What is the BSB file format?
A .bsb file is the chart-set descriptor for a BSB/KAP nautical chart - a raster chart format developed by Maptech (originally Nautical Technologies Inc.) and adopted by NOAA for its official Raster Navigational Charts (RNCs) from the mid-1990s onward. The actual chart imagery is stored in companion .kap files; a single .bsb descriptor can reference multiple .kap panels that together form one navigational chart.
The format uses a mixed structure: an ASCII text header containing chart metadata (version, projection, datum, scale, color palette, and georeferencing control points), followed by a binary raster body where scan lines are palette-indexed and compressed with run-length encoding. The palette is limited to 127 colors - sufficient for nautical charting symbology. Georeferencing is achieved through REF entries in the header that map pixel coordinates to latitude/longitude positions, enabling chart plotters to overlay a vessel's GPS position accurately.
BSB versions 1 through 3 are unencrypted; version 4.0 introduced an encrypted variant using the .cap extension. The format was widely supported by marine navigation software and embedded chartplotters for three decades.
NOAA completed its raster chart sunset program in December 2024, canceling the remaining 1,007 RNC charts in favor of vector-based Electronic Navigational Charts (ENCs) in S-57/S-101 format. BSB/KAP files remain in use in legacy chartplotters and offline navigation applications, and GDAL provides open-source read support via its BSB raster driver.
Security & safety
RISK: LOWRaster image/data files; cannot execute code. Encrypted BSB v4.0 files (.cap) require a license key to decrypt; unauthorized distribution of licensed chart content may violate copyright. NOAA-produced RNCs are public domain.
Format details
in a nutshellPrograms that open BSB files
Windows4 apps
OpenCPN
Open-source Add BSB/KAP charts via Options > Charts > Chart Files; supports .kap and .bsb natively.
GDAL (gdal_translate)
Open-source gdal_translate -of GTiff chart.kap output.tif - convert to GeoTIFF from command line. 
macOS1 app
Linux2 apps
Technical details
deep spec| Header format | ASCII text block containing version, copyright, projection, scale, datum, color table (RGB), and georeferencing REF control points |
| Raster encoding | Palette-indexed (7-bit); scan lines compressed with per-row run-length encoding (RLE) |
| Color depth | 7-bit palette - maximum 127 distinct colors per chart |
| Encryption | None in BSB v1-3.x; BSB v4.0 introduced encryption (.cap extension used for encrypted charts) |
| Byte order | Big-endian (raster row data) |
| Georeferencing | REF entries in ASCII header map pixel (x, y) to WGS-84 lat/lon; Mercator projection standard |
| Primary vs. descriptor file | .bsb is the chart-set descriptor; .kap files contain the chart imagery (one .kap per panel) |
| Typical chart file size | 1 MB - 50 MB per chart; full-scale NOAA RNCs average 5-15 MB |
| Chart pixel dimensions | Approximately 3,000x4,000 to 10,000x15,000 pixels for NOAA charts |
| Format versions | 1.0, 1.1, 2.0, 3.0, 4.0 (v4 adds encryption; encrypted charts use .cap extension) |
| Format status | Legacy - NOAA completed raster chart cancellation December 2024; replaced by S-57/S-101 ENC vector format |
| MIME type | image/x-bsb |
| Open-source support | GDAL BSB raster driver provides read support on all platforms; OpenCPN reads BSB/KAP natively as a cross-platform chartplotter |
| Released | 1990 (Maptech/BSB Electronic Charts, Version 1.0); NOAA adopted for RNCs circa 1994 |
| Latest version | BSB Version 4.0 (introduced encrypted .CAP variant) |
| Specification | gdal.org |
BSB conversions
Community Q&A
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