Lidar point classification
Every lidar point can have a classification assigned to it that defines the type of object that has reflected the laser pulse. Lidar points can be classified into a number of categories including bare earth or ground, top of canopy, and water. The different classes are defined using numeric integer codes in the LAS files.
Classification codes were defined by the American Society for Photogrammetry and Remote Sensing (ASPRS) for LAS formats 1.1, 1.2, or 1.3.
Only LAS 1.1 and higher has a predefined classification system in place. Unfortunately, the LAS 1.0 specification does not have a predefined classification scheme, nor do the files summarize what, if any, class codes are used by the points. You need to be given this information from the data provider.
Classification Bit Field Encoding
When a classification is carried out on lidar data, points may fall into more than one category of the classification. Classification bit fields are used to provide a secondary description or classification for lidar points. With the LAS version 1.0, a lidar point could not simultaneously maintain two assigned classification attributes.
For example, a lidar return from water may need to be removed from the final output dataset, but it still should remain and be managed in the LAS file as a collected lidar point. Using LAS version 1.0, this point could not be set as both water and withheld from analysis.
In later versions (LAS 1.1, 1.2, and 1.3), the original eight-bit encoded field was split to solve this problem. The lower five bits were used to define the classification codes 0 through 31, and the three high bits were used for flags. Three classification flags were added to the LAS standard to flag points with information additional to the traditional classification. Synthetic, key-point, and withheld flags can be set for each lidar point. These flags can be set along with the classification codes.
For example, in LAS versions 1.1, 1.2, and 1.3, a water record could be given a classification code for water (9), as well as a withheld flag. The point will remain in the dataset but will be withheld from any additional analysis on the LAS files.
The following table describes the bit definition for LAS classification.
Bits | Field Name | Description |
|---|---|---|
0–4 | Classification | The ASPRS standard classification shown below |
5 | Synthetic | A point that was created by other than lidar collection, such as digitized from a photogrammetric stereo model |
6 | Key-point | A point considered to be a model key-point and should not be withheld in any thinning algorithm |
7 | Withheld | The point should not be included in processing |
Classification Codes
If you are working with the LAS 1.1, 1.2, or 1.3 specification, refer to the predefined classification schemes defined by American Society for Photogrammetry and Remote Sensing (ASPRS) for the desired data category. The following table lists the LAS classification codes defined by ASPRS.
Classification Value (bits 0-4) | Meaning |
|---|---|
0 | Never classified |
1 | Unassigned |
2 | Ground |
3 | Low Vegetation |
4 | Medium Vegetation |
5 | High Vegetation |
6 | Building |
7 | Noise |
8 | Model Key |
9 | Water |
10 | Reserved for ASPRS Definition |
11 | Reserved for ASPRS Definition |
12 | Overlap |
13–31 | Reserved fro ASPRS Definition |
Modifying classification codes
You are able to use either the Change LAS Class Codes or the Set LAS Class Codes Using Features geoprocessing tools. For interactive editing, you can use the LAS Dataset Profile View to modify the current classification of the LAS files contained in a LAS dataset. For example, you can update to the current class code of 2 to represent bare earth, if your LAS files contain a different class code for bare earth.
Flags can be set or cleared in any combination using ArcGIS.
For more information on working with classification codes in ArcGIS, see Working with LAS classifications.