Describing the profile of a vessel in a digital format¶

The most difficult part of the story is finding an acceptable digital format for describing the profile of a vessel.

For drawing purposes, it is common to use vector graphics software or CAD systems. It is clear that it is highly desirable to represent the profile as a vector geometry, that is, a sequence of points and lines in a cartesian space.

However, all those programs are made for drawing, and not for describing geometric entities in a purely abstract fashion. We don’t need line widths, colors and other such features. Furthermore, we don’t want to work in a coordinate system where the origin (the 0,0 point) is in the top left corner, like for example SVG does. We don’t need anything that has to do with graphics, just a compact way of representing a (tiny) set of ordered geometric coordinates.

Making sense of different standards¶

To add more fun, profile drawings can be both right-handed or left-handed. We could force people to mirror their images before feeding them into Kotyle, but why would you do that ? Computers should be working for us, not the opposite !

It’s quite easy to manage this disparity, even taking into account that people might start drawing from the rim (top) or from the bottom of the vessel.

	x0 > xN	x0 < xN
y0 > yN	↙	↘
y0 < yN	↖	↗

x0 is the ascissa of the first point in the sequence, and xN the ascissa of the last point. Same for y0 and yN. Arrows indicate in which direction the profile was drawn. “Same sign” combinations give the same result.

The standardized view is with the 0,0 point at the bottom of the profile, where it crosses the rotation axis. But it’s easy to reverse() those profiles that follow the opposite standard, mirroring them.

CSV: a basic approach¶

Until now, the most convenient way for doing so may be a good old CSV file:

5989847716,11.4316005076
4873096447,11.3938563452
4263959391,11.387836802
6700507614,11.4760149746
1675126904,11.6587162437
7512690355,11.7511243655
9441624365,12.0639779188
7563451777,12.6677218274
3147208122,13.4012926396
6446700508,14.008127665
7715736041,14.1675642132
7969543147,14.2635515228
7969543147,14.3917515228
923857868,14.8075880711

While it might seem awkward at first sight, it contains all the information we need. The only problem with this approach is that there is no place for metadata like the name and description of the vessel.

GeoJSON: going further¶

A slighly more advanced encoding of the same data can be expressed as GeoJSON:

{ "type": "Feature",
  "geometry": {
    "type": "LineString",
      "coordinates": [
            [11.5989847716,11.4316005076],
            [12.4873096447,11.3938563452],
            [13.4263959391,11.387836802],
            [14.6700507614,11.4760149746],
            [16.1675126904,11.6587162437],
            [16.7512690355,11.7511243655],
            [17.9441624365,12.0639779188],
            [18.7563451777,12.6677218274],
            [19.3147208122,13.4012926396],
            [19.6446700508,14.008127665],
            [19.7715736041,14.1675642132],
            [19.7969543147,14.2635515228],
            [19.7969543147,14.3917515228],
            [19.923857868,14.8075880711]
      ]
    },
  "properties": {
    "name": "Hayes 61 B",
    "description": "A quite small version of 61 B.",
    "author": "Stefano Costa"
  }
}

This format is cheap, and it can be read directly by dedicated geometry libraries. It’s not easy to type, but it’s definitely easy to read. Plus, the idea is that you don’t have to type it, because a dedicated program can do the work for you.

If you use the GIMP, Kotyle has GNU Image Manipulation Program plugin that export vector paths to GeoJSON and facilitate creating digital profiles.

GeoJSON profiles can be analysed with Μετρω, the Kotyle measurement tool.