Statistical analysis for LAS datasets

The ability to produce statistical information for LAS files referenced by the LAS dataset is essential to better understand the lidar data you are working with. When statistics are calculated, a LAS auxiliary file (.lasx) is created for each LAS file. The auxiliary file is given the same name as the source LAS file and is stored in the same location on disk. It stores the calculated statistical information and the spatial index for each LAS file.

Statistical analysis allows you to conduct initial QA/QC on lidar data to ensure that the data deliverables are exactly what was expected. It also allows you to determine the overall quality and extent of the data prior to conducting time-consuming processes on the lidar data. The LAS dataset provides the ability to generate a fast and in-depth statistical analysis on your lidar data. For more information on assessing the coverage and density of your lidar data, see Assessing lidar coverage and sample density.

When statistics are calculated, the point spacing for each lidar file is estimated by using a binning method that incrementally evaluates small areas of the file to determine the final value. Without statistics. a less accurate estimate is used.

Statistics can be computed for an entire LAS dataset containing several LAS files. or they can be computed for a single file.

Note:

Point spacing is not the same as point density. Point spacing (PS) is defined as linear units per point, and point density (PD) is defined as points per square unit area. To convert point spacing to point density, use the following equation: PD = 1/(PS)^2. A higher point density means lower values for point spacing.

How to generate statistics on a LAS dataset

Statistics can be generated and updated in several ways for the LAS dataset. You can use any of the following methods to calculate statistics on your LAS dataset:

• Create LAS Dataset geoprocessing tool
• LAS Dataset Statistics geoprocessing tool
• LAS Dataset Properties dialog box in ArcCatalog or the Catalog window

Note:

If LAS files that are added to a LAS dataset already contain a LAS auxiliary file (.lasx), statistics have already been calculated and will not need to be recalculated.

Create LAS Dataset geoprocessing tool

The Create LAS Dataset geoprocessing tool quickly builds a LAS dataset referencing LAS files by scanning the header information of the LAS files. It will optionally scan through the lidar files referenced by the LAS dataset and calculate statistics about each LAS file individually, as well as the collection of LAS files. The results can be reviewed through the LAS Dataset Properties window in ArcCatalog or the Catalog window.

To generate statistics using the Create LAS Dataset geoprocessing tool, check the Compute Statistics parameter prior to executing the tool.

Note:

Calculating statistics requires a complete scan of the referenced LAS files. The time it takes to compute statistics will correspond to the number of points in the LAS files, but will yield much more information. Gathering statistics is recommended if you are not in a time-critical situation. Keep in mind that statistics can always be calculated at a later time using either the LAS Dataset Statistics geoprocessing tool or the LAS Dataset Properties.

LAS Dataset Statistics geoprocessing tool

The LAS Dataset Statistics geoprocessing tool generates statistics for LAS datasets and optionally outputs the statistical analysis to a text file. The tool provides the ability to generale an output report that can then be loaded into a spreadsheet or database program. The tool scans through the lidar files referenced by the LAS dataset and calculates statistics about each LAS file individually, as well as the collection of LAS files.

If statistics were not generated using the Create LAS Dataset tool, use the LAS Dataset Statistics geoprocessing tool to calculate statistics.

The results can be reviewed through the LAS Dataset Properties window in ArcCatalog or the Catalog window.

ArcCatalog or the Catalog window

The LAS Dataset Properties dialog box from ArcCatalog or the Catalog window also allows you to calculate or update statistical information for the LAS dataset. This dialog box reports information about the LAS dataset as a whole, plus details about the specific LAS files that participate in the dataset.

If statistics were not generated using either the Create LAS Dataset or the LAS Dataset Statistics geoprocessing tool, you can quickly calculate statistics for a LAS dataset interactively or an individual LAS file using the LAS Dataset Properties dialog box.

The statistical results can be reviewed immediately for the LAS dataset through the Statistics tab of the LAS Dataset Properties window in ArcCatalog or the Catalog window. For individual LAS files, the statistics can be reviewed from the LAS Files Properties and Statistics window.located on the LAS Files tab. The LAS Dataset Properties dialog box cannot be used to export the statistics to a text file; use the LAS Dataset Statistics geoprocessing tool to create an output report.

