Repository navigation
MapPanel plots correctly with isobars but not isotherms #4192
Description
What went wrong?
With the code below, the first figure that pops up looks like this:
Note the lack of Canadian provinces despite the mp.layers setting, as well as the relatively coarse boundaries.
The second figure looks like what I would expect:
As far as I can tell, I'm setting up the MapPanel and PanelContainers the same for both plots, yet the map is different. Is this a bug?
Operating System
Linux
Version
1.7.1
Python Version
3.13.16
Code to Reproduce
import xarray as xr
import metpy.calc as mpcalc
from metpy.cbook import get_test_data
from metpy.plots import ContourPlot, MapPanel, PanelContainer
from metpy.units import units
nam = xr.open_dataset(get_test_data('NAM_test.nc', as_file_obj=False)).metpy.parse_cf()
isen_lev = [300] * units('K')
smooth = 10
hgt = mpcalc.smooth_gaussian(nam['Geopotential_height_isobaric'], smooth).squeeze().rename('hgt')
tmp = mpcalc.smooth_gaussian(nam['Temperature_isobaric'], smooth).squeeze().rename('tmp')
nam_isen = mpcalc.isentropic_interpolation_as_dataset(isen_lev, tmp, hgt,
bottom_up_search=False)
nam_isen = nam_isen.metpy.assign_latitude_longitude()
lev = nam_isen.isentropic_level.values[0]
cp = ContourPlot()
cp.data = nam_isen
cp.field = 'temperature'
cp.contours = range(100, 400, 10)
cp.clabels = True
cp.plot_units = 'K'
mp = MapPanel()
mp.layers = ['coastline', 'borders', 'states']
mp.layers_edgecolor = 'saddlebrown'
mp.title = f'{lev}-K'
mp.plots = [cp]
pc = PanelContainer()
pc.size = (12,8.5)
pc.panels = [mp]
pc.draw()
pc.figure.tight_layout()
pc.show()
cp = ContourPlot()
cp.data = nam_isen
cp.field = 'pressure'
cp.contours = range(100, 1050, 50)
cp.clabels = True
cp.plot_units = 'hPa'
mp = MapPanel()
mp.layers = ['coastline', 'borders', 'states']
mp.layers_edgecolor = 'saddlebrown'
mp.title = f'{lev}-K'
mp.plots = [cp]
pc = PanelContainer()
pc.size = (12,8.5)
pc.panels = [mp]
pc.draw()
pc.figure.tight_layout()
pc.show()
Activity
dcamron commentedon Nov 3, 2026
You found yourself right at an odd Cartopy defaults quirk! By default, the named Natural Earth features from Cartopy are filtered through an "auto scaler" setting their scale from coarse to fine based on the extent of the axes. We mirror this explicitly in MapPanel for our named layers, but the default hard cutoff to go from a medium scale to a coarser one is over 50 deg of lat and lon extent. You'll notice that your temperature image is slightly zoomed out with an extent that, surprise, barely grows larger than that 50 deg/each threshold.
You can verify this by inspecting the extents directly, or just changing your
mp.contoursvalues. I tried 9, 15, etc. Or you can just crank up the total number of contours egmp.contours = 25. Note the extent shrinks and the features pop in. Blame upstream contour autoscaling :)You can avoid this by either fixing your extent using
mp.area, by name ('uslcc'is close here) or bytuple : (lon_left, lon_right, lat_bottom, lat_top); or by passing in features with specific scales toMapPanel.layers, egWe might look to make the interface to
MapPanel.areaa bit cleaner, as it was messy when I was playing around trying to pull lon/lat extents from the DataSet. Or we could take an opinionated approach on the feature scaling on our named features. But otherwise call it luck of thedraw()!I'll leave this open a bit in case we decide to open any new issue on those points. Thanks for the clear report as ever.
sgdecker commentedon Nov 4, 2026
Hmm, I guess I'm concerned that the extent of the map changes based on the presence or absence of certain contours. I haven't tested this, but I imagine the same plot settings, but at different times, would shift around the map a bit (and therefore cause Canadian provinces to appear or disappear seemingly at random because of the auto-scaler) as the isotherms change from one time to the next. That's not good.
I'm surprised the map changes at all depending on the contours. I had hoped that not specifying
MapPanel.areawas equivalent to GEMPAK's GAREA=grid, but evidently this is not the case, and that means #4161 really is a feature request (create a new aliasMapPanel.area = 'grid'that does exactly what GEMPAK does, that is, show the entire grid, no more, no less, assuming you leave the projection unspecified) rather than just a documentation update.dcamron commentedon Nov 5, 2026
With that captured in the other issue, I'll close this.