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LAS To Raster function

Overview

The LAS To Raster function is used to render lidar data stored using the LAS file format. The function will be used when you add lidar data to a mosaic dataset using the LAS raster type. With this function, you need to specify both input and output properties. Also, due to the resolution of the data and the time it can take to convert the point data to raster data, this function can write preprocessed raster data files to an output location (cache).

Notes

The function will be used when you add multipoint data to a mosaic dataset. When adding the data to a mosaic dataset, you need to open the properties to define some of the input and output properties, such as pixel size.

LAS is an industry format created and maintained by the American Society for Photogrammetry and Remote Sensing (ASPRS). This function supports version 1.0, 1.1, 1.2, and 1.3.

Input Properties control the selection of different aspects of the LAS dataset to be processed, such as the type of return, classification type, and data type.

The LAS To Raster function allows you to add the LAS data by selecting individual LAS files or selecting one or more folders containing LAS files. When adding a folder, all LAS files in the folder will be added to the mosaic dataset as individual items. Therefore, you will see each LAS file's extent and properties (such as average point distance) in the mosaic dataset. In cases where folders have hundreds or thousands of LAS files, you may want to add the LAS folder as one dataset, thereby creating only one item in the mosaic dataset. To do this, check Treat each folder as a dataset. When using this option, the LAS files must all use the same spatial reference system; otherwise, they will not be added correctly.

It's important to understand that the point spacing estimates are for either all points or only the points per return type or class type. For example, with first or last return types, the point density is high, whereas if you select fifth return types only, the point density will be much less and the average point spacing will be much higher. Typically, the Ground class type has many points, but there will be many voids due to buildings or trees that are removed. If you select buildings only or large trees only, there are even more voids and, therefore, a smaller point density and larger average point spacing.

It is better to go with a pixel size that is several times larger than the average point spacing but small enough to identify gaps or voids. A reasonable size is four times the point spacing. For example, if your data is sampled at 1 meter and your pixel size is 4, you can expect, on average, to get 16 points in a pixel.

Output Properties affect how the LAS data is displayed and how the points are converted to raster.

The output properties are unique to the LAS, LAS dataset, and Terrain raster types. Since the inputs involve some sort of interpolation from points, this can be quite computationally intensive and therefore slow to display. The option to create caches at the base pixel size for the inputs exists to improve performance. Without the cache, you may have to wait several minutes for some surfaces to display.

The output location for the preprocessed raster data files defaults to the location next to the geodatabase where the mosaic dataset is stored for file geodatabases. When using an enterprise geodatabase, the files are stored in the geodatabase by default. This location can be changed on the General tab of the LAS To Raster Properties dialog box.

LAS files added directly to a mosaic dataset will not use the spatial indexes from the LAS auxiliary files (.lasx).

Parameters

PropertiesDescription
Las File Or Folder

The path and name of the LAS files or folder containing the LAS files. You can modify this value if the input is moved. Using a folder is recommended when using many LAS files as multiple partial signals.

Pixel Size

The pixel size must be specified when adding the LAS data to the mosaic dataset

Generally, if the pixel size is three times greater than the point spacing, the voids in the data should be filled (unless, for example, the voids are due to water). When specifying a pixel size that is smaller, you will want to use void filling.

Data Type

Defines the value to represent when generating the surface. Two data type choices are available:

  • Elevation—A height (elevation) value will be used.
  • Intensity—Intensity is a measure, collected for every point, of the return strength of the laser pulse that generated the point. It is based, in part, on the reflectivity of the object struck by the laser pulse. Other descriptions for intensity include return pulse amplitude and backscattered intensity of reflection. Keep in mind, reflectivity is a function of the wavelength used, which is most commonly in the near infrared. Intensity is used as an aid in feature detection and extraction, in lidar point classification, and as a substitute for aerial imagery when none is available. If your lidar data includes intensity values, you can make images from them that look something like black-and-white aerial photos.

The default is Elevation.

Classification

Filters are defined for the points by the provider of the LAS files managed within the LAS dataset. For Classification filters, you can select Any to add all the points regardless of their classification; you can also select more than one. The classification types are listed below.

  • Any
  • Never classified
  • Unclassified
  • Ground
  • Low vegetation
  • Medium vegetation
  • High vegetation
  • Building
  • Noisy low point
  • Model key point
  • Water
  • Rail
  • Road surface
  • Reserved12
  • Wire guard
  • Wire conductor
  • Transmission tower
  • Wire structure connector
  • Bridge deck
  • High noise
Returns

A single pulse from the lidar sensor can be returned more than once as it reflects off objects at different heights on or above the ground, resulting in pulses returning to the sensor at different times. Therefore, the return type can be used to differentiate ground returns from other returns, such as tree canopy.

