public static ViewOrientation3D ConvertBasePoint(Document doc, XYZ c, XYZ view, XYZ up, bool negative)
{
//UIDocument uidoc = uiapp.ActiveUIDocument;
//Document doc = uidoc.Document;
//ElementCategoryFilter filter = new ElementCategoryFilter(BuiltInCategory.OST_ProjectBasePoint);
//FilteredElementCollector collector = new FilteredElementCollector(doc);
//System.Collections.Generic.IEnumerable<Element> elements = collector.WherePasses(filter).ToElements();
double angle = 0;
double x = 0;
double y = 0;
double z = 0;
//VERY IMPORTANT
//BuiltInParameter.BASEPOINT_EASTWEST_PARAM is the value of the BASE POINT LOCATION
//position is the location of the BPL related to Revit's absolute origini!
//if BPL is set to 0,0,0 not always it corresponds to Revit's origin
ProjectLocation projectLocation = doc.ActiveProjectLocation;
XYZ origin = new XYZ(0, 0, 0);
ProjectPosition position = projectLocation.get_ProjectPosition(origin);
int i = (negative) ? -1 : 1;
x = i * position.EastWest;
y = i * position.NorthSouth;
z = i * position.Elevation;
angle = i * position.Angle;
if (negative) // I do the addition BEFORE
c = new XYZ(c.X + x, c.Y + y, c.Z + z);
//rotation
double centX = (c.X * Math.Cos(angle)) - (c.Y * Math.Sin(angle));
double centY = (c.X * Math.Sin(angle)) + (c.Y * Math.Cos(angle));
XYZ newC = new XYZ();
if (negative)
newC = new XYZ(centX, centY, c.Z);
else // I do the addition AFTERWARDS
newC = new XYZ(centX + x, centY + y, c.Z + z);
double viewX = (view.X * Math.Cos(angle)) - (view.Y * Math.Sin(angle));
double viewY = (view.X * Math.Sin(angle)) + (view.Y * Math.Cos(angle));
XYZ newView = new XYZ(viewX, viewY, view.Z);
double upX = (up.X * Math.Cos(angle)) - (up.Y * Math.Sin(angle));
double upY = (up.X * Math.Sin(angle)) + (up.Y * Math.Cos(angle));
XYZ newUp = new XYZ(upX, upY, up.Z);
return new ViewOrientation3D(newC, newUp, newView);
}