charts

I currently use a C-Map Max N chart that I got as a free download when I bought my plotter and I'm getting frustrated that the contour resolution is abysmal, which makes it hard to find likely holding spots and features to investigate. I've fiddled with all the settings but the detail just isn't...

Sorry if this is an obvious question and I'm being a bit thick but what is the dm in the measurements charts in the Tide Tables booklets?