This question comes up a fair bit so I wanted to explain how it works.

A surface once formed has a set of properties that can be reviewed. These properties are derived initially from the Project Settings - Computations - Surfaces function which defines the defaults for all new surfaces that get created.

The Adjust Flat Triangles setting can be set to No or Yes. If set to Yes you are asked for a tolerance - the default is 1’ or 30cm. So what does this do and what is the tolerance value.

This function was initially developed to fix surface models created from contours, to force the triangles in the model to link between contours rather than between nodes on the same contour which effectively creates flatspots in the model - see example below

The above represents a series of contours on a drainage swale or on a crest of a sloping hillside or berm etc. The intent is pretty clear when you look at the contours - If you were to draw a profile through the apex of the contours you would expect a fairly uniform slope from top to bottom - however if you have the Adjust Flat triangles set to No you can get the following TIN Model

If we then cut a profile along the pink line we would get the following result vs the desired or closer to reality result

If you change the setting for Adjust Flat Triangles to Yes and then enter a value of 1’ for the tolerance (an acceptable value for contour based models where the contours have a constant height), you would see the TIN model change to something like the following

And if you now cut a profile you would get closer to the desired outcome.

So the Adjust Flat Triangles setting forces triangles to form so that their 3 points connect between 2 separate lines when the elevation range on one of the lines is less than the tolerance specified e.g. 1’ or 30cm.

This approach can also be used to force triangles to form in the same way between points on a line around the base of a stockpile and the top of a stockpile or around the lip of a hole and the base of a hole. The tolerance value is the range of elevations on the lines that are included in the surface model. So if the stockpile is built on the side of a hill, and the range of elevations on either of the two lines is e.g. 6.8’ (the Min to Max elevations of a line) then you would need to enter e.g. 7.0 as the tolerance value for it to solve flat triangles in the model as shown below.

Adjust Flat Triangles = No or Yes and Tolerance = 1.0’

Adjust Flat Triangles = Yes - Tolerance = 8.0’

Note that the range of the base of the pile is from 100.0 to 107.0 and for the top of the pile is from 116 to 120 so the value of 8.0’ for the tolerance covers both line min max elevation ranges.

Hope that this helps

Alan