I have often read and heard it stated that the seabed is less well mapped than the surface of other planets. However, I had not noticed papers that provided evidence for this or cited primary sources. I thank Peter Harris and David Sandwell for providing links to the evidence that shows indeed maps of other planets (and at least one asteroid) are at a finer spatial resolution and with better coverage than our oceans.
Peter drew to my attention that:
You can download from the web a 50 m Digital Terrain Model (DTM) of the surface of Mars. The website states “The high-resolution images used have a resolution of 10 m/pixel. The DTM elevation data derived from these images is provided in pixels of up to 50 m, with a height accuracy of 10 m.”
You can download a 100 m Digital Terrain Model for the Moon. Its website states “Wide-angle Camera (WAC) …. stereo images from the one-year nominal mission and the first months of the science mission phase are combined to produce a near-global digital terrain model (DTM) with a pixel spacing of 100 m, the Global Lunar DTM 100 m, or GLD100. It covers ….. , 98.2% of the entire lunar surface.”
“For the ocean floor we can’t even get close to the resolution of these models. The resolution of the SRTM 30 arc second (Shuttle Radar Topography Mission) DTM for the ocean is supposedly 1 km (Becker et al., 2009). A resolution of 1 km is 100 times lower resolution than the 100 m DTM of the moon and 400 times worse than Mars!
But we don’t really have a 1 km map for the ocean because even though the grid is 1 km, the underlying data are spaced much further apart in most places . The satellite altimetry data used by Smith and Sandwell (1997) to fill in the holes where actual soundings are missing provides +/- 100 m vertical estimates at 12.5 km spacing. That is the worst-case scenario but still applies to big chunks of the ocean. 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 of for the whole of the surfaces of Mars and the Moon.”
David Sandwell brought to our attention that Mercury has also been mapped at a resolution of 665 m, ten times better than the seabed. For the latest bathymetry, “11% of the oceans is mapped by ships at 500 m or better resolution. The other 89% is mapped by satellite altimetry at a ½ wavelength resolution of 6 km. Mercury, Mars, and the Moon all have topographic maps that have much better than 1 km spatial resolution. Venus was mapped by radar altimetry where the footprint of the radar is ~10 km so the map is provided on about a 10 km grid but the ½ wavelength resolution is only about 20 km. This is slightly worse than the Earth at ~6 km. Note that in terms of imaging Venus has about 200 m resolution globally which is much better than the coverage of the oceans.”
Even some asteroids are mapped at better resolution than our seabed, for example, Phobos has a 100 m/pixel DTM (Willner et al. 2014).
I also thank Roger Sayre and Dawn Wright for the above conversation.
References and further reading
Becker, J. J., D. T. Sandwell, W. H. F. Smith, J. Braud, B. Binder, J. Depner, D. Fabre, J. Factor, S. Ingalls, S. H. Kim, R. Ladner, K. Marks, S. Nelson, A. Pharaoh, R. Trimmer, J. Von Rosenberg, G. Wallace and P. Weatherall (2009). Global bathymetry and elevation data at 30 arc seconds resolution: SRTM30_PLUS. Marine Geodesy 32(4): 355-371.
Ford, P.G. and Pettengill, G.H., 1992. Venus topography and kilometer‐scale slopes. Journal of Geophysical Research: Planets 97 (E8): 13103-13114.
Mayer, L., Jakobsson, M., Allen, G., Dorschel, B., Falconer, R., Ferrini, V., Lamarche, G., Snaith, H. and Weatherall, P., 2018. The Nippon Foundation—GEBCO seabed 2030 project: The quest to see the world’s oceans completely mapped by 2030. Geosciences, 8(2), p.63.
Smith, W. H. and D. T. Sandwell (1997). Global sea floor topography from satellite altimetry and ship depth soundings. Science 277 (5334): 1956-1962.
Smith, D. E., Zuber, M. T., Neumann, G. A., and Lemoine, F. G. (1997). Topography of the Moon from the Clementine lidar. Journal of Geophysical Research E: Planets 102 (E1): 1591-1611.
Smith, D. E., Head, J. W., Garvin, J. B., Banerdt, W. B., Muhleman, D. O., Pettengill, G. H., Neumann, G. A., Lemoine, F. G., Abshire, J. B., Aharonson, O., Brown, C. D., Hauck, S. A., and Ivanov (1999). The global topography of Mars and implications for surface evolution. Science 284 (5419): 1495-1503.
Tozer, B., Sandwell, D.T., Smith, W.H.F., Olson, C., Beale, J.R. and Wessel, P., 2019. Global bathymetry and topography at 15 arc seconds: SRTM15+. Earth and Space Science. https://doi.org/10.1029/2019EA000658
Willner, K., Shi, X., and Oberst, J. (2014). Phobos’ shape and topography models. Planetary and Space Science 102 (C): 51-59.
 Becker et al. (2009) state that about 10% of the cells in the SRTM30_PLUS 1 km grid are constrained by one or more soundings. If the grid size is increased to 2 km then about 24% of the cells are constrained by one or more soundings.