Check the Depths

The grid

The sea floor and the land under every frame come from NOAA's DEM global mosaic, which stitches NOAA's coastal relief models along US shores to GEBCO's global grid elsewhere. For a spot frame up to 400 km across our server also asks GMRT, Lamont-Doherty's synthesis of ship multibeam surveys, and lays its cells over the mosaic wherever a ship has swept the floor. If the mosaic doesn't answer, ETOPO 2022 at 15 arc-seconds stands in, and the sheet says so.

The share of the frame drawn from multibeam prints in the margin. Off Monterey it is most of the frame. In the open ocean it can be under a tenth: the rest is predicted from satellite gravity and older soundings, a good model, and not a measurement.

The frame

Every sheet is drawn in a Lambert azimuthal equal-area projection centred on the frame, named in the margin, so a canyon at the edge keeps its true area. The width printed on the sheet is measured on the WGS84 ellipsoid along the geodesic. Your pin is kept as a place on the drawing, two numbers between 0 and 1, and never as a coordinate.

Twenty named deeps

Before the first print was sold we chose twenty named deeps with a depth published by a survey or a paper, read our receipt grid round the place each source gives, and set the deepest cell there beside the published figure. The tolerance was fixed first and has not moved: the larger of 3% and 150 m. If fewer than twelve had landed inside it, no sheet would print a published depth at all.

The waters

A country frame can run to the hairline of its exclusive economic zone, from the Flanders Marine Institute's Marine Regions. We measure each zone's area on the ellipsoid and hold it to 2% of the area Marine Regions publishes. The drawn line is thinned to about a thousandth of the frame's width; the area is measured on the whole zone.

The shelf edge

The Drop and every pinned sheet print the distance from your pin to the first point 200 m deep along the bearing, read every few hundred metres along the section. Where the pin is already past 200 m the figure is left off rather than guessed.

The floor under Satellite

The Satellite style's land comes from the sharpest licensed pass for the frame's width. A print needs about 150 dots an inch to hold up at arm's length, so each source fills a print only from a frame so wide.

Sharper pictures, 30 to 50 cm, are sold by the square kilometre under a written licence. We have not bought them, so no print here uses them.

The styles

Bathymetric

The grid's depth on a curve from the shore colour to the deep colour, banded at each isobath when bands are on, the land lit from the north-west at a quarter strength.

Chart

The sea pale, the shelf above 200 m (or the second isobath) tinted, each isobath labelled in the sheet's unit, soundings sampled on a jittered grid.

Contour

The isobaths as lines in the sea and the land's own contours at a step fitted to its highest point.

Survey

The coast, which is the grid's zero line, and every isobath in one ink, nothing filled.

Linocut

The sea one solid block of ink, the isobaths cut out of it in the paper's colour, the land's shadows past a fixed light cut in the same ink.

Gilt

The coast, the isobaths and the land's contours in gold on black or navy, the lit faces of the land glazed, a gold keyline round the sheet.

Watercolour

Two washes deepening with the depth, laid at part strength so each pools, and a cool shadow on every land face the sun at 315 degrees misses.

Relief

The floor and the land in the shadows and lights of a sun at 315 degrees and 40 above the horizon, the relief steepened to suit the frame.

Oblique

The frame seen from the south, 60 degrees down, the ground raised until its whole relief stands a quarter of the drawing tall; the land from the Terrarium tiles at a spot frame.

Satellite

The sea from the grid, lit at 315 and 45 degrees and coloured in four bands that land at 0, 200, 3,000 and 8,500 m; the land from Sentinel-2, NAIP or NASA's GIBS, laid with a one-pixel feather at the coast, its cloud left as the satellite saw it.

The tones

The tone colours the sea and the pin. Each ground offers only the tones that hold 3:1 contrast against it, measured on every build.