BAG (Bathymetric Attributed Grid)
Dec 5,2025

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Introduction

BAG (Bathymetric Attributed Grid) is an open, standardized data format used for marine surveying and seabed terrain modeling, developed and maintained by the Open Navigation Surface Working Group. It employs an HDF5 container to organize data, with core components including the bathymetric grid elevation layer (storing seabed depth values) and the uncertainty layer (recording depth measurement errors), while also supporting embedded metadata (e.g., coordinate reference systems, acquisition device parameters) and optional layers (such as sounding point density and seabed classification). The format utilizes tile-based compression technology to optimize the storage and transmission efficiency of large-scale seabed terrain data and is compatible with multi-resolution pyramid structures. It is one of the marine bathymetric data exchange standards recommended by the International Hydrographic Organization (IHO).

File Structure

BAG uses an HDF5 container with a layered structure:

1. Core Data Layers

Elevation layer: Stores seabed depth grid data
Uncertainty layer: Records error range for each depth value

2. Metadata Section

Coordinate system definition (e.g., WGS84)
Data extent, resolution, acquisition information
Global attributes including format version

3. Optional Extension Layers

Sounding point density distribution
Seabed classification
Data quality flags

4. Optimization Structures

Tiled storage: Divides large datasets into blocks for compression and fast access
Multi-resolution pyramids: Stores data at different zoom levels for rapid visualization

Pros

High Standardization and Openness: Compliant with International Hydrographic Organization (IHO) standards, based on the open-source HDF5 format, supporting cross-platform data exchange.
Complete and Reliable Data: Mandatory inclusion of elevation and uncertainty layers, with comprehensive metadata recording, ensuring data quality and traceability.
Efficient Storage and Access: Utilizes tile-based compression technology to reduce storage space and supports multi-resolution pyramids for rapid browsing and online publishing.
Broad Application Scenarios: Extends functionality through optional layers, meeting diverse needs such as marine surveying, navigation safety, scientific research, and engineering planning.
Excellent Software Compatibility: Widely supported by mainstream GIS software (e.g., QGIS, ArcGIS) and web mapping services (e.g., Geoserver).

Cons

High Technical Requirements: Relies on the HDF5 technology stack, making direct operation challenging for non-professionals and requiring specialized tools for data processing and maintenance.
Limited Structural Flexibility: The fixed "elevation + uncertainty" dual-layer structure may be overly complex for simple applications and does not support streaming transmission.
Limited Processing Performance: Pyramid construction for extremely large datasets is time-consuming, lacks distributed storage support, and single-file processing is constrained by hardware limitations.
Restricted Application Domains: Primarily serves the marine surveying field, with relatively weak application ecosystems and support in other geospatial information domains.
Version Compatibility Challenges: Differences in metadata fields between versions may cause compatibility issues, and custom extensions lack unified standards.

Application Scenario

The BAG format serves as a core data carrier for marine surveying, navigation safety, and underwater engineering, primarily used in high-precision seabed terrain modeling and visualization, nautical chart production and obstacle detection, route planning for underwater pipelines/cable laying, marine scientific research (e.g., seabed geomorphological evolution analysis), and 3D reconstruction of shipwrecks/underwater cultural heritage sites.