How to extract the Gebco data upto 30m depth contour from the coastline (-1m) for specific region from whole region

Dear All,

I have Gebco data in .xyz format (lon,lat, height/depth) for the region which covers both land and ocean, in which, i want to extract the bathymery(ocean) from coastline(-1m) upto -30m(ocean) contour for specific region in term of latitude (i.e. starting and ending of the coastline) from whole data.

First I want to extract the data from -0 to -30m contour and then have to extract that -30m contour data from one starting point of coast (lat) to another ending point of coast (lat).

Thanks for your attention. I am looking forward for your reply.