For the Returns filter, you can select Any to add all the lidar returns, or you can also select more than one. The Return types are First through Fifteenth and Last.

Interpolation Types

There are two interpolation types available:

  • Binning—The process of determining the value of a pixel by examining the points that fall within the pixel to determine the final value

  • Triangulation—Uses Delaunay triangulation to create a surface from a network of triangular facets defined by nodes and edges that cover the surface, which is then rasterized. This is recommended for low-density lidar data, when binning can't be used to create an appealing surface, or when zooming in to an area that will cause a low-density lidar surface to be displayed.

The default is Binning.

Cell assignment type

Determines which z-value to use when generating the raster surface when there is more than one point to consider.

  • Mean—Uses an average z-value

  • Maximum—Uses the largest z-value

  • Minimum—Uses the smallest z-value

  • Sum—Uses the sum of all the z-values

  • Mean Distance Weighted—Uses the z-value resulting from a weighted mean distance

The default is Mean.

Void filling

Voids occur when there are no points collected within the area represented by a pixel in the resultant raster. Voids are often caused by water bodies or by class type selection or exclusion. Void filling is most commonly used when generating a ground surface. Void filling options are listed below:

There are three options for void filling:

  • None—No voids will be filled. This is the default.

  • Simple—Computes the average using up to eight neighboring pixels (with values). Only small voids will be filled.

  • Plane Fitting / IDW—A Simple method is applied first and a plane fitting method is used; however, if the fitting error is too large, an inverse distance weighted algorithm is applied. If the width or height of the bounding box around the void is larger than the Maximum width value, the void is not filled.

The default is None.

Maximum Width

The width value used for void filling when using the Plane Fitting / IDW void filling method. This is defined in the units of the LAS file's spatial reference system. No maximum width will be used if this is blank or a value of 0 is entered.

This parameter is only exposed when Plane Fitting / IDW is selected for Void filling.

Interpolation method

Used when the interpolation type is Triangulation.

The estimation of surface values at unsampled points based on known surface values of surrounding points. Two interpolation methods are available:

  • Linear—Estimates the z-value from the plane defined by the terrain triangle that contains the x,y location of a query point.

  • Natural Neighbor—Estimates the z-value by applying area-based weights to the terrain's natural neighbors of a query point.

The default is Linear.

Z Factor

The scaling factor used to convert the z-values. The scaling factor has two purposes:

  1. To convert the elevation units (such as meters or feet) to the horizontal coordinate units of the dataset, which may be feet, meters, or degrees.
  2. To add vertical exaggeration for visual effect.

When you specify a Z factor value, the Arithmetic function is added to the function chain for the item in the mosaic dataset.

Cache Path

The location where the cached surfaces will be stored. By default, the cache is generated and stored in a folder next to where the mosaic dataset resides. This folder has the same name as the geodatabase, with a .cache extension. However, if the mosaic dataset is created in an enterprise geodatabase, the cache will be created within that geodatabase.

Number of cached surfaces

The maximum number of caches that can be created using different input properties for this surface, for example, using one interpolation method versus another. Entering a value of 0 will disable caching and will clear an existing cache.

The default value is 10.

Disconnect from Las files

Check this box if you do not want your mosaic dataset to communicate with the source LAS files. If you use this option, the cache will be used to render the mosaic dataset. Cached files are faster than accessing the source LAS files; this is especially true when serving a mosaic dataset that has the source LAS files experiencing low performance due to a poor network connection or a slowdown with the data server. If the service becomes too slow, it may become unusable.

Learn more about the LAS To Raster function

Lidar is an active sensor that sends out a laser pulse at a particular frequency and wavelength. This pulse is absorbed or reflected by different surfaces and received by the sensor. A single pulse from a laser can be returned as multiple signals as it reflects off objects at different heights on or above the ground, resulting in pulses returning to the sensor at different times. Therefore, the return type can be used to differentiate ground returns from other returns, such as tree canopy. The LAS To Raster function allows you to select one or more return values.

Diagram of return types
Comparing the timing and intensity of return signals.

Classifications are defined for the points by the provider of the LAS files. You can select Any to add all the points regardless of their classification; you can also select more than one. The classification types from the LAS specification 1.3 .pdfLink to ASPRS website are Any, (0) Never Classified, (1) Unclassified, (2) Ground, (3) Low Vegetation, (4) Medium Vegetation, (5) High Vegetation, (6) Building, (7) Noisy Low Point, (8) Model Key Point, (9) Water, (10) Rail, (11) Road surface, (12) Reserved12, (13) Wire guard, (14) Wire conductor, (15) Transmission tower, (16) Wire structure connector, (17) Bridge deck, and (18) High noise.

