Creating intensity images from lidar in ArcGIS

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 back scattered 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. ArcGIS provides the ability to creating intensity imagery from lidar data.

Lidar intensity image

1. Loading points with intensity

The first step in creating an intensity image in ArcGIS is to import your source points into the geodatabase as multipoints. The source lidar dataset must be in either LAS or ASCII XYZI file format.

If your data is in LAS format, use the LAS To Multipoint geoprocessing tool with intensity selected as an attribute to import, as shown in the image below. If your data is in ASCII XYZI format, use the ASCII 3D To Feature Class geoprocessing tool.

It is most common to use first return lidar data to create intensity images. If your lidar data is in ASCII format, format the ASCII files representing first return point data. If your lidar data is in LAS format, specify the class code on the LAS to Multipoint dialog box to represent first return lidar data. The tool will then filter the points and only import the first return points into the multipoints.

The data loading process results in a multipoint feature class with an attribute field containing the intensity values (see below). You cannot read these attributes directly because they are packed into binary large objects (BLOBs) in the multipoint feature class. Every vertex of a multipoint has intensity mapped into the BLOB and is stored for that multipoint record.

Intensity values stored in BLOBs

2. Generating an intensity image

Use the Point To Raster geoprocessing tool to generate an intensity image from the multipoint feature class. Populate Input Features with the name of the multipoint feature class and specify Intensity BLOB for Value field. Select MEAN from the Cell assignment type options for a rasterization method. Set a cell size that is approximately four times larger than the average point spacing. Once the geoprocessing tool has been run, the resulting raster is the lidar intensity image.

You might find one of the other Cell assignment type options useful for analysis, for example, RANGE of intensity as a variable used in feature detection.

Point To Raster tool

Finally, review the presence of NoData areas. Significant numbers of NoData cells will result if the output cell size you specified is too small relative to the density of the lidar points. You can see NoData cells by assigning a color to them on the Symbology tab of the raster's Layer Properties dialog box. If there are several areas with no data available, the easiest thing to do is go back and rerun the Point To Raster geoprocessing tool with a larger cell size. Alternatively, you can fill in missing values using an expression in the Spatial Analyst calculator. See Creating raster DEMs and DSMs from large lidar point collections.

Image display

The range of values in the image is hard to predict without knowing details about how your data was collected and processed. For one thing, the original intensity values are sensor dependent. Secondly, the values may have been adjusted by the vendor (for example, normalized to a range of 0–255). Due to these variables, it is hard to determine what the best display options are for intensity imagery. Experiment with the intensity raster layer stretch type and contrast. Turning on bilinear resampling is probably a good idea. If you are looking for more display possibilities, consider combining intensity with another variable such as hillshade. The intensity image below is displayed with 50 percent transparency on top of a hillshade of the first return surface.

Intensity image draped on a hillshade

The return intensity collected for every lidar point can be used to make images. These images have a variety of uses in GIS applications including feature detection and extraction. ArcGIS provides tools to make these images.