Tile Schema

The baked vector tiles use a fixed set of layers and fields. The web viewer depends on this schema, so the names are a contract. Do not rename a layer or a field without updating the frontend to match.

Every tile uses an extent of 4096 and a buffer of 64. The PMTiles metadata lists all seven layer ids under vector_layers.

Zoom levels and navigational bands

A nautical chart is not one map at one scale. NOAA compiles each ENC cell for a navigational purpose — from a wide overview to a close-in berthing plan — and the right cell to show depends on how far you are zoomed in. chartplotter mirrors that scheme instead of baking every cell at every zoom.

Each cell gets a band

Every cell carries a compilation scale (its CSCL, a 1:N denominator). The baker maps that scale to one of six bands:

Band	Compilation scale (1:N)
Overview	coarser than 1:2,300,000
General	1:500,000 – 1:2,300,000
Coastal	1:130,000 – 1:500,000
Approach	1:32,000 – 1:130,000
Harbor	1:8,000 – 1:32,000
Berthing	1:8,000 and finer

Each band bakes over its own zoom range

A band bakes only the Web-Mercator zoom levels that match its scale. Coarse bands fill the low zooms; fine bands fill the high zooms. Adjacent bands overlap by one zoom so there is no gap at the handoff.

Band	Baked zoom range
Overview	0 – 8
General	8 – 10
Coastal	10 – 12
Approach	12 – 14
Harbor	14 – 16
Berthing	16 – 18

This keeps the bake small. A large-scale harbor cell covers a tiny area, so baking it down to zoom 0 would do nothing useful; a coarse overview cell baked up to zoom 18 would emit millions of redundant tiles. Each cell only produces tiles where its scale is the right one to read.

The client overzooms above a band's max

Vector tiles scale crisply, so the viewer overzooms the top baked level rather than baking more. Zoom past a band's max and MapLibre stretches its highest tiles; the chart stays sharp because the geometry is vector, not pixels. The baker may add a level or two past a band's native max to keep the boundary against the next finer band clean, but the rest is free on the client.

Best-available display across bands

When you bake one archive per band (bake --bands), the viewer loads each band as its own source and shows the finest band that actually covers a point. Where a fine band has no data — open water off the edge of a harbor cell — the next coarser band shows through the gap. Bands never bleed into each other.

Within a band: SCAMIN

A single band still holds more detail than fits at its low zooms. Each feature carries an S-52 SCAMIN (the scale below which it should disappear), and the viewer honors it exactly with a per-SCAMIN layer minzoom. So as you zoom within a band, minor features drop out at their own thresholds and the chart never clutters.

A rule that runs through the schema

Color is always a name, not an RGB value. Fields like color_token and halo_color_token hold S-101 color names. The browser looks them up in colortables.json to get the right Day, Dusk, or Night color. This is what lets the viewer restyle the chart without re-baking.

The seven layers

areas

Filled polygons, such as depth areas and land.

Field	Type	Meaning
color_token	string	Fill color name.
class	string	S-57 object class.
draw_prio	int	Draw priority.
cat	—	Category.
bnd	—	Boundary-pass marker.

area_patterns

Polygons filled with a repeating pattern instead of a flat color.

Field	Type	Meaning
pattern_name	string	Name of the fill pattern.
class	string	S-57 object class.
draw_prio	int	Draw priority.
cat	—	Category.
bnd	—	Boundary-pass marker.

lines

Stroked lines, such as depth contours. Sector-light legs and arcs also go here.

Field	Type	Meaning
class	string	S-57 object class.
color_token	string	Stroke color name.
width_px	int	Stroke width in pixels.
dash	—	Dash pattern.
cat	—	Category.
bnd	—	Boundary-pass marker.

complex_lines

Lines drawn with a named, repeating line style.

Field	Type	Meaning
class	string	S-57 object class.
linestyle_name	string	Name of the line style.
cat	—	Category.
bnd	—	Boundary-pass marker.

point_symbols

Single symbols placed at a point, such as buoys and beacons.

Field	Type	Meaning
class	string	S-57 object class.
symbol_name	string	Name of the symbol.
rotation_deg	number	Rotation in degrees.
scale	number	Scale factor.
offset_x, offset_y	number	Pixel offset from the point.
halo_color_token	string	Halo color name.
halo_width	number	Halo width.
draw_prio	int	Draw priority.
cat	—	Category.
bnd	—	Boundary-pass marker.

When the symbol carries a depth, two more fields appear: danger_depth and sym_deep.

soundings

Depth soundings, drawn as digit symbols.

Field	Type	Meaning
class	string	S-57 object class.
symbol_names	string	The digit symbols that make up the sounding.
scale	number	Scale factor.
draw_prio	int	Draw priority.
cat	—	Category.
bnd	—	Boundary-pass marker.

When the depth is known, three more fields appear: depth, sym_s, and sym_g.

text

Text labels.

Field	Type	Meaning
class	string	S-57 object class.
text	string	The label text.
font_size_px	number	Font size in pixels.
color_token	string	Text color name.
halign, valign	—	Horizontal and vertical alignment.
offset_x, offset_y	number	Pixel offset from the anchor.
halo_color_token	string	Halo color name.
halo_width	number	Halo width.
draw_prio	int	Draw priority.
cat	—	Category.
bnd	—	Boundary-pass marker.