Before laying out platforms on hilly ground, we want to know how much of the site is too steep to build on as it stands, and where those areas are. Terrain > DEM > Slope Classification answers this in a 2D view: it colours every triangle of the terrain mesh by its gradient and tabulates the area that falls in each gradient class. The same view can also show the site after development, with the platforms in place, so the two states can be compared.

This article covers that 2D view. For the quick overlay on the Input view, see Slope Classification (Design Aid); for the 3D View layers, see Contour Line and Slope Classification in 3D View.

Opening the view #

Go to Terrain > DEM > Slope Classification and pick one of the two items:

• OGL classifies the original ground, the terrain built from the active survey point group.
• Post-Dev classifies the ground after development: the platform surfaces, together with the original terrain left around them (see Post-development terrain below).

Both open the same Slope Classification tab, so we can switch between OGL and Post-Dev from the drop-down without closing it. The view needs an active survey point group; without one, the tab is unavailable.

What the colours and the table mean #

The terrain mesh is made of triangles. For each triangle, MiTS computes its gradient as an angle in degrees from the horizontal, so 0° is flat and 90° is vertical. The triangle is then filled with the colour of the class its gradient falls in.

The default classes are:

Beside the drawing, the legend table lists each class with its colour, its area in m² and a comment (the class name). The area column is either the 3D Surface Area, measured along the sloping triangles, or the Projected Plan Area, measured on plan. On steep ground the two differ noticeably: a 35° slope has about 22% more surface area than its plan area, so choose the one that matches the quantity being reported.

The legend table can be dragged to another position in the drawing, and MiTS remembers where we put it.

Settings on the right-hand panel #

The Slope Classification settings panel on the right controls the view:

• Surface Area 3D: Yes to report 3D surface area in the legend, No to report projected plan area. The default is Yes.
• Show breakdown: Yes to add one legend table per selected boundary (titled with its boundary mark), so the class areas can be read zone by zone. The default is No.
• Classification Mode: Equal Interval or Quantile, explained below.
• Include Surrounding Terrain (Post-Dev only): see Post-development terrain.
• The class table for the current mode, where class names, colours and (for Equal Interval) gradient ranges are edited.
• The boundary list, which chooses the area to classify (see Classifying part of the site).

The same options are stored with the project under Options > Project Settings > Earthworks > Terrain Analysis > Slope Classification Parameters, with the class tables under Equal Gradient Parameters and Equal Area Parameters. These are separate from the Slope Classification Parameters (Design Aid) used by the Input view overlay.

Equal Interval #

In Equal Interval mode we set the gradient range of each class ourselves, in degrees. This suits a design rule with fixed limits, for example a planning guideline that treats ground steeper than 25° differently from gentler ground. Editing the upper limit of one class moves the lower limit of the next, so the ranges stay continuous. Classes can be added or removed, and each has its own name and colour.

Quantile #

In Quantile mode we choose only the number of classes, their names and their colours. MiTS sorts the gradients of all the terrain triangles and splits them into that many groups with the same number of triangles each, then shows the resulting gradient range for every class. This is useful for seeing how the slopes on this particular site are distributed, when no fixed limit applies.

It should be noted that the groups hold equal numbers of triangles, not equal areas, even though the settings page is named Equal Area Parameters. Triangles in a mesh vary in size, so the areas in the legend table will generally differ from class to class.

Post-development terrain #

Post-Dev classifies the ground as it will be after the platforms are built. With Include Surrounding Terrain set to Yes (the default), MiTS combines the platform and sub-platform surfaces with the original terrain that remains outside them, so the view covers the whole site. Set it to No to classify the platforms alone, without the terrain around them.

Comparing the OGL and Post-Dev legend tables shows how much steep ground the development removes or creates, for example how much Class IV area remains once the platforms are in place.

Classifying part of the site #

The boundary list on the right chooses the area to classify:

• The Survey Points boundary, the convex hull of the imported survey points, classifies the whole terrain.
• Each project boundary, listed by its mark, classifies only the terrain inside it.
• One further entry covers the terrain outside all the project boundaries.

With Show breakdown set to Yes, each selected boundary gets its own legend table. See Boundary for creating boundaries.

Exporting #

The Slope Classification view can be exported like the other output drawings, with Export for the current view or Batch Export together with the other outputs. See Results for the export buttons.