var doc = commandData.Application.ActiveUIDocument.Document;
var basePoint = new FilteredElementCollector(doc).OfClass(typeof(BasePoint)).Cast<BasePoint>().Where(p => !p.IsShared).ToList().FirstOrDefault();
if (basePoint != null)
{
basePoint.get_Parameter(BuiltInParameter.BASEPOINT_NORTHSOUTH_PARAM).Set(111);
basePoint.get_Parameter(BuiltInParameter.BASEPOINT_EASTWEST_PARAM).Set(222);
basePoint.get_Parameter(BuiltInParameter.BASEPOINT_ELEVATION_PARAM).Set(11);
basePoint.get_Parameter(BuiltInParameter.BASEPOINT_ANGLETON_PARAM).Set(0.707);
}