1. Right-click the LAS dataset in ArcCatalog or the Catalog window and select Properties.
2. Select the Statistics tab on the LAS Dataset Properties dialog box.
3. If the LAS dataset has not had statistics calculated, click the Calculate button to calculate statistics. If the LAS dataset has already had statistics calculated, click the Update button to update statistics.

Note:

The Calculate button will compute statistics for the first time, or recompute statistics if they are out of date as a result of editing.

To learn more about the LAS Dataset Properties dialog box, see LAS Dataset Properties.

How to generate statistics on a single LAS file

The LAS Dataset Properties dialog box reports information about the LAS files that participate in the LAS dataset. Follow the steps below to calculate statistics for a single LAS file referenced by the LAS dataset.

1. Right-click the LAS dataset in ArcCatalog or the Catalog window and select Properties.
2. Select the LAS Files tab on the LAS Dataset Properties dialog box.
3. From the list of LAS files, select the button under the Statistics column that corresponds to the LAS file for which statistics will be generated.
4. From the LAS Files Properties and Statistics dialog box, click the Calculate button to calculate the statistics for the specific LAS file.

Note:

This button will compute statistics for the first time or recompute statistics if they are out of date. You can also use the Prev File and Next File buttons to compute statistics for other LAS files referenced by the LAS dataset.

To learn more about the LAS Dataset Properties dialog box, see LAS Dataset Properties.

Accessing statistical information

LAS dataset statistics can be calculated and displayed using the LAS Dataset Properties dialog box in ArcCatalog or the Catalog window or output to a report file through the LAS Dataset Statistics geoprocessing tool. Statistical information for individual LAS files can be accessed through the LAS Files tab on the LAS Dataset Properties dialog box in ArcCatalog or the Catalog window.

Understanding statistics

LAS Dataset - Statistics tab

• Returns—Statistical information is calculated based on the return values for the lidar points of the LAS dataset. The statistics include the point count, the percentage of points, the z minimum, and the z maximum.
• Attributes—Statistical information is calculated from the lidar attributes for the LAS dataset. The statistics include the minimum and maximum values.
• Classification Codes—Statistics are calculated based on the classification completed on the LAS dataset. The statistics include the point count, the percentage of points in each classification code, the z minimum, the z maximum, the minimum intensity, the maximum intensity, and the synthetic point count.
• Points With Encoding Flag—Synthetic, key-point, and withheld flags can be set for each lidar point. These flags can be set along with the classification flag. The statistics include the point count and the percentage of the lidar points in each encoding flag for the LAS dataset.

LAS Files tab - LAS File Properties and Statistics

• General—The general section contains general information about the specific LAS file. Most of the general information is obtained from the header section of the individual LAS file.

  The general section includes the file name, LAS version and point record format, point count, file size, spatial reference, date the LAS file was created, the number of model key points, the number of synthetic points, the number of withheld points, the number of overlap points, if the LAS file has RGB, if the LAS file has GPS time, whether or not it is in standard time, the system ID, the generating software, the project ID, and the file source ID.

• Extent—The extent of the LAS file.
• Returns—Statistical information is calculated based on the lidar return values in the LAS file. The statistics include the point count, the percentage of points, the z minimum, and the z maximum.
• Attributes—Statistical information is calculated from the lidar attributes for the LAS file. The statistics include the minimum and maximum values.
• Classification—Statistics calculated based on the classification completed on the LAS file. The statistics include the point count, the percentage of points in each classification code, the z minimum, the z maximum, the minimum intensity, the maximum intensity, and the synthetic point count.
• Points With Encoding Flag—Synthetic, key-point, and withheld flags can be set for each lidar point. These flags can be set along with the classification codes. The statistics include the point count and the percentage of lidar points in each encoding flag in the LAS file.

Exporting statistical information

A text file can be output detailing the statistics computed for the LAS dataset or the individual LAS files. Use the LAS Dataset Statistics geoprocessing tool to output a text file with the statistical information.