I have a bathymetery data which contains positive values for depths and zero for land regions. I need to plot borders between see and lands.
I don't think of a clean way. One idea is to assign a negative value, say −1 m, to land and plot the contour lines corresponding to depth = 0. If you do that, the contour lines tend to be significantly different from the real coast lines. This is because we miss the actual altitude data of the land.
So, a better way may be to plug in real land topography data to the land points of your depth data.
set data ETOPO5
let alt_regridded = rose[gx=depth,gy=depth] ! positive for land
let dep_ocean_land = if depth ge 0 then (-1)*alt_regridded else depth
contour/lev=(0) dep_ocean_land
If your depth data and the topography data don't match, the above method may result in some noisy coast lines.
Ryo