## 1. General Features


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### Introduction

The Grid Analysis and Display System ([GrADS](http://cola.gmu.edu/grads/) or its open-source version [OpenGrADS](http://www.opengrads.org/)) is a widely used software for easy access, manipulation, and visualization of earth science data.  Its targeted users are mainly meteorological scientists (but not limited).  It uses a [descriptor (or control) file with a suffix `.ctl`](http://cola.gmu.edu/grads/gadoc/descriptorfile.html) to  describe a raw binary 4D dataset.  The `ctl` file is similar to the header information of a [NetCDF](https://www.unidata.ucar.edu/software/netcdf/docs/file_structure_and_performance.html) file, containing all the information about dimensions, attributes, and variables except for the variable data.

This python package [xgrads](https://github.com/miniufo/xgrads) is designed for parse and read the `.ctl` file commonly used by [GrADS](http://cola.gmu.edu/grads/).  Right now it can parse various kinds of `.ctl` files.  However, only the commonly used raw binary 4D datasets can be read using [dask](https://dask.org/) and return as a [`xarray.Dataset`](http://xarray.pydata.org/en/stable/)  Other types of binary data, like `dtype` is `station`, may be supported in the future.

![3D plot](https://raw.githubusercontent.com/miniufo/xgrads/master/pics/3D.png)

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### Features
#### Fast and efficient
[GrADS](http://cola.gmu.edu/grads/) is written using C program language so that it is fast and efficient.  In batch mode, [GrADS](http://cola.gmu.edu/grads/) can generate tens of plots in one second.  This is really amazing when compared to its counterparts like [Matlab](https://www.mathworks.com/products/matlab.html) and [Python-matplotlib](https://matplotlib.org/).

This [xgrads](https://github.com/miniufo/xgrads) package also utilized the functionalities of [xarray](http://xarray.pydata.org/en/stable/), basically the underlying [numpy](https://numpy.org/) and [matplotlib](https://matplotlib.org/), to support fast and efficient data access and plotting.  When data loading and ploting only need **2 lines of codes**, you have been in the games!

#### Automatic map plotting
[GrADS](http://cola.gmu.edu/grads/) is good at dealing with geographic data defined on a latitude/longtiude grid.  Its `xdef` and `ydef` keywords in `.ctl` file commonly refer to longitude and latitude.  As a result, the map will be plotted automatically when some horizontal data are displayed.  One have to explicitly turn off the map drawing using the command `set mapdraw off`.  So if your data are defined on Cartesian coordinates, it still show you the map under the plot.  This is handy compared to other softwares when dealing with lat/lon data.  For example, when you use [Matlab](https://www.mathworks.com/products/matlab.html) or [Python-matplotlib](https://matplotlib.org/) to plot the lat/lon data, you need a lot of extra scripts to add a map (see [m_map](https://www.eoas.ubc.ca/~rich/map.html) and [cartopy](https://scitools.org.uk/cartopy/docs/latest/)).  However, automatic map plotting is sometimes annoying when you deal with non-lat/lon (e.g., Cartesian coordinates) data.

This is still not easy with [xgrads](https://github.com/miniufo/xgrads) as it is only an I/O package now.  Plots with map requires the extra package [cartopy](https://scitools.org.uk/cartopy/docs/latest/).

#### Zonal periodicity
This is amazing feature when your data cover the whole latitude bands.  If you want to display from 0 to 400E, [GrADS](http://cola.gmu.edu/grads/) will automatically reuse the data from 0 to 40E as 360E to 400E so that the whole plot is seamless from 0 to 400E.  Similarly, you can set the longitude range within -360E to 0, or even unbounded.  [GrADS](http://cola.gmu.edu/grads/) will automatically adjust the range for display.  However, this is really some kind of disaster when you use [cartopy](https://scitools.org.uk/cartopy/docs/latest/) for map plotting, especially labelling in units of E or W (maybe I'm not so good at this).  But [cartopy](https://scitools.org.uk/cartopy/docs/latest/) is still evolving.