for (int tl = 0; tl < ntls; ++tl) {
          for (int vi = 0; vi < groupdata.numvars; ++vi) {
            groupdata.valid.at(tl).at(vi) = valid_t();
            poison_invalid(leveldata, groupdata, vi, tl);

        if (ntls > 1) { // assume one timelevel only means constant data
          for (int tl = 0; tl < ntls; ++tl) {
            for (int vi = 0; vi < groupdata.numvars; ++vi) {
              groupdata.valid.at(tl).at(vi) = valid_t();
              poison_invalid(leveldata, groupdata, vi, tl);
            }