Yesterday, a paper was published in Science on the creation of new, higher resolution maps of the ocean floor using gravity anomaly data collected via satellite. The authors write, “at scales smaller than 200 km, variations in marine gravity primarily reflect sea-floor topography.” Since rock has a different density than water, precise gravity measurements allow for an estimation of ocean depth. One of my first thoughts was to use this data in a map.
The authors provide their data for download; however, it is provided as GMT grid files, which are apparently common for oceanography research, instead of something more standard for GIS such as GeoTIFF. Using GMT, I was able to convert the data to the GDAL supported NetCDF format, although I had to reencode the file with CDO before GDAL would handle it. Using GDAL, I then reprojected the data in EPSG:3857, converted it from floating point to integer, and created a GeoTIFF file to be more friendly to web mapping applications.
Since Mapbox Streets doesn’t include a layer for land, I masked out land masses from the gravity anomaly map with Natural Earth’s 10m land polygons in TileMill and created tiles down to zoom level 7. Below, the tiles are displayed using Mapbox GL JS with a slightly modified outdoors style (if the map displays as a solid color, try clicking and dragging).
Download the GeoTIFF Marine Gravity Anomaly Map (417 MB).
Also, a 32-bit floating point version (3.5GB), and a 32-bit floating point GeoTIFF of the vertical gravity gradient map (3.5GB).
Matthew-
I have been struggling for several weeks to get the new Sandwell gravity grid into a usable format for ArcGIS. Your GeoTIFF was just what I have been looking for! Any chance you could share a 32-bit floating point version of the GeoTIFF?
Thank You
BD Andrews
I just uploaded a copy and added a link. It didn’t compress well, so the file is rather large.
Matthew – Thank You, this process step is exactly what I’ve been looking for. Cheers to you for sharing. – BJ
Any thought on converting the Marine Vertical Gravity dataset to GeoTiff/Floating Point? Cheers
I just added a link.
Matthew-
Thank you for converting both grids to 32-bit FP tiffs. It appears the grav-fp.tif might be corrupt?, but the curv-fp.tif downloads and displays fine.
Thanks again-very helpful
BDA
The image seems fine to me, although, for whatever reason, the download took a very long time to start.
Hi Matthew,
With
img2grdin GMT you can directly convert to geotiff. To get a geographic lon/lat grid use:img2grd curv.img.23.1 -Gcurv.tiff=gd: GTiFF -Rd. So you don’t have to reproject and you can do it in one step…Cheers Anna
Thanks for the tip. I’m familiar with GDAL but not GMT, so I only used GMT for what I couldn’t do with GDAL.
Hi Matthew,
I loaded your curv-fp.tif-file over the GEBCO bathymetry in ArcGIS and find there is a (I think)
continuous offset of about 20km. Your data seems to be shifted 20km to the north. I compared with
my curv-tif-file I converted in GMT and find the same offset.
I don’t understand what is happening when you reencode the data in CDO, but maybe something during
projecting back and forth produced that offset.
Or it is an issue of the different definitions of web mercator in ESRI and GDAL ?!- see:
https://trac.osgeo.org/gdal/ticket/3962
Maybe you could check that and maybe tell me I am wrong :),
cheers,
Anna
I’m guessing it has to do with the ESRI / GDAL difference you mention. I just loaded the image and the Natural Earth coastline data in QGIS, which uses GDAL, and the datasets match up.