What is the AT5 file format?
An .at5 file is a Navico Atlas Type 5 chart used by Lowrance, Simrad, and B&G GPS chartplotters for marine navigation and topographic mapping. The format stores tiled vector and raster chart data - depth contours, navigation aids, shaded relief, and optionally satellite imagery - in a proprietary binary container.
A complete AT5 chart set delivered on a microSD card consists of an at5.xml manifest file (which indexes the chart collection), one or more large .at5 data files, and a ShadedRelief/ folder holding raster hillshade tiles. The binary container uses little-endian byte order and is identified by the magic bytes 55 AA 05 00 at the file start.
Commercial Insight charts sold by Navico are encrypted and device-locked to a specific chartplotter serial number, making them non-transferable. Custom charts created with tools such as GPSMapEdit or ReefMaster are generally unencrypted and freely transferable between compatible devices.
The AT5 format succeeded the earlier AT4 format and is compatible with a wide range of Navico-family hardware. For chart creation and PC-side management, Navico Insight Map Creator and ReefMaster can produce AT5 chart sets. The format has no direct equivalent in standard GIS interchange formats such as KML or GPX, and dedicated tools are required to work with AT5 data outside Navico hardware.
Security & safety
RISK: LOWAT5 is a chart data file with no executable code. The main caution: purchasing chart files from unofficial resellers may violate Navico licensing and the device-lock may prevent them from working anyway.
Format details
in a nutshellPrograms that open AT5 files
Windows4 apps
GPSMapEdit
Freemium Open or create vector map data and export as AT5 for Navico chartplotters. Used for custom land/water chart creation.
MapCreate
Paid Legacy Lowrance PC software for managing and deploying charts to Lowrance devices. Largely superseded by Insight Map Creator.
Navico Insight Map Creator
Free Use Insight Map Creator to create, edit and export custom AT5 chart sets for Lowrance/Simrad/B&G devices. Download from Navico/Navionics support pages.
ReefMaster
Paid Import sonar/bathymetric data and export as AT5 chart to load on Lowrance/Simrad chartplotters. Primary tool for custom fishing depth maps. Technical details
deep spec| File encoding | Binary, little-endian |
| Magic bytes | `55 AA 05 00` at byte offset 0 |
| Data types | Tiled vector layers (depth contours, navigation aids) combined with raster tiles (shaded relief, satellite imagery) |
| Companion files | `at5.xml` manifest (chart index) + `ShadedRelief/` folder of raster hillshade tiles on the same microSD card |
| Typical file size | 1 MB - 500 MB (varies with geographic coverage and resolution) |
| Tile compression | Internal per-tile compression (zlib/deflate per tile) |
| Encryption | Commercial Insight charts are device-locked (bound to chartplotter serial number); custom/community AT5s are typically unencrypted |
| Hardware targets | Lowrance, Simrad, and B&G GPS chartplotters (delivered via microSD card) |
| Format predecessor | AT4 (earlier Navico chart format) |
| Raster tile color depth | 24-bit RGB (shaded relief and satellite imagery tiles) |
| Chart creation tools | Navico Insight Map Creator, GPSMapEdit, ReefMaster |
| Platform | Windows (creation tools); Lowrance/Simrad/B&G chartplotter hardware (deployment) |
| Released | Approximately 2008-2010 (Navico Insight mapping era) |
| Latest version | AT5 format still current for compatible Lowrance/Simrad/B&G chartplotters as of 2026 |
| Specification | www.memotech.franken.de |
AT5 conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about AT5 files.
