Welcome, guest | Sign In | My Account | Store | Cart
#!/usr/bin/env python3

'''An alternative way to draw parallels and meridians with basemap.

Basemap is a toolkit of matplotlib used to plot geographic maps.

With this function you can:

* Draw the latitude/longitude grid easily in one line of code, specifying the
  lat/lon intervals.
* Use rcParams, so all your figures will look more consistents.
* Specify the label pad in points (instead of fraction of width/height).
* Place the labels indicating which margins will be used.


import numpy as np

def latlon_grid(bmap, lon_int, lat_int, labels='lb', **kwargs):
    '''Draws a lat-lon grid in an easy way.

    Some default values are taken from rcParams instead of 'black' (color) and
    1.0 (linewidth) which is the default in Basemap.

    In Basemap, the label pad is computed applying a factor to the dimensions
    of the figure. Now you can use the keyword argument 'labelpad' to control
    this separation in points. If not specified, this value is taken from


    bmap -- Basemap object.
    lon_int, lat_int -- Difference in degrees from one longitude or latitude to
                        the next.
    labels -- String specifying which margins will be used to write the labels.
              If None, no label will be shown.
              It is assummed that left/right margins correspond to latitudes
              and top/bottom to longitudes. It is valid every combination of
              the characters 't' | 'b' | 'l' | 'r' (top|bottom|left|right).
              Ex: 'lrb' means than the longitude values will appear in bottom
              margin and latitudes in left and right.
    **kwargs -- Other arguments to drawparallels, drawmeridians and plt.text.
                labelpad has units of points.

    # Proccesses arguments and rcParams for defult values
    if 'color' not in kwargs:
        kwargs['color'] = plt.rcParams['grid.color']
    if 'linewidth' not in kwargs:
        kwargs['linewidth'] = plt.rcParams['grid.linewidth']
    if 'labelpad' in kwargs:
        padx = pady = kwargs['labelpad']
        del kwargs['labelpad']
        pady = plt.rcParams['xtick.major.pad']
        padx = plt.rcParams['ytick.major.pad']
    if 'size' in kwargs:
        xfontsize = yfontsize = kwargs['size']
        del kwargs['size']
    elif 'fontsize' in kwargs:
        xfontsize = yfontsize = kwargs['fontsize']
        del kwargs['fontsize']
        xfontsize = plt.rcParams['xtick.labelsize']
        yfontsize = plt.rcParams['ytick.labelsize']
    # Vectors of coordinates
    lon0 = bmap.lonmin // lon_int * lon_int
    lat0 = bmap.latmin // lat_int * lat_int
    lon1 = bmap.lonmax // lon_int * lon_int
    lat1 = bmap.latmax // lat_int * lat_int
    nlons = (lon1 - lon0) / lon_int + 1
    nlats = (lat1 - lat0) / lat_int + 1
    assert nlons / int(nlons) == 1, nlons
    assert nlats / int(nlats) == 1, nlats
    lons = np.linspace(lon0, lon1, int(nlons))
    lats = np.linspace(lat0, lat1, int(nlats))
    # If not specified, computes de label offset by 'labelpad'
    xos = yos = None
    if 'xoffset' in kwargs:
        xos = kwargs['xoffset']
    if 'yoffset' in kwargs:
        yos = kwargs['yoffset']
    if xos is None and yos is None:
        # Page size in inches and axes limits
        fig_w, fig_h = plt.gcf().get_size_inches()
        points = plt.gca().get_position().get_points()
        x1, y1 = tuple(points[0])
        x2, y2 = tuple(points[1])
        # Width and height of axes in points
        w = (x2 - x1) * fig_w * 72
        h = (y2 - y1) * fig_h * 72
        # If the aspect relation is fixed, compute the real values
        if bmap.fix_aspect:
            aspect = bmap.aspect * w / h
            if aspect > 1:
                w = h / bmap.aspect
            elif aspect < 1:
                h = w * bmap.aspect
        # Offset in projection units (meters or degrees)
        xos = padx * (bmap.urcrnrx - bmap.llcrnrx) / w
        yos = pady * (bmap.urcrnry - bmap.llcrnry) / h
    # Set the labels
    latlabels = [False] * 4
    lonlabels = [False] * 4
    if labels is not None:
        pst = {'l': 0, 'r': 1, 't': 2, 'b': 3}
        lst = {'l': latlabels, 'r': latlabels, 't': lonlabels, 'b': lonlabels}
        for i in labels.lower():
            lst[i][pst[i]] = True
    # Draw the grid
    bmap.drawparallels(lats, labels=latlabels, fontsize=yfontsize,
                       xoffset=xos, yoffset=yos, **kwargs)
    bmap.drawmeridians(lons, labels=lonlabels, fontsize=xfontsize,
                       xoffset=xos, yoffset=yos, **kwargs)


if __name__ == '__main__':
    import matplotlib.pyplot as plt
    import mpl_toolkits.basemap as basemap
    # Some rcParams (example)
    plt.rc('grid', linewidth=0.5, color='g')
    plt.rcParams['xtick.major.pad'] = 6
    plt.rcParams['ytick.major.pad'] = 12
    plt.rcParams['xtick.labelsize'] = 'small'
    plt.rcParams['ytick.labelsize'] = 'x-small'
    # Basemap
    m = basemap.Basemap(projection='merc', llcrnrlon=-120, llcrnrlat=30,
                        urcrnrlon=120, urcrnrlat=70)
    # Plots a figure to compare
    x = np.linspace(-10, 10, 10000)
    y = np.sin(x)
    plt.plot(x, y)
    plt.title('Example of figure using only rc values')
    # Draws the grid (using the function)
    latlon_grid(m, 30, 10, labels='lb', dashes=[1, 3])
    plt.title('Using latlon_grid()')