Re: NSIDC data, mapproject, and invproj reduex

JJ,

The "a" and "b" values of those two versions of the ellipsoid that you quote differ by order 100 m, so it is not surprising that your results differ by 100 m. The values given by NSIDC header look to me like standard modern earth ellipsoids (computing the flattening from (a-b)/a I get something on the order of 1/298.257___, and the a value is the standard one). The values given for the EPSG 3412 are very different numbers.

W

On Apr 24, 2014, at 4:20 PM, "Becker, Joseph J." <[email protected]> wrote:

> 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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