Satellite imagery and other datasets come together to show our home planet from mountaintop to ocean bottom.
Earth is our home. It's the planet we know best, our terra cognita. And for centuries, cartographers have attempted to portray what we've learned about this world — first as maps (some more fanciful that others) and eventually as round globes.
Having produced globes of the Moon, Venus, Mars, and Mercury, the Sky & Telescope staff thought it was high time to add a version of Earth to our growing collection. However, rather than mimicking the usual geopolitical format, with lines identifying national boundaries and labels for countries and major cities, we opted for a version that shows our planet's best features: its mountains, valleys, and other major geologic structures.
The project began with a search for the best portrayals of both land and sea. The goal was to show land masses very close to their natural color, while simultaneously depicting the fascinating topography hidden beneath the oceans and seas.
Data for the land portion comes from a mosaic of thousands of images, known as the Blue Marble, acquired by MODIS instruments (short for Moderate Resolution Imaging Spectroradiometer) aboard NASA's Terra and Aqua satellites. The MODIS instruments make their observations in 36 spectral bands at a resolution of 500 meters (0.3 mile) per pixel and with consistent solar illumination. Our globe depicts Earth as it appears in September — the transition from summer to autumn in the Northern Hemisphere and the onset of spring in the Southern Hemisphere.
For the 71% of the globe not involving land, we turned to bathymetry from the British Oceanographic Data Centre. BODC scientists amass these data using "acoustic radar" — by generating sound waves at the surface that travel through the water, bounce of the seafloor, and return upward to waiting detectors. These acoustic echoes also reveal details about the size and shape of seamounts (underwater mountains) and seafloor roughness. Darker blue shadings indicate greater depths.
No sea ice is shown, which makes it easy to distinguish water from land. Nor are there clouds — though at any given moment they hide about 70% of our planet.
S&T illustration director Gregg Dinderman then combined these databases into a single base map. But a big rectangle doesn't conform to a sphere seamlessly — a special projection was needed. For that we turned to cartographer Michael Zeiler of Eclipse-Maps.com, who created a unique "daisy-shaped" projection for each hemisphere using Esri's ArcGIS software.
"I received the map in a Plate-Carree projection, a rectilinear portrayal that treats geographic coordinates (latitude and longitude) as Cartesian (x,y) coordinates," explains Zeiler. Then he transformed the map's gores, each 30° of latitude wide, in a polyconic projection, while utilizing azimuthal equidistant projections for the arctic and antarctic areas (latitudes above 80°).
"These were quite large files," Zeiler adds, "because we wanted to work at a very high resolution so that the stretching and compressing of pixels from the Plate-Carree to the polyconic projection would not be visible." Dinderman then assembled the pieces, added labels for key features, and delivered the huge files to Replogle Globes for printing and globe production. (Here's a fascinating 5-minute video showing how Replogle assembles its globes.)
We're pleased with how it all turned out. Our hope is that, by scrutinizing our custom Earth Globe — and especially by comparing it with those of other worlds — you'll gain a deeper appreciation for our planet's geologic features and its unique status among the solar system's many diverse worlds.