Lab 5: LiDAR

Goals and Background:

The main goal of this assignment was to obtain basic skills on LiDAR data structure and processing. In doing so two main objectives were to be accomplished within the three part assignment. They are:

• Finding and processing surface terrain models such as a Digital Surface Model (DSM), and a Digital Terrain Model (DTM).
• Using point cloud, LAS formatted data to create an intensity image.

Within each of the three parts, smaller objects are met to work towards the two main objectives. They are as follows:

Part 1:

• Use Erdas Imagine to input and visualize LiDAR point clouds.
• Examine different tile boundaries for overlap, to ensure to data voids within the image. 

Since this part is used only as a way to familiarize with the data that will be used, and no resulting product is derived from it, it will not be discussed within further sections of this post.

Part 2:

• Preform quality check by verifying LiDAR statistics and coordinate systems for point cloud data of the City of Eau Claire, WI.

Part 3:

• Create both a DSM and a DTM from LAS files.
• Using "hillshade" to enhance both DSM and DTM.
• Create an intensity image from the LAS files.

Methods:

For each part, separate headers will be listed with associated methods underneath. However, before any of the assignment's parts were completed, data management was first be dealt with. To do so, a separate folder needed to be created to be used as an active work space to store all associated output files that were created. This file is labeled "lab5". As previously mentioned, part one will not be discussed as it was used only to help in associating with the data that was to be used in further parts of the assignment.

Part 2:

The first step in this part was to first create a sub folder within "lab5" named LAS_data. Then file connections were to be made in both ArcMap, and ArcCatalog. Furthermore, the workspace within environments, on ArcMap needed to be designated to "lab5".

Within ArcCatalog, a new LAS dataset (Eau_Claire_City) was created within the LAS_data folder. LAS data files of the City of Eau Claire, which were provided by the instructor, were then uploaded into this dataset. Within the dataset properties window the statistics tab was selected, and statistics were calculated for the new LAS dataset.

The next step in setting up the new dataset was to assign a projected coordinate system. To accomplish this, first the metadata was consulted to verify both horizontal (X,Y), and vertical (z) coordinate systems. Once this was done, the XY coordinate system's tab was opened under dataset properties. From here, horizontal coordinates were set to NAD 1983 HARN Wisconsin CRS Eau Claire (US Feet) and vertical coordinates were set to NAVD 1988 US Feet.

Once this was completed, the new LAS data set was uploaded and displayed in ArcMap. In order to view point access dataset properties, the number of viewing classes was changed from nine to eight. Then the LAS dataset toolbar was activated within ArcMap. On this toolbar "points" was set to elevation and "filter" was set to first return. To finish up part two, the profile view tool, located on LAS dataset toolbar, was used to create a profile of one of the bridges featured on the image. This profile is listed as "Figure 1" , under results.

Part 3:

This portion of the assignment was meant for the creation of the DSM, DTM, and intensity images. The first on that was created was the DSM. To do so, the "LAS Dataset to Raster Tool" was used. Within the tool's window, the file was outputted into the previously designated workspace, and the following parameters were set.

• Value field was set to elevation.
• Cell type was set to maximum.
• Void filling was set to Natural Neighbor.
• Sampling type was set to cell size.
• Sampling value was set to 6.56168 ft. (This is equal to 2 meters, which is what was really desired, however units must correlate to the coordinate system, thus the conversion of values.)

Once the tool was ran, the output was added into ArcMap, and the "Hillshade Tool" was used to create a more defined image. This final product is listed as figure 2 within the results section.

The next image created was the DTM. The process for this was much of the same for the DSM image. However, within the  "LAS Dataset to Raster Tool" window, the parameters were altered as follows:

• Value field was set to elevation.
• Cell type was set to minimum.
• Void filling was set to Natural Neighbor.
• Sampling type was set to cell size.
• Sampling value was set to 6.56168 ft.

Again, the output was added into ArcMap and the "Hillshade Tool" was used to enhance the new DTM image. This can be seen as figure 3 within the results section.

The final portion of this assignment was associated with making an intensity image from the LAS dataset that was previously created. The first step in doing so was to adjust the dataset to points and the filter to first return within the LAS toolbar. Then the "LAS Dataset to Raster Tool" was again used. The parameters for this is as follows:

•  Value field was set to intensity.
• Cell type was set to average.
• Void filling was set to Natural Neighbor.
• Sampling type was set to cell size.
• Sampling value was set to 6.56168 ft.

This image is listed as figure 4 within the results section.

Results:

Within part 2, statistics were calculated from the newly developed LAS dataset, including the maximum height and minimum height. They are as follows:

• Maximum = 1845.92 feet above sea level. (This is most likely associated with very tall or elevated vegetation within the area.)
• Minimum = 517.85 feet above sea level. (This is most like associated with low lying features such as roads or river ways.) 

While looking at the metadata for the LAS files, it was determined that the data used feet for its units of measurement. This then became the basis for the rest of the data processed. 

As previously mentioned, the end of part one consisted of a profile image of a bridge within the associated data. Figure 1 is that resulting image.

Figure 1: LAS dataset with associated profile of bridge using the profile view tool located on the LAS dataset tool.

 Part 3 consisted of three separate results. The first was the DSM image with the hillshaded image overlayed (figure 2). The second was the DTM image with the hillshaded image overlayed (figure 3). Then finally, the last image was of the intensity raster that was created (figure 4).

Figure 2: DSM raster image with overlaying hillshade showing the first return LiDAR data for the City of Eau Claire.

Figure 3: DTM raster image with overlaying hillshade showing the ground return LiDAR data for the City of Eau Claire.

Figure 4: Intensity raster image showing first return LiDAR data for the City of Eau Claire.