Consider any random 2D point in a space, and user wants to calculate its elevation based on some terrain surface. Project the point on to terrain surface to check whether it is inside terrain boundary or its external to terrain surface. The below code snippet shows how to calculate elevation of a 2D point which exists inside terrain boundary.

internal void CalculateEleForPntInsideTerrain()
        {
            //Get active dgn model
            Bentley.DgnPlatformNET.DgnModel dgnModel = Bentley.MstnPlatformNET.Session.Instance.GetActiveDgnModel();
            //Create connection to dgn model 
            Bentley.CifNET.SDK.ConsensusConnection consensusConnection = new ConsensusConnection(dgnModel);
            //Get active geometric model 
            Bentley.CifNET.GeometryModel.SDK.GeometricModel geometricModel = consensusConnection.GetActiveGeometricModel();
            foreach (Bentley.CifNET.GeometryModel.SDK.TerrainSurface ts in geometricModel.TerrainSurfaces)
            {
                //Get terrain model from terrain surface
                Bentley.TerrainModelNET.DTM dtm = ts.DTM;

                //Point 2D
                Bentley.GeometryNET.DPoint3d currentPoint = new DPoint3d(70774.916616, 1210021.6587936000433);

                Bentley.TerrainModelNET.DTMDrapedPoint drappedPoint = null;
                double pntElevation = 0.0;

                //project 2D point on 3D terrain and get drapped point/projected point
                drappedPoint = dtm.DrapePoint(currentPoint);

                if (drappedPoint != null)
                {
                    //Check points position, projected point is not within terrain boundry
                    if (drappedPoint.Code == Bentley.TerrainModelNET.DTMDrapedPointCode.External)
                    {
                        //Point is external to terrain 
                        return;
                    }
                    else //projected pt is inside terrain boundry
                    {
                        //Get elevation for point
                        pntElevation = drappedPoint.Coordinates.Z;
                    }
                }
            }
        }


		

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UseDrapePoint() of DTM to project a 2D point on terrain surface, the projected points Z coordinate will be the elevation for 2D point.