Importing bathymetry, the basis of your model

Comment on Last Week's video

Last week we uploaded the first video in a series of videos aiming at be a step by step example of dBSea features and workflow. In the video i demonstrate one way of getting bathymetry into dBSea. In this post I'll comment a little further on the subject.

Finding bathymetry data

Depending on what part of the seabed you are interested in you could face very different challenges when trying to come by good bathymetry data. In Europe, as in the video, a very good place to start is the "European Marine Observation and Data Network" - EMOD. Most data from European countries are collected and put into this user-friendly database. The most recent data is not available as it takes some time for new data to be incorporated.
Especially the "Download area of interest" option is great for getting a limited area of interest.
Depending on your selections in the viewer you'll download a file that is immediately compatible with GIS software and dBSea. 
You'll often end up having to find better resolution data, and this is a bit of a skill in its own right, and can be downright frustrating until you figure it out.

Whats under the ASCII label?

The ASCII format is very basic and consists of a simple grid of depth values preceded by a six-line preamble.
A bathymetry file in ASCII grid format
The preamble gives you (and any software) some simple information:
  • Width - ncols, number of columns
  • Height - nrows, number of rows
  • Lower left corner longitude - xllcorner
  • Lower left corner latitude - yllcorner
  • Resolution - cellsize, degrees per data point (this can be meters if x-y corner is in meters)
  • No data value - Allows for designation of a number to be interpreted as "not surveyed", Often this would be "-9999", my GIS-software chooses -3.4e38 as default (!?)
The one downside of the ASCII grid format is that it is uncompressed and therefore takes up a lot of space. 
If you will do a lot of manipulation with you bathymetry you can download it as GEOTiff as this format only take up about a third of the space (when uncompressed), and works with all GIS-software as well. You'll need to convert back to ASCII grid before import to dBSea.

Top tip: I often use a simple program in R or Octave to replace NODATA_values with an arbitrary land height value, as the unsurveyed areas are often land anyway. For smaller data sets you can use the "find/replace" method in notepad for the same thing.

Many more data formats exist, and often you need some skills in GIS-software to convert this into either ASCII grid or xyz format (QGIS, SAGA and GRASS are three great choices for open source GIS-tools).

Data resolution ≠ display resolution

Once imported into dBSea you might find that the display is not the same resolution as the bathymetry, this is to conserve memory. The calculations are done on the full resolution data, but the graphics display only shows a compressed version of the final result. (change this in Preferences/Graphics/Max bathymetry graphics data)
Left picture has a resolution of 4,000 pixels (200x200), while the right picture is 400,000 (2000x2000). The calculations will be the same, but the rendering is much faster in the left picture.
No point in setting the resolution higher than the screen you're working on (unless you need to zoom a lot). Only the bathymetry resolution is affected by this, the results are presented depending on your calculation grid resolution.

Thanks for reading!