millibar surface, for example, is located at a higher altitude along a ridge or within a high center than at other surrounding locations. The height contours used to show the pattern of highs and lows, ridges and troughs would then correspond nicely to the way an elevation contour map illustrates variations in topography of the land and sea surfaces.