2025 FOSS4G NA | Is Zarr the New COG - Jarrett Keifer & Julia Signell

In this session from FOSS4G NA 2025, Julia Signell and Jarrett Keifer of Element 84 present Is Zarr the New COG? The presenters tackle the growing debate around whether the Zarr data format is poised to replace Cloud Optimized GeoTIFFs (COGs) by providing a technical comparison of how each format handles multi-dimensional arrays, chunking, and compression.

Julia and Jarrett break down the "apples to oranges" comparison, explaining that while COG is a specialized specification built on top of the TIFF format, Zarr is a generic format for storing hierarchies of n-dimensional arrays. They explore the critical roles of chunking and compression codecs in data performance and demonstrate how, with identical settings, the bytes on disk for a Zarr chunk and a COG tile can be virtually the same. The talk also looks ahead to emerging Zarr conventions, such as GeoZarr and multi-scales, that aim to bring COG-like features to the Zarr ecosystem.

Highlights:

🛰️ Defining the Formats: A look at the structural differences between Cloud Optimized GeoTIFFs and the hierarchical array format of Zarr

🔄 Linearization and Chunking: How data is projected from multi-dimensional space to a single-dimension string of bytes on disk

🧩 Compression Codecs: A deep dive into the five types of codecs—entropy, predictive, structural, quantization, and mapping—used to optimize data storage

⚙️ The Faster Myth: Why performance differences are often due to default compression and chunking settings rather than the format itself

🌐 Interoperability vs. Multi-dimensionality: When to stick with the portability of TIFF and when to leverage the n-dimensional power of Zarr

🏢 Zarr Conventions: Understanding how new, community-driven specifications are breaking the deadlock for Zarr's geospatial adoption

🏗️ Virtual Zarr Stores: Using references to access legacy COG data through a modern Zarr interface without duplicating storage

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