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Better return values for grdinfo #593

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@weiji14

Description

@weiji14

Description of the desired feature

Certain pygmt.grdinfo return values can be in an array form rather than a string. This would be a good first issue to take up, and would be similar to #575 but for grids instead of tables.

Originally posted by @liamtoney in #147 (comment)

tl;dr

After reading the above discussion, I would like to work on more fully wrapping grdinfo. It seems like a potentially good "first issue," and I can see a benefit in wrapping it even with Python containers also available. See below, for example.

Motivation

In my use case, I want to easily see the min/max z-values of a DEM so that I can choose my colorbar limits.

1. No specified region

With GRIDFILE being the full path to my grid file, I could use

import gmt
print(gmt.grdinfo(GRIDFILE).strip())
...
../srtm15_plus_w180n90.nc: z_min: -7840.125 z_max: 6057.64355469 name: z
...

or (with xarray, for example)

import xarray
print(xarray.open_dataset(GRIDFILE).z.actual_range)
[-7840.125       6057.64355469]

These are about equal effort (and xarray returns things in a more usable output).

2. Specified region of interest

For viewing the min/maz z-values within a specified REGION = (lonmin, lonmax, latmin, latmax), I think that grdinfo makes this easier:

import gmt
print(gmt.grdinfo(GRIDFILE, R='{}/{}/{}/{}'.format(*REGION)).strip())
...
../srtm15_plus_w180n90.nc: z_min: -4996.11816406 z_max: 6057.64355469 name: z
...

vs.

import xarray
z = xarray.open_dataset(GRIDFILE).sel(lon=slice(*REGION[0:2]), lat=slice(*REGION[2:4])).z.values
print(z.min(), z.max())
-4996.118 6057.6436

Plus grdinfo is more format agnostic. By minimally including the @use_alias and @kwargs_to_strings decorators, the grdinfo command above could be just

print(gmt.grdinfo(GRIDFILE, region=REGION).strip())

and (with more work) could perhaps output a dictionary? Please let me know if this is a worthwhile effort and I'll make a PR for modules.py.

Are you willing to help implement and maintain this feature? Yes/No

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