# Removing noise

This exercise uses PDAL to remove unwanted noise in an airborne LiDAR collection.

## Exercise

PDAL provides the outlier filter to apply a statistical filter to data.

Because this operation is somewhat complex, we are going to use a pipeline to define it.

{
"pipeline": [
"./exercises/analysis/denoising/18TWK820985.laz",
{
"type": "filters.outlier",
"method": "statistical",
"multiplier": 3,
"mean_k": 8
},
{
"type": "filters.range",
"limits": "Classification![7:7],Z[-100:3000]"
},
{
"type": "writers.las",
"compression": "true",
"minor_version": "2",
"dataformat_id": "0",
"filename":"./exercises/analysis/denoising/clean.laz"
}
]
}


Note

This pipeline is available in your workshop materials in the ./exercises/analysis/denoising/denoise.json file.

### Pipeline breakdown

After our pipeline errata, the first item we define in the pipeline is the point cloud file we’re going to read.

"./exercises/analysis/denoising/18TWK820985.laz",


#### 2. filters.outlier

The PDAL outlier filter does most of the work for this operation.

{
"type": "filters.outlier",
"method": "statistical",
"multiplier": 3,
"mean_k": 8
},


#### 3. filters.range

At this point, the outliers have been classified per the LAS specification as low/noise points with a classification value of 7. The range filter can remove these noise points by constructing a range with the value Classification![7:7], which passes every point with a Classification value not equal to 7.

Even with the filters.outlier operation, there is still a cluster of points with extremely negative Z values. These are some artifact or mis-computation of processing, and we don’t want these points. We can construct another range to keep only points that are within the range $$-100 <= Z <= 3000$$.

Both ranges are passed as a comma-separated list to the range filter via the limits option.

{
"type": "filters.range",
"limits": "Classification![7:7],Z[-100:3000]"
},


#### 4. writers.las

We could just define the clean.laz filename, but we want to add a few options to have finer control over what is written. These include:

{
"type": "writers.las",
"compression": "true",
"minor_version": "2",
"dataformat_id": "0",
"filename":"./exercises/analysis/denoising/clean.laz"
}

1. compression: LASzip data is ~6x smaller than ASPRS LAS.

2. minor_version: We want to make sure to output LAS 1.2, which will provide the widest compatibility with other softwares that can consume LAS.

3. dataformat_id: Format 3 supports both time and color information

Note

writers.las provides a number of possible options to control how your LAS files are written.

### Execution

Invoke the following command, substituting accordingly, in your  Shell:

\$ pdal pipeline ./exercises/analysis/denoising/denoise.json


### Visualization

Use one of the point cloud visualization tools you installed to take a look at your clean.laz output. In the example below, we simply opened the file using the Fugro Viewer

## Notes

1. Control the aggressiveness of the algorithm with the mean_k parameter.

2. filters.outlier requires the entire set in memory to process. If you have really large files, you are going to need to split them in some way.