FIELD NOTES · NASA BLUE MARBLE 2004

The colours, and where they come from

Average Earth is an atlas of image colours. Knowing how the images were prepared matters as much as the colours themselves.

The source: twelve maps from 2004

The active edition uses NASA Blue Marble Next Generation’s twelve monthly base maps for 2004. We use the base-map images, without the optional extra topographic or bathymetric shading variant. These are processed display composites, not raw reflectance data or a live satellite feed.

Source: NASA’s Blue Marble base-map downloads. Background: NASA’s project description. NASA credit does not imply endorsement of Average Earth.

From an image to a colour sample

The display maps are prepared offline. A separate 720 × 360 RGB grid stores one sample per 0.5° cell. The app reads this grid for swatches and exports; it does not sample the scaled screen canvas. A half-degree is roughly 56 km north–south, so a sample describes a broad area rather than an address.

Zooming changes what you can see, not the resolution of the underlying measurements. The current view, temporal swatches and exports all use the same prepared dataset.

How the averages work

We treat source RGB as display sRGB, convert to linear light for averaging, and convert the result back to RGB bytes. Global and longitude averages use spherical cell-area weights. Across a single latitude row, the area factor is common to the row and cancels out.

The annual view is a day-weighted mean of the twelve monthly display images, using all 366 days of 2004. It is not a long-term average of Earth’s reflectance.

Two different kinds of spatial ribbon

Band averages

The right ribbon averages each full latitude band. The bottom ribbon averages each full longitude band with area weighting. Zooming does not redefine these as averages of the visible viewport.

Cross-sections

The ribbons show the actual sequence of cells through the selected longitude and latitude. They are slices through the map, not averages along those slices.

Land, oceans and mixed coasts

Land only applies a coarse land-fraction mask derived from public-domain Natural Earth geometry. Whole image includes all supplied pixels, including rendered oceans. A coastal pixel can contain both land and water; the mask cannot recover a pure land colour from that mixture.

Ocean colours in the source rendering should not be interpreted as measured water RGB values. There is no valid local land sample in a cell that contains no land.

Sharing and using the results

Shared links retain coordinates, period, scope and view settings. CSV timeline exports and JSON colour exports include information about the dataset. Keep the source, period and scope with any colours you reuse so another person can reproduce the view.

Average Earth is useful for exploring patterns and colour palettes. It is not a tool for diagnosing climate change, crop health, water quality or conditions at a particular property.