The Region Control field allows you to control that area over which prism volume calculations take place using line features and groups. You should note that the region control algorithm assumes that the segments of the line features or boundaries of the groups are also present as triangle edges in the current DTM. There are four options.
All the triangles in the current DTM will be used.
Those triangles that are within a boundary defined by a line feature will be used. You will be asked to indicate the bounding line feature.
Those triangles that are within a boundary defined by a DTM group in a third DTM will be used. You will be asked to select the third DTM and one of its groups.
This option is similar to the previous option with the exception that all of the DTM groups in the third DTM will be used. You will be asked to select the third DTM and the results for each of its groups will be presented.