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====== Seabed Ocean Bottom Data Tutorial ======

**2018-03-10** Seabed ocean bottom heightmap in Global Mapper (GM) and [[tools:l3dt|L3DT]] tutorial by Ice

  - Download the seabed data from opentopo.
  - Export your heightmap and seabed data like normal.
  - Use L3DT and the "lower to" brush and set it to -6. Lower all the coast etc on the seabed heightmap to -6. You should be left with big deep bits, but shallow areas are -6. Otherwise I find you end up with 20m cliffs along the coast, due to it being low res!
  - depending how it goes:
    - Open the seabed data back into GM and make a shape along the coast and where you want the seabed. That should work.... if not ->
    - The way I do it, is make a mask using the coastline shape from OSM or elsewhere. So seabed is black and island etc is white or whatever colour you choose. Import that to GM and in the control center, right click it and choose "create area feature from equal values" in the pop window. Pick "only create areas for selected colours" and choose the colour you set in the image for the seabed. This then makes a shape for you of only that colour.
  - Select the shape, then select the seabed layer in the control center. Click options at the bottom then the cropping tab. Click crop to selected polygon. Now the seabed data is cropped to that shape! So...
  - Import the main heightmap again and put your newly cropped seabed data on top of it (I think its whatever is at the bottom in the control center is on top)
  - Export heightmap like normal, but now its has the real life seabed data merged with it so its semi accurate!
  - Use L3DT and its smooth brush to go along the edge and just clean it up a bit.
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