Global DEM and Bathymetry Rendering

I have prepared global DEM and Bathymetry images based on color tables that mimic those you might find in an atlas (remember those?).

This is a two-step process – firstly prepare a raster rendering algorithm that can be displayed in MapInfo Pro and then use the MapInfo Pro Advanced “Export Image” operation to render this algorithm out to located images at a variety of resolutions.

I have used two raster datasets. The first is the GEBCO_2020 DEM, which you can find here. GEBCO_2020 is a 15 arc-second resolution DEM that provides global coverage of both land RL and ocean bathymetry. The second dataset is MERIT DEM, which you can find here, is described here and compared against another global DEM here. MERIT is a 3 arc-second resolution DEM that provides coverage of land RL only, above 59.5 degrees South (it does not include coverage of Antarctica).

The rendering algorithm consists of three layers. The bottom layer uses GEBCO_2020 and is used to render all oceanic bathymetry as well as Antarctica. The second layer uses MERIT and is used to render all land north of 59.5 degrees south. The top layer also uses MERIT and renders Greenland in a white color scheme, to match the white color scheme used for Antarctica.

I use two color tables that provide both color and data transform information. The first table is used for bathymetry which is presented in shades of blue. Above 0m RL it uses shades of white. This is applied to Antarctica and also to Greenland. The second table is used for land, which is presented in green-brown shades similar to the kind of color scheme you will see in an atlas. For terrain below sea level (0m RL) I use a special grey-green color.

As Greenland is taken from the MERIT DEM, by default it will be rendered in the green-brown land shades. This seems inappropriate and inconsistent with the rendering of Antarctica, which uses shades of white. To resolve this I use a virtual raster that clips MERIT to a high-resolution polygonal outline of Greenland and apply the bathymetry color table. This layer overprints the standard land rendering of Greenland.

Along the coastal edges, I want to use the high-resolution MERIT raster to represent the land-ocean boundary. However, the low-resolution GEBCO_2020 raster lies underneath the MERIT layer and so terrain ‘bleeds’ out and blurs the coastal boundary. To prevent this, I use a virtual raster that applies a calculator expression to the GEBCO_2020 raster. The expression retains all RL values below 0m RL and replaces all other RL values with -1m RL. It only does this North of 59.5 degrees South, which means we retain the Antarctic terrain but remove everything else.

In summary, the algorithm uses three layers, two raster sources, and one clipping polygon. Two of the layers use raster data that is processed on-the-fly via a MapInfo Virtual Raster (MVR) – firstly to clip out terrain above sea level and north of 59.5 degrees South and secondly to clip a global raster to a polygon of Greenland. You can download a 5-minute resolution located image in GeoTIFF format here. You can download a PDF of this article, including the color table, virtual raster and rendering scripts here. If you are interested in acquiring a higher resolution image or you are interested in the rendering algorithm or virtual raster scripts, you can email me.