Steven Decker

I haven't tried implementing this yet, but I think the general solution would be to accept the data CRS object as an optional argument, rather than more specific things such...

The equation way back in the first comment is valid for grid-relative `u` and `v`. If you used earth-relative winds on, say, a polar stereographic projection where North America is...

@dcamron Yes, I can join in on July 8.

Using GEMPAK's value for the radius of the Earth marginally improves (by 0.02% or so, consistent with the radius differing in the fifth significant digit) the correspondence between MetPy and...

Calculating the map factors as ``` m_x = 1 + (factors.parallel_scale - 1) / mpconst.earth_avg_radius.m m_y = 1 + (factors.meridional_scale - 1) / mpconst.earth_avg_radius.m ``` brings things closer to GEMPAK...

Makes sense that the spherical correction is no good for NAM 218, as that isn't a spherical grid. The fact that the last approach based on the map factors doesn't...

My investigations are cataloged here: https://github.com/sgdecker/testing/blob/188ed819b4b464d5a3c1b83034435f0e6b9d800c/MapProjectionExploration.ipynb

I can now reproduce the GEMPAK calculation on Grid 218 using the proj4 map factors. I am not sure what I am doing differently from @kgoebber actually, but it works!...

Summary notebook demonstrating a working vorticity function for Grid 218 is [here](https://github.com/sgdecker/testing/blob/05efd2cfbfe0021cf65917f489dd9158087f5765/PrototypeVort.ipynb). Still need to revisit GFS global data and look at a stereographic grid.

It is at heart differential geometry, but you can see the general expression in the table [here](https://en.wikipedia.org/wiki/Curvilinear_coordinates#Differentiation) (good luck going through the derivations to get to that table!), where h1...