private Cell combineZooms(List<Cell> byTile) {
      // aggregate Cells over zoom levels, but respect the global zoom-level weights
      // from TileWeights
      double sumWeight = 0;
      double preSumWeight = 0;
      for (int z = 0; z < byTile.size(); z++) {
        var cell = byTile.get(z);
        long zoomWeight = tileWeights.getZoomWeight(z);
        if (cell.numTiles() > 0 && zoomWeight > 0) {
          sumWeight += zoomWeight;
          preSumWeight += cell.totalWeight;
        }
      }
      boolean noData = sumWeight == 0 || preSumWeight == 0;
      Cell result = new Cell();
      for (int z = 0; z < byTile.size(); z++) {
        var cell = byTile.get(z);
        long zoomWeight = tileWeights.getZoomWeight(z);
        if ((cell.numTiles() > 0 && zoomWeight > 0) || noData) {
          double weight = noData ? 1 : (zoomWeight / sumWeight) / (cell.totalWeight / preSumWeight);
          result.mergeIn(cell, weight);
        }
      }
      return result;
    }
Returns the minimum zoom a specific layer appears at in the tileset.
planetiler-core/src/main/java/com/onthegomap/planetiler/util/TilesetSummaryStatistics.java