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id node_id number title user state locked assignee milestone comments created_at updated_at ▲ closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
332018176 MDU6SXNzdWUzMzIwMTgxNzY= 2231 Time bounds returned after an operation with resample-method rpnaut 30219501 open 0     8 2018-06-13T14:22:49Z 2022-04-17T23:43:48Z   NONE      

Problem description

For datamining with xarray there is always the following issue with the resampling-method. If i resample e.g. a timeseries with hourly values to monthly values, the netcdf-standards tell us to put into the result file information about:

  1. the bounds for each timestep over which the aggregation was taken (for each month the beginning and the end of the month)
  2. the method which was used for aggregation decoded by the variable attribute 'cell_method' (e.g. 'time: mean').

The recent implementation should be improved which is proven by the following data example.

Data example

I have a dataset with hourly values over a period of 5 month. python <xarray.Dataset> Dimensions: (bnds: 2, time: 3672) Coordinates: rlon float32 22.06 rlat float32 5.06 * time (time) datetime64[ns] 2006-05-01 2006-05-01T01:00:00 ... Dimensions without coordinates: bnds Data variables: rotated_pole int32 1 time_bnds (time, bnds) float64 1.304e+07 1.305e+07 1.305e+07 ... TOT_PREC (time) float64 nan nan nan nan nan nan nan nan nan nan nan ... Attributes: Doing a resample process using the mean operator gives

In [36]: frs Out[36]: <xarray.Dataset> Dimensions: (bnds: 2, time: 5) Coordinates: * time (time) datetime64[ns] 2006-05-31 2006-06-30 2006-07-31 ... Dimensions without coordinates: bnds Data variables: rotated_pole (time) float64 1.0 1.0 1.0 1.0 1.0 time_bnds (time, bnds) float64 1.438e+07 1.438e+07 1.702e+07 ... TOT_PREC (time) float64 12.0 nan nan nan nan

Here the time_bnds is still in the file but the content is very strange: In [37]: frs["time_bnds"] Out[37]: <xarray.DataArray 'time_bnds' (time: 5, bnds: 2)> array([[ 1.438020e+07, 1.438380e+07], [ 1.701540e+07, 1.701900e+07], [ 1.965060e+07, 1.965420e+07], [ 2.232900e+07, 2.233260e+07], [ -6.330338e+10, -6.330338e+10]]) Coordinates: * time (time) datetime64[ns] 2006-05-31 2006-06-30 2006-07-31 ... Dimensions without coordinates: bnds So, he still knows that time_bnds is related to the coordinate time. However, the values are not correct. The first time_bnds entry should be [1.5.2006 00:00,31.5.2006 23:00]. That is definitely not the case, i.e. the numbers here are related to the original file (seconds since 2005-12-01), but they do not match to my expection. 1.438020e+07 equals "Dienstag, 16. Mai 2006, 10:30:00" and 1.438380e+07 equals "Dienstag, 16. Mai 2006, 11:30:00". Moreover, the xarray's do not consider to change the unit of the time_bnds according the unit of the variable 'time' if data is written to netcdf. Output of the program ncdump reveals that time was changed to days since but time_bnds seems to be still coded in "seconds since".

``` ncdump -v time_bnds try.nc netcdf try { dimensions: time = 5 ; bnds = 2 ; variables: double rotated_pole(time) ; rotated_pole:_FillValue = NaN ; double time_bnds(time, bnds) ; time_bnds:_FillValue = NaN ; double TOT_PREC(time) ; TOT_PREC:_FillValue = NaN ; int64 time(time) ; time:units = "days since 2006-05-31 00:00:00" ; time:calendar = "proleptic_gregorian" ; data:

time_bnds = 14380200, 14383800, 17015400, 17019000, 19650600, 19654200, 22329000, 22332600, -63303379200, -63303379200 ; } ```

Is there a recommendation what to do?

 
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     xarray 13221727 issue

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