May 2010

More on new shorelines

A couple of weeks ago I found a new dataset describing the coastal outlines of landmasses and contained seas and islands. The "Global Self-consistent, Hierarchical, High-resolution Shoreline Database" (GSHHS) comes in five resolutions ranging from extremely detailed to crude.

The GSHHS dataset was obtained from
NOAA's National Geophysical Data Center and will replace my use of a dataset obtained from NASA's Goddard Institute for Space Studies. The NASA dataset would seem to be oriented more towards presenting fairly gross-scale overlays for maps; the GSHHS dataset is more detailed and complete in terms of small islands, inland seas and lakes, and wide rivers. The latest version (2.0) of the GSHHS data was released in July 2009.

Satellites models the globe in (16-bit) integer Kilometers. The second least detailed GSHS dataset is still too detailed for Satellites and I've processed it in a couple of ways to make it work better:
  • Remove the smallest islands and lakes. This has the effect of 'de-cluttering' the globe -- in fact, it mostly removes lakes which has an especially good effect in areas of the world where there are so many small lakes that the model is almost solid outline. It also removes small islands, but since the NASA map had a lot of islands missing, even this removal restores many new islands.
  • Remove all the vertices in the model that become the same when converted to integer Kilometers. This is for efficiency and allows many vertices to be deleted.
In general the match between the GSHHS outline and the NASA ones is good. The largest difference is the coastline of Antarctica -- the GSHHS outline has several large bays filled and may represent the edge of the ice sheet, rather than the land. In this image, the orange outline is the NASA dataset, the green is the GSHHS. Though hard to see at this scale, you may notice more islands in green in the Antarctic Peninsula, though the continental outline is cruder in places.

Another example of the differences is shown here. The GSHHS outline restores several large islands (Isle of Wight, Angelsey, and more of the Hebrides, Orkney, Shetland and Faroe Islands) to the United Kingdom, adds large lakes in Ireland, and more detail in the Norwegian fiords:

This new GSHHS data will be used in the next version of Satellites.