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Lesson 2: Explore and Classify Lidar Data

This tutorial illustrates upgrading LAS versions, viewing LAS classifications and manually classifying LAS class metadata.

For more lessons visit Working with Lidar Point Clouds.

The sample data used in this lesson is 3DEP lidar data in two LAZ files, containing LAS version 1.2, located in Madison, MS, USA. Prior to commencing this lesson, please download the sample data:

This lesson will take approximately 20 minutes.

The following video executes the steps in the tutorial.

1. LAS Classifications

LAS(er) files are an open format defined by ASPRS. LAS files contain the spatial location of points along with additional attribution stored in metadata fields for each point.

" The LAS file is intended to contain LIDAR (or other) point cloud data records. The data will generally be put into this format from software (e.g. provided by LIDAR hardware vendors) which combines GPS, IMU, and laser pulse range data to produce X, Y, and Z point data. The intention of the data format is to provide an open format that allows different LIDAR hardware and software tools to output data in a common format.* "

The American Society for Photogrammetry & Remote Sensing (2013). LAS SPECIFICATION VERSION 1.4 – R13. Retrieved from https://www.asprs.org/a/society/committees/standards/LAS_1_4_r13.pdf

LAZ files are a losslessly compressed version of LAS files created and maintained by rapidlasso GmBH which significantly reduce the storage requirements of LAS files. uniDEM uses LASzip to read and write LAZ files.

The following section discusses some key differences between LAS version 1.2 and version 1.4 by focusing on LAS CLASS and Flag differences.

uniDEM Support Notes:

LAS Flags

  • Synthetic
  • Key-point
  • Withheld
  • Overlap (v1.4)

Compare LAS Class between v1.2 and v1.4

Value LAS Class v1.2 LAS Class v1.4
0 Created, never classified Created, never classified
1 Unclassified Unclassified
2 Ground Ground
3 Low Vegetation Low Vegetation
4 Medium Vegetation Medium Vegetation
5 High Vegetation High Vegetation
6 Building Building
7 Low Point (noise) Low Point (noise)
8 Model Key-point (mass point) Reserved
9 Water Water
10 Reserved for ASPRS Definition Rail
11 Reserved for ASPRS Definition Road Surface
12 Overlap Points Reserved
13 Reserved for ASPRS Definition Wire – Guard (Shield)
14 Reserved for ASPRS Definition Wire – Conductor (Phase)
15 Reserved for ASPRS Definition Transmission Tower
16 Reserved for ASPRS Definition Wire-structure Connector
17 Reserved for ASPRS Definition Bridge Deck
18 Reserved for ASPRS Definition High Noise
19-31 Reserved for ASPRS Definition Reserved
32-63 Exceeds maximum value Reserved
64-255 Exceeds maximum value User definable

2. Load and upgrade to LAS 1.4

  1. Click uniDEM icon
  2. Select Load LIDAR
  3. Select both LAZ files and click OK
  4. Select Upgrade file format to LAS version 1.4
  5. Click Yes to all files into single dataset
  6. Enter Madison for the combined dataset name
  7. Data is displayed in the Table Of Contents (TOC)
upgrade to las v1.4

Help references:

Load LIDAR | Data Management | Table of Contents

3. Display Lidar Point Cloud

  1. Right Click Madison in the TOC
  2. Select Check All
  3. Lidar points are colored by LAS class
  4. Within the LIDAR tab
    • Set (Point) Density to Normal
    • Set Point Size to Large
show lidar dataset

Help references:

Table of Contents | Point Density | Point Size

4. Profile LAS Classes

  1. Within the LIDAR tab
  2. Click Class Profile
  3. Click OK to use the Madison dataset
  4. Click Select All to select both LAZ files
  5. Click OK

Classes in Madison dataset:

  • 1 - Unclassified
  • 2 - Ground
  • 3 - Low Vegetation
  • 4 - Medium Vegetation
  • 7 - Low Point (Noise)
  • 9 – Water
  • 11 – Road
las class profile

Note: Only Ground and Water represent the defined LAS class accurately.

