Code Sample, a copy-pastable example if possible

```python import xarray as xr

Load a global grid with 1 degree spacing distributed by GMT

whole = xr.open_dataarray('earth_relief_60m.grd') print("Global grid:\n", whole)

The grid coordinates have metadata regarding the range (actual_range).

This is used to detect if the grid is pixel or node registered.

print("\nMetadata for global coordinates:", whole.lat.attrs)

Select only between latitudes -40 and 40

part = whole.sel(lat=slice(-40, 40)) print("\nSliced grid:\n", part)

Slicing preserves the coordinate metadata and now the actual_range is incorrect.

This is preserved when saving to netCDF and causes errors that are difficult to

diagnose and fix when passing it along to GMT for plotting.

print("\nMetadata for sliced coordinates:", part.lat.attrs) Output: Global grid: <xarray.DataArray 'z' (lat: 181, lon: 361)> array([[ 2762., 2762., 2762., ..., 2762., 2762., 2762.], [ 2983., 2980., 2977., ..., 2989., 2986., 2983.], [ 3074., 3074., 3074., ..., 3072., 3073., 3074.], ..., [-3727., -3715., -3706., ..., -3759., -3742., -3727.], [-2294., -2282., -2271., ..., -2322., -2308., -2294.], [-4181., -4181., -4181., ..., -4181., -4181., -4181.]], dtype=float32) Coordinates: * lon (lon) float64 -180.0 -179.0 -178.0 -177.0 -176.0 -175.0 -174.0 ... * lat (lat) float64 -90.0 -89.0 -88.0 -87.0 -86.0 -85.0 -84.0 -83.0 ... Attributes: long_name: z actual_range: [-8425. 5551.]

Metadata for global coordinates: OrderedDict([('long_name', 'latitude'), ('units', 'degrees_north'), ('actual_range', array([-90., 90.]))])

Sliced grid: <xarray.DataArray 'z' (lat: 81, lon: 361)> array([[-3062., -3451., -3695., ..., -1504., -3226., -3062.], [-3559., -3515., -3773., ..., -128., -2991., -3559.], [-3552., -3498., -3459., ..., 519., -1149., -3552.], ..., [-5425., -5389., -5268., ..., -5162., -5399., -5425.], [-5385., -5499., -5580., ..., -5407., -5497., -5385.], [-5325., -5937., -5557., ..., -5602., -5572., -5325.]], dtype=float32) Coordinates: * lon (lon) float64 -180.0 -179.0 -178.0 -177.0 -176.0 -175.0 -174.0 ... * lat (lat) float64 -40.0 -39.0 -38.0 -37.0 -36.0 -35.0 -34.0 -33.0 ... Attributes: long_name: z actual_range: [-8425. 5551.]

Metadata for sliced coordinates: OrderedDict([('long_name', 'latitude'), ('units', 'degrees_north'), ('actual_range', array([-90., 90.]))]) ```

Problem description

Indexing seems to preserve the attrs. If it contains information about the values, then this information will be outdated. Some software, like GMT rely on this information for certain operations. It can manage missing metadata but there is no way to guard against incorrect metadata.

Expected Output

I would expect indexing to drop attrs unless keep_attrs is specified. It's better to have no metadata than to have incorrect metadata.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.4.final.0 python-bits: 64 OS: Linux OS-release: 4.15.18-3-MANJARO machine: x86_64 processor: byteorder: little LC_ALL: None LANG: en_US.utf8 LOCALE: en_US.UTF-8 xarray: 0.10.2 pandas: 0.22.0 numpy: 1.14.2 scipy: 1.0.1 netCDF4: 1.3.1 h5netcdf: 0.5.0 h5py: 2.7.1 Nio: None zarr: None bottleneck: 1.2.1 cyordereddict: None dask: 0.17.2 distributed: 1.21.4 matplotlib: 2.2.2 cartopy: None seaborn: None setuptools: 39.0.1 pip: 9.0.1 conda: None pytest: 3.5.0 IPython: 6.2.1 sphinx: 1.7.2