Extract Ground Scan

(This functionality is available with the Point Cloud module)

Allows a new ground scan and a new object scan to be extracted from an existing scan. The process uses progressive morphological filtering and requires the user to enter parameters which will be based upon the type of terrain to be extracted.

Note: This process is very time consuming. Typically, it may take 20 minutes or more to extract the ground scan from a source scan of about 150,000 scan points. The time required increases exponentially as scan size increases. As such, using Downsample Scan to create a suitably less dense source scan prior to extracting the ground may be preferable.

Downsample Scan dialog box

Item	Used to
Current Scan Name	Select an existing scan to be separated into the two new scans.
New Ground Scan Name	Enter in a new scan name for the new ground scan.
New Object Scan Name	Enter in a new scan name for the new object scan. The user is NOT required to enter the name of the object points scan. If this is blank, no object scan will be created.
Initial Window Size	Enter a positive value for Initial Window Size. The window size refers to the size of the filtering window, where points are collected and elevations determined for use in the filtering process. This is not the size of the entire dataset, the window moves over the entire dataset. If a small window size is used, only small non ground objects (cars, trees) will be removed, but most ground points will remain. If a large window size is used, larger objects such as buildings will be removed, but the risk is greater that more ground points will be removed too. Since there is no one window size that will do the job perfectly, the process will iterate through different window sizes, starting with the initial Window size and increasing until the maximum Object Length specified is reached. The maximum object length should typically be the size of the largest building to be removed. The typical initial size will depend upon how close the scan points are to each other. If a downsampled scan to 0.1 metres is used to extract ground from, then a good starting point for the Initial Window size may be 0.5.
Max. Terrain Slope	Enter a positive value for the maximum terrain slope applicable to the scan. This assists in determining that steeper areas of the scan are not part of the ground.
Max. Object Length	Enter a positive value for maximum Object Length. The maximum object length should typically be the length of the largest building to be excluded from the resulting ground scan.
Initial Hgt. Threshold	Enter a positive value for Initial Height Threshold. This is the maximum allowable difference in height between points in each initial window groups, that could still be classified as ground points. Any points of a greater height difference than this threshold will be classed as part of an object.
Min. Object Height	Enter a positive value for minimum Object height that could be treated as object. This could be set to the lowest building or tree height to be treated as not ground. It is expected that the user should experiment with these parameters to get the best results depending upon the terrain of the area in question.
Apply	Create the new scans. The time taken to perform the ground extraction can be measured in hours. The time taken depends on the number of iterations the process must go through to start from the initial window size up to the maximum object length specified. The window size will increase exponentially, but as the window size gets greater and more points are sampled in the window size the time taken is greater. For example, if an initial window size is 1.0, and the maximum object length is 20.0, the iterations will proceed from 3, 5,9, 17,33. For a scan size of 400K, this can take several hours to perform.