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rspatial / terra

R package for spatial data handling https://rspatial.github.io/terra/reference/terra-package.html
GNU General Public License v3.0
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Offset of “time” dim for terra packages when reading nc file #1599

Open Breeze-Hu opened 1 week ago

Breeze-Hu commented 1 week ago

When I open an nc file using terra, the time dimension of the read data is off by 3 days. The description of the data source should read: 1961-01-01 to 2022-12-31. But the result read using terra is: 1961-01-03 to 2023-01-02 UTC. The same file was read using xarray in python. The time dimension of the data is normal.

In seeking a solution to the problem, I may have seen a similar situation with other people's data (I'm not sure): [R terra not reading attributes from NetCDF file](https://gis.stackexchange.com/questions/435439/r-terra-not-reading-attributes-from-netcdf-file).Though this is monthly data, so it doesn't have an impact on what's in the study.

But, this issue would be very problematic for daily data.

operating system： Snipaste_2024-09-05_17-42-02

image

macOS：14.6.1

packageVersion("terra") [1] ‘1.7.78’

terra::gdal(lib="all") gdal proj geos "3.5.3" "9.1.0" "3.11.0"

Breeze-Hu commented 1 week ago

I tried to find the cause of this problem in other ways. I checked ‘Panoply’ Panoply netCDF, HDF and GRIB Data Viewer read of this file and the result is consistent with xarray (1961-01-01).

But when using the python base package 'datetime' to do the calculations, the results are consistent with terra. I looked at the documentation for xarray, and it points to the use of CF conventions rules in the date calculation process.

When reading or writing netCDF files, xarray automatically decodes datetime and timedelta arrays using CF conventions cf-conventions

Perhaps this discrepancy is the result.

rhijmans commented 1 week ago

Can your provide an example file?

Breeze-Hu commented 1 week ago

Hello developer! I've now uploaded a sample datasetexample.nc and to reduce the size of the dataset, I've cut the original dataset of the question in time. And I checked to be able to reproduce the question.

image image

The reason for the problem was given a very detailed response by the author of another package https://github.com/pvanlaake/CFtime/issues/9.

This proves that it's not a problem with the terra package (maybe this isn't really a bug, but rather a habit of creating datas)