What are LAS and LAZ files and how do you open them? Reading and checking a point cloud
A LiDAR flight ends up as one file holding millions of points.
Field takeaways
- LAS is an open format carrying position, elevation and extra attributes for every point.
- LAZ is the same data compressed losslessly; once opened, it is identical to the LAS.
- The class code says what a point is; ground is class 2.
- If the coordinate system is not in the file, the data will not sit in place however good it looks.
- Check density, coverage and classification before you accept a delivery.
What is inside a LAS file
LAS is an open point-cloud format published by ASPRS. The file opens with a header: the number of points, the bounds of the area it covers, and scale and offset values. Then come X, Y and Z for every point, alongside the return intensity, the return number, a class code and often GPS time and colour. Point clouds from laser scanners, drone LiDAR and photogrammetry all travel in this format.
LAZ: the same data, a smaller file
LAZ is a LAS file compressed losslessly with LASzip. When it is opened, the points are identical to the LAS; no coordinate is rounded. Large projects are usually delivered as LAZ because the file shrinks several times over and is easier to move. If the software reading it supports LAZ, you do not need to convert it to LAS first.
Class codes: what each point is
Every point carries a class code. In the standard ASPRS codes, 1 means unclassified, 2 ground, 3 to 5 low, medium and high vegetation, 6 building, 7 noise and 9 water. Terrain models, cut-and-fill and contours are built from ground points only. In a cloud that has not been classified, treetops and roofs behave like ground, which is why the ground class is the first step of any terrain work.
The coordinate system has to be in the file
From LAS 1.4 onwards the coordinate system can be stored inside the file as WKT. Older files may have this information incomplete or missing altogether. A cloud with an unknown system looks fine on screen, but shifts by metres when laid over the cadastre, the project or another flight. Height is a separate question: are the values ellipsoidal or orthometric heights? Ask for it to be stated in the delivery.
Five checks before you accept a file
Before accepting a point cloud, look at these: does the coverage fully include the required boundary; does the point density per square metre meet the contracted value; has the ground class been done, and are ground points left on bridges or buildings; are the coordinate and height systems written down; is the height difference at a few known control points reasonable? These five questions largely tell you whether the data is fit for the job.
Open the point cloud, pull out the ground.
MapLab Survey 3D views LAS and LAZ files in 3D, takes sections and filters by class, suggests a ground classification you can correct by hand, and produces a terrain surface, cut-and-fill volume and contours as DXF. Windows, on request.
Frequently asked questions
Do I have to convert a LAZ to LAS before opening it?
Not if the software reading it supports LAZ. LAZ is lossless; once opened, the points are the same as the LAS.
Which software opens a LAS file?
Point-cloud viewers and LiDAR software open LAS. MapLab Survey 3D also views LAS and LAZ files in 3D, takes sections and filters by class.
What if there is no ground class?
Ground classification has to come first. Automatic classification is a good start, but bridges, slopes and building edges need checking by hand.
Is a cloud from drone photogrammetry also LAS?
Yes, photogrammetry software usually delivers its point cloud as LAS or LAZ. These clouds cannot see beneath vegetation; ground is only measured in open areas.
Technical references
The field guidance in this article is aligned with the technical and official references below.
Related: Drone land measurement · Cut-and-fill calculation · What are contours? · MapLab Survey