NSIDC data, mapproject, and invproj reduex

In a nut shell, these results differ in the 4th decimal of latitude. I guess that is only about 100m, but I was expecting much better...

a) grdreformat NSIDC_Ant500m_wgs84_elev_cm.dat=gd foo.nc

b) grd2xyz foo.nc | head -1 | mapproject -R-180/180/-90/0 -Js0/-90/-70/1:1 -C -F -I


	-50.7255753273	-57.3452848192	0
 

c) grd2xyz foo.nc | head -1 | invproj +init=epsg:3412 -f "%.9f"


	-50.725575327	-57.345939985	0


d) There seems to be some difference of opinion about what "a" and "b" values for EPSG 3412 are

	Proj4js.defs["EPSG:3412"] = "+proj=stere +lat_0=-90 +lat_ts=-70 +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs";

NSIDC header says 

	projection info = {31, 6378137.0, 6356752.3, -70.000000, 0.000000, 0.0, 0.0, WGS-84, Polar Stereo South GLAS, units=Meters}

close but different "a, b"


e) Further more how does one specify a semi-major and minor axis in mapproject ???


thanks 

-jj

================================




A few years ago I asked about a similar issue and got some great advice. 

I was looking at some NSIDC data for Greenland, and got this handy snippet back with essentially perfect agreement

> mapproject -R-180/180/0/90 -Js-45/90/70/1:1 -C -F -I <<< "209000  -2780000"
> -40.7005994538	64.6687837499
> 
> invproj +init=epsg:3413 -f "%.9f" <<< "209000  -2780000"
> -40.700599454	64.668783750
> 
> The parameters for WGS 84 / NSIDC Sea Ice Polar Stereographic North are: +proj=stere +lat_0=90 +lat_ts=70
> +lon_0=-45 +k=1 +x_0=0 +y_0=0 +da
> 

Now I'm working with

	https://nsidc.org/data/docs/daac/nsidc0422_antarctic_1km_dem/

I was expecting I could just use the same sort of call to mapproject,  but using the (to me odd) -71 rather than -70 true scale lat.  

I got a reasonable answer, but just to be sure I tried the test at the top of this post and noticed a slight difference between map project and invproj

Anyone got the "right answer" for making a geographic version of ERS-1 Radar & ICESat-1 data ???

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