cdm/src/main/java/ucar/nc2/dt/grid/GridCoordSys.java
@Override
  public boolean isZPositive() {
    if (vertZaxis == null) return false;
    if (vertZaxis.getPositive() != null) {
      return vertZaxis.getPositive().equalsIgnoreCase(ucar.nc2.constants.CF.POSITIVE_UP);
    }
    if (vertZaxis.getAxisType() == AxisType.Height) return true;
    return vertZaxis.getAxisType() != AxisType.Pressure;
  }
@Override
  public boolean isZPositive() {
    if (vertZaxis == null) return false;
    if (vertZaxis.getPositive() != null) {
      return vertZaxis.getPositive().equalsIgnoreCase(ucar.nc2.constants.CF.POSITIVE_UP);
    }
    if (vertZaxis.getAxisType() == AxisType.Height) return true;
    return vertZaxis.getAxisType() != AxisType.Pressure;
  }
true if increasing z coordinate values means "up" in altitude