What is a "ranked" peak?
A ranked peak is a summit that rises at least 300 feet above the low point of the highest connecting ridge (called the "connecting saddle") to another peak of greater elevation. The difference between the elevations of the summit and the connecting saddle is called the "prominence". So if the prominence is greater than 300 feet, then the summit is ranked. Prominence is an objective way of determining the height of a peak compared to its surroundings.
Perhaps a simpler way to picture prominence is to follow the contours surrounding the peak of interest, stepping down one contour at a time until you find the highest contour that surrounds another higher peak. Take peak 8,132' below on the right for an example.
Peak 8,132' is the second highest peak in the example here. But is it ranked?
Dropping down one contour at a time, we see that the 7,760' contour (red) is the highest contour that surrounds both peak 8,132' and another higher peak (8,205'), while the lowest contour that doesn't surround the next highest peak is the 7,800' contour (blue). Therefore, we know that the connecting saddle is located at the col between these two contours, and ends up being at the point marked as 7,793'. The prominence of peak 8,132' is then calculated as the difference between the summit elevation (8,132') and the connecting saddle elevation (7,793'), or 339'. Since 339' is greater than 300', the peak is "ranked".
Often peaks are not given a specific elevation on USGS maps (see the example map below).
In this case, the elevation of the peak on the right is somewhere between 8,120' and 8,159'; the elevation of the peak on the left is somewhere between 8,200' and 8,239'; and the connecting saddle elevation is somewhere between 7,760' and 7,799'. Therefore, we know that the peak on the right is lower, and that its prominence falls somewhere between 321' and 399'. Thus, it is certain to be ranked.
Typically what is done in estimating the prominence for such a peak is to estimate the elevations by interpolating between contours. So, for this example, the peak on the right is interpolated to 8,140', the peak on the left is interpolated to 8,220', and the saddle is interpolated to 7,780'. This makes the interpolated prominence of the peak on the right to be 360'.
Occasionally there will be a peak with an interpolated prominence of slightly less than 300', while its possible prominence range encompasses 300', so we are unsure if it is ranked or not. These peaks are considered to be "soft ranked". On the other hand, peaks with an interpolated prominence of 300' or slightly more, but with a possible prominence range encompassing 299', are considered to be ranked until which time measurements either prove or disprove this classification.
For an absolutely excellent resource listing all ranked peaks for the western United States (and eventually the entire U.S.), go to Lists of John