LAS example representing elevation

LAS Data Intensity is a measure, collected for every point, of the return strength of the laser pulse that generated the point. It is part of the light cone that is reflected back to the plane. Its value depends on the portion of the cone that was reflected back (for example, a roof is 100 percent, while a leaf is much less) and the reflectivity of the surface hit. If the cone hits a mirror at an angle, nothing comes back. Intensity is used as an aid in feature detection and extraction, in lidar point classification, and as a substitute for aerial imagery when none is available. If your lidar data includes intensity values, you can make images from them that look similar to black-and-white aerial photos.

LAS example representing intensity

Pixel size

Because different features will reflect, scatter, or absorb the lidar laser pulse differently, lidar data contains data voids and irregular spacing between data points. You can compensate for data voids by assigning a Pixel size to create the raster. Generally, if the pixel size is three times greater than the point spacing, the voids in the data should be filled (unless, for example, the voids are due to water). Therefore, the pixel size must be specified when adding the LAS data to the mosaic dataset

It's important to understand that the point spacing estimates are for either all points or only the points per return type or class type. For example, with first or last return types, the point density is high, whereas if you select fifth return types only, the point density will be much less and the average point spacing will be much higher. Typically, the Ground class type has many points, but there will be many voids due to buildings or trees that are removed. If you select buildings only or large trees only, there are even more voids and, therefore, a smaller point density and larger average point spacing.

It is better to go with a pixel size that is several times larger than the average point spacing but small enough to identify gaps or voids. A reasonable size is four times the point spacing. For example, if your data is sampled at 1 meter and your pixel size is 4, you can expect, on average, to get 16 points in a pixel.

In most cases, the point spacing is supplied by the vendor along with the point data files and can be found in a metadata file. If the point spacing is unknown and you have the ArcGIS 3D Analyst extension, you can use the Point File Information tool to obtain a point spacing for the supplied data files; otherwise, enter 1, add the LAS files, and check the mosaic dataset's attribute table for the correct value. If necessary, you can edit the value you entered in the LAS To Raster function.

Output properties

If the pixel size is three to four times larger than the average point distance, you can safely use binning. If the pixel size is smaller than that, you can try binning with void filling turned off. If the resulting raster mainly contains voids and only a few single data cells, binning generally does not produce a meaningful elevation raster. You need to either increase the pixel size or switch to triangulation. If the resulting raster shows enough content with some salt and pepper voids, and maybe a few larger voids, you can use binning with void filling turned on. Click the Void filling drop-down arrow and select either Simple or Plane Fitting / IDW.

Conversion between units and map projections

The scaling factor used to convert the z-values. The scaling factor has two purposes:

  1. To convert the elevation units (such as meters or feet) to the horizontal coordinate units of the dataset, which may be feet, meters, or degrees.
  2. To add vertical exaggeration for visual effect.

To convert from feet to meters or vice versa, see the table below. For example, if your z-units are feet and your mosaic dataset's units are meters, you would use a value of 0.3048 to convert your z-units from feet to meters (1 foot = 0.3048 meters).

This is also useful when you have geographic data (such as GCS_WGS84 using latitude and longitude coordinates) where the z-units are in meters. In this case, you need to convert from meters to degrees (0.00001; see below). The values for degree conversions are approximations.

Conversion factor between feet and meters

FromTo

Feet

Meters

Feet

1

0.3048

Meters

3.28084

1

Conversion factor between feet and meters

To apply vertical exaggeration, you must multiply the conversion factor by the exaggeration factor. For example, if both z-values and dataset coordinates are meters and you want to exaggerate by a multiple of 10, the scaling factor would be the unit conversion factor (1 from the table) multiplied by the vertical exaggeration factor (10), or 10. As another example, if the z-values are meters and the dataset is geographic (degrees), you would multiply the unit conversion factor (0.00001) by 10 to get 0.0001.

Cache

Rendering the LAS dataset can be computationally intensive. Without the cache, you may have to wait several minutes for some surfaces to display. The cache is generated when the following occur:

  • You view the mosaic dataset where the LAS dataset is used to generate the mosaicked image.
  • The overviews are built.
  • The Synchronize Mosaic Dataset tool is run with Build Item Cache checked.

The cache will be updated in the following scenarios:

  • The input has been updated.
  • The cache has been deleted or is missing.
  • The function parameters are set to define a different surface than the one that matches the cache (for example, use a different Return type).