Map scalebar always horizontal even for projections not preserving angles (Mercator vs. RGF93)

[salahelfarissi]

I am using your library to plot maps, currently I'm using Web Mercator as a projection (Below is a map in web mercator).
Recently I was asked to use a more precise projection. For France, we use 2154. However, when I do that the Scalebar is not horizontal anymore. Is there a solution to my problem.

Thank you

[raphaelquast]

Hey, nice to see you're using EOmaps!

the scalebar has a "rotation" argument that sets the azimuth angle in which the scalebar points
See docs for add_scalebar

As this angle is relative to the direction of latitude lines, a scalebar with rotation=0 is not necessarily "horizontal" in the plot (except for 4326 and 3857).

As a quick suggestion i see 2 possible solutions:

• the simple way: manually adjust the angle until the scalebar is approx. horizontal for the desired the position (you can use +- keys to interactively rotate the scalebar and get the active rotation with scalebar_instance.get_position)
• the nice but complex way: use m.ax.projection.get_geod() to get the pyproj.geod instance for the used crs and then use pyproj methods to determine the desired orientation angle for a given position...

[raphaelquast]

Hey, I had a proper look at the problem and actually found a quite nice and simple solution!
It will be available as ScaleBar.set_rotation with the next release!

In the meantime, here's a quick monkey-patch version:

from eomaps import Maps
import numpy as np

def set_rotation(self, ang=0):
    """
    Set the absolute rotation angle of the first segment of the scalebar.
    
    Note
    ----
    This method sets the "absolute rotation angle" in display units, 
    not the "azimuth angle" which can be set with :py:meth:`ScaleBar.set_position`.
    
    Parameters
    ----------
    ang : float
        The rotation angle.
    
    """
    lon, lat, _ = self.get_position()
    x0, y0 = self._m._transf_lonlat_to_plot.transform(lon, lat)
    x1, y1 = self._m._transf_lonlat_to_plot.transform(
        *self._geod.fwd(lon, lat, -ang, self.get_scale())[:2]
        )
    azim = np.rad2deg(-np.arctan2(x1 - x0, y1 - y0))
    self.set_position((lon, lat), azim=azim)

m = Maps(2154)
m.add_feature.preset.coastline()

scb1 = m.add_scalebar((-6, 43.45))
set_rotation(scb1, 0)

scb2 = m.add_scalebar((2.5, 46))
set_rotation(scb2, 45)

scb3 = m.add_scalebar((-7, 51))
set_rotation(scb3, 90)

[salahelfarissi]

Thank you.
We use your library in a celery context to produce gis maps.
I will test this and give you my feedback.
I hope to be able to contribute to your project very soon.

One last remark that I got from some of our collaborators.
Is there a way to have rounded values, preferable to x5 / x10 / x100 ...

Currently I'm planning to do that using the x limits of the map, to specify directly the segment length, I'm obliged to have 4 segments alternating in color.

[raphaelquast]

Hey,

I hope to be able to contribute to your project very soon.

🚀 Nice to hear! I very much appreciate any efforts to improve the package!

Is there a way to have rounded values, preferable to x5 / x10 / x100 ...

There is an example in the docs on how to fully customize scalebars:
Example - Scalebars

The "scale" (e.g. the length of one segment of the scalebar) is only dependent on the axes-limits if you do not specify it explicitly.
Just use m.add_scalebar(scale=29) and you should be good to go!

For example:

from eomaps import Maps
lon, lat = -2.562, 49.4653
extent = (-2.565, -2.554, 49.4642, 49.4682)

m = Maps(2154)
m.add_feature.preset.coastline()
m.set_extent(extent)

scb1 = m.add_scalebar((lon, lat))
scb2 = m.add_scalebar((lon+0.002, lat), scale=29, n=4,  
                      scale_props=dict(colors=("r", "g")))
scb3 = m.add_scalebar((lon+0.004, lat), scale=29, n=4, 
                      scale_props=dict(colors=("k", ".75")), 
                      patch_props=dict(fc="none", ec="none"))

[raphaelquast]

Hey, I've converted this to a discussion to preserve the answers for later!
Feel free to mark as accepted if you think all your questions are answered!

[raphaelquast]

With an update to EOmaps v8.3 you can use:

eomaps import Maps

m = Maps(2154)
m.add_feature.preset.coastline()
scb = m.add_scalebar((-6, 43.45), scale=5e4)
scb.set_rotation(90)