issues: 1768118776

What happened?

I'm not sure if this is a bug, normal behaviour or a feature request but if it's expected behaviour, it should be better documented.

I want to interpolate a 2D dataset to a finer spatial grid. I've put together a reproducible example based on the tutorial here: https://docs.xarray.dev/en/stable/user-guide/interpolation.html

The result is this:

When I do this, the edge points are lost (as you can see on the west/north boundaries here).

What did you expect to happen?

I'd expect the interpolation to protect the boundary points and not lose them.

Minimal Complete Verifiable Example

```Python ds = xr.tutorial.load_dataset("air_temperature") subplot_kw = dict(projection=ccrs.PlateCarree()) new_lon = np.linspace(ds.lon[0], ds.lon[-1], ds.dims["lon"] * 4) new_lat = np.linspace(ds.lat[0], ds.lat[-1], ds.dims["lat"] * 4) dsi = ds.interp(lat=new_lat, lon=new_lon, method='nearest', kwargs={'fill_value':None})

fig, axes = plt.subplots(1, 2, figsize=(18, 12), subplot_kw=subplot_kw) axes = np.ravel(axes)

ax = axes[1] ds.air.isel(time=0).plot(x='lon', y='lat', ax=ax, add_colorbar=False) ax.set_extent([ds.lon.min()-5., ds.lon.min()+20., ds.lat.max()-20, ds.lat.max()+5]) ax.coastlines()

ax = axes[0] dsi.air.isel(time=0).plot(x='lon', y='lat', ax=ax, add_colorbar=False) ax.set_extent([ds.lon.min()-5., ds.lon.min()+20., ds.lat.max()-20, ds.lat.max()+5]) ax.coastlines()

plt.show() ```

MVCE confirmation

• [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [X] Complete example — the example is self-contained, including all data and the text of any traceback.
• [X] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.7.8 | packaged by conda-forge | (default, Jul 31 2020, 02:25:08) [GCC 7.5.0] python-bits: 64 OS: Linux OS-release: 3.10.0-1160.90.1.el7.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8 libhdf5: 1.10.6 libnetcdf: 4.7.4 xarray: 0.16.1 pandas: 1.1.3 numpy: 1.19.2 scipy: 1.5.2 netCDF4: 1.5.4 pydap: None h5netcdf: None h5py: 2.10.0 Nio: None zarr: None cftime: 1.2.1 nc_time_axis: 1.2.0 PseudoNetCDF: None rasterio: 1.1.8 cfgrib: None iris: 2.4.0 bottleneck: None dask: 2.30.0 distributed: 2.30.0 matplotlib: 3.3.2 cartopy: 0.18.0 seaborn: 0.11.0 numbagg: None pint: None setuptools: 49.6.0.post20201009 pip: 20.2.3 conda: None pytest: None IPython: 5.8.0 sphinx: None