Help references:

Class Profile

5. View Specific Classification Values

View Overlap

Road (11) has been used to represent Overlap (12) in this dataset.
  1. Within the Lidar tab
  2. Click Show/Hide Classes button
  3. Select Never Classified (0) and Road (11)
  4. Click OK
Observe: Only the "Overlap" points are displayed in the data view.
view overlap class

Open Multiple Data Views

  1. Within the Home Tab
  2. Press Ctrl-N 3 times to create a total of four data views
  3. Press Ctrl-T to organize the windows
  4. Click into the top right window
  5. Right click the Madison dataset in the TOC
  6. Select Check All
  7. Repeat for bottom two windows
  8. All windows are linked for movement
show 4 lidar data views

Vary Classes per Window

  1. Click into top right window
  2. Within the LIDAR tab
  3. Click Show/Hide Classes
  4. Select Ground (2) and Water (9)
  5. Click into bottom left window
  6. Click Show/Hide Classes
  7. Select Low Vegetation (3) and Medium Vegetation (4)
show 4 windows with different classifications

Help references:

Table of Contents | Show/Hide Classes | New Window (Ctrl-N) | Tile Windows (Ctrl-T)

6. Reassign Road (Overlap) Classification

Road (11) has been used to represent the overlap. First reclassify to Never Classified (0) and then assign the Overlap Flag.

Reclassify Overlap

  1. Click into top left window
  2. Press R to enable the rectangle drawing tool
  3. Hold Shift + left click while moving the mouse to draw rectangle around one overlap region
  4. Within the LIDAR tab
  5. Click Set From Drawing
  6. Click OK to use the Madison dataset
  7. Click Select All to use both LAZ files
  8. Select Road (11) class for reassignment
  9. Click OK
  10. Select Never Classified (0) for the new class
  11. Click OK
  12. Click OK to accept all geometries ABOVE
  13. Click OK to accept all geometries BELOW
reclassify LAS Road to Never Classified Observe: Overlap region is colored white represents class Never Classified (0) for the selected region.

Set Overlap Flag

  1. Within the LIDAR tab
  2. Click Tranfer to Metadata
  3. Click Yes to acknowledge Metadata overwrite warning
  4. Click OK to select the Madison dataset
  5. Click Select All to select both LAZ files
  6. Select 0: Never Classified as the class to Apply to Metadata
  7. Select Overlap as the Metadata field to adjust from the dropdown
  8. Enter 1 for the New Metadata Value indicating the region is an overlap
Observe:
  • Mouse over modified overlap region and observe LAS Overlap flag value of 1 in Info box
  • Mouse over unmodified region and observe LAS Overlap flag value of 0 in Info box
set LAS overlap flag

Help references:

Rectangle Drawing Tool | Set (LAS class) From Drawing | Transfer (LAS class) to Metadata | Table of Contents

7. Correct Classifications Manually

Low Vegetation (3) and Medium Vegetation (4) have been used to classify both buildings and vegetation. Polygons can be drawn around buildings to correct the LAS Classification.

Reclassify Building

  1. Click into bottom right window
  2. Press P for the polygon drawing tool
  3. While holding Shift right click to add vertices around one building
classify LAS Building

Reclassify Building

  1. Within the LIDAR tab
  2. Click Set From Drawing
  3. Select the Madison dataset and LAZ files
  4. In "Select existing classes to reassign:" pick
    • 3: Low Vegetation
    • 4: Medium Vegetation
  5. In "Select new class:" pick
    • 6: Building
  6. Click OK
  7. Click OK to accept all geometries ABOVE
  8. Click OK to accept all geometries BELOW
classify LAS Building
Observe: Medium and High Vegetation reclassified to Building (6), but Ground (2) is unchanged.

Help references:

Polygon Drawing Tool | Set (LAS class) From Drawing | Transfer (LAS class) to Metadata | Table of Contents