LIDAR Classification

Purpose

To identify different types of LIDAR return and what they relate to. For instance the basic groups will be: ground, vegetation, buildings and noise. For the purpose of ARSF LIDAR deliveries we just classify noisy points. These typically tend to be due to the atmospheric conditions (cloud and haze), signal noise or single isolated points.

Method

We can use the TerraScan software or the in-house classify_las program for point classification. The TerraScan usage can be found on other pages of the wiki.

classify_las.out

This is a c++ utility written to do basic classification on LAS files. The current classification algorithms include:

• Isolated points - identify points as noise (classification 7) if there is no other point within x metres of it.
• Absolute elevation - identify points as noise (classification 7) if they are above or below a given elevation.

A fence can be used to only classify points within a rectangular boundary.

Usage

YOU MUST BE ON A FEDORA 12 MACHINE TO RUN THIS

 -lasfilein <LASFILENAME>    The LAS file to be imported and classified
 -lasfileout <LASFILENAME>   The name of the output LAS file 

optional flags

 -isolated x                 Run the isolated points algorithm using a distance of x metres
 -above x                    Run the absolute elevation algorithm classifying points with elevation above x metres
 -below x                    Run the absolute elevation algorithm classifying points with elevation below x metres   

 -fence minx miny maxx maxy  Use a fence whose bounds are given by minx,miny maxx,maxy   

Currently only one method can be used per run of the software. This is due to be changed shortly. Also, the output LAS file will no longer have its points ordered by time, but rather by the quadtree / pointbucket ordering which is used to speed up searching within the software.

Example - to classify isolated points

classify_las.out -lasfilein example.LAS -lasfileout classified.LAS -isolated 5