You may have recently read or heard that the Earth’s ocean floor is less well mapped than the surface of other planets in our solar system. Were you sceptical of such claims? Don’t be. 

Here you can download a 50 m Digital Terrain Model (DTM) of the surface of Mars. The DTM elevation data were produced by the Mars Express High Resolution Stereo Camera with a height accuracy of 10 m.

Here you can download a 5 km DTM of Venus; there are also 300 m resolution data available for around 70 per cent of the planet. This data set is based on the Magellan Mission to Venus launched on 4 May 1989 on board the Space Shuttle Atlantis and covers 98 per cent of the planet’s surface. 

Here you can download a 100 m DTM for the Moon. The model is based on wide-angle camera stereo images combined to produce a DTM with a pixel spacing of 100 m, covering 98.2 percent of the lunar surface.

For the Earth’s ocean floor, the highest resolution we have is the SRTM 15 arc second DTM, which is equal to a horizontal resolution of 500 m. A resolution of 500 m is 25 times lower than the resolution we have for the Moon and 100 times worse than the resolution we have for Mars! For Venus we have 300 m resolution data for most of the surface and a 5 km resolution model for the whole planet, which is lower in resolution than the 500 m DTM we have for the ocean floor. 

Or is it? 

In fact we don’t actually have 500 m resolution data for the whole ocean because even though the grid is 500 m, the underlying data are spaced much further apart in many places. The satellite altimetry data used to fill in the holes where actual soundings are missing provides +/- 150 m vertical estimates at 6.25 km spacing. That is the worst-case scenario, but still applies to more than 80 per cent of the ocean. According to the SeaBed 2030 mapping effort, we have only mapped around 20 percent of the ocean floor with high-quality modern echo-sounders. 

So it is very safe to say that our maps of most of the ocean floor don’t even come close to the resolution of maps we have for the whole of the surfaces of Mars, Venus, and the Moon.

At GRID-Arendal, we have followed in Marie Tharp’s footsteps by using seafloor mapping information to assist developing countries in a number of different ways, including ocean conservation, improved marine spatial planning decisions on use of resources, and supporting claims for ocean territory under the UN law of the sea (UNCLOS). Tharp's work is remembered in the book “Mysterious Ocean” published this year.

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Peter Harris is managing director of GRID-Arendal.