Break lines

When you want to filter out large stockpiles of debris to find the underlying real digital terrain model (DTM), use the Break Line tool.

Some common examples in which you can use this tool are when you want to:

Calculate the amount of quarry stockpile on a lot
Measure the volume of earth displaced after a mudslide

For example, large, often man-made, landforms can lean up against the side of a cliff, or sprawl above the top of the cliff. Typically, the edges of the cliff are averaged to give a rough estimate of the DTM. However, by using break lines, you can get a much more accurate calculation of the volume of the debris stockpile in question.

Start by creating a working-area polygon at the area of interest (AOI). When you create the working-area polygon, it is best to entirely surround the stockpile, with the top of the cliff and the bottom of the cliff at the edges, as shown conceptually in Figure 1.

Figure 1. Conceptual depiction of using break lines when cliff obscured by debris stockpile

Using break lines on cliff with debris pile

Follow the contour of the cliff base and the cliff rim. You can draw several break lines in succession to preserve more detail of the cliff, if necessary.

Note: When drawing break lines, the vector must begin and end outside the bounds of the editing polygon

In Figure 2, image A shows a DSM with a stockpile of earth, rock, or other debris that obscures the cliff top and the cliff bottom.

Figure 2. Images A, B, and C comparing filtering without (B) and with (C) break lines

Images A, B, and C comparing filtering without and with break lines

Image B is without using break lines and with the Fill from edges filter applied. Image C is with break lines and the Fill from edges filter applied.

Filters you can use with break lines

Terrain filter (flat)
Terrain filter (rough)
Remove pits
Remove bumps
Remove bumps (keep linear)
Fill from edges

The filter used most commonly is Fill from edges.