.LAZ

Introduction

LAZ is a LAS lidar point cloud compressed with LASzip, created by Martin Isenburg of rapidlasso and released as open source in 2011. It is lossless: decompressing gives back exactly the original LAS file, usually from a file a fifth to a tenth of the size.

Because lidar surveys run to billions of points, LAZ has become the usual form for publishing and downloading them, including many national elevation datasets.

Structure

A LAZ file keeps the LAS layout. It starts with the same public header block and its LASF signature, and the header and variable length records are not compressed, so a reader can see the point count, bounds and coordinate system without decompressing anything. A variable length record from LASzip, with the user ID laszip encoded, describes the compression, and the point format number in the header has its top bit set to mark the points as compressed.

The points are compressed in chunks, by default of 50,000 points, each predicted from the points before it and coded with arithmetic coding. A chunk table at the end lets a reader jump to any chunk.

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