[PROJ] Fwd: proj list issue

[Jeff McKenna]

I imagine that user is trying to send the email to the incorrect address 
proj at lists dot maptools dot org

-jeff



On 2019-04-17 1:24 PM, Even Rouault wrote:
> Forwarding post from a subscriber of the list
> 
> ----------  Forwarded Message  ----------
> 
> Subject: proj list issue
> Date: mercredi 17 avril 2019, 09:14:44 CEST
> From: Ken Mankoff <mankoff at gmail.com>
> 
> Dear Proj list,
> 
> Please excuse duplicates - my last email was returned with a bounce error so
> I'm sending a second one.
> 
> I'm following up on a helpful thread from this list from 2017: https://
> lists.osgeo.org/pipermail/proj/2017-November/007496.html
> 
> I'm calculating map projection errors, and want to check my interpretation of
> the results with someone who knows proj better. I'm using EPSG:3413 projection
> and the following proj string:
> 
> +proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +no_defs
> +a=6378137 +rf=298.257223563 +towgs84=0.000,0.000,0.000 +to_meter=1
> 
> To estimate the error in Southern Greenland, I do:
> 
> echo -40 60 | proj -VS ${PROJSTR}
> 
> and see
> 
> #Stereographic
> #       Azi, Sph&Ell
> #       lat_ts=
> # +proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +no_defs
> # +a=6378137 +rf=298.257223563 +towgs84=0.000,0.000,0.000 +to_meter=1
> #Final Earth figure: ellipsoid
> #  Major axis (a): 6378137.000
> #  1/flattening: 298.257224
> #  squared eccentricity: 0.006694379990
> Longitude: 40dW [ -40 ]
> Latitude:  60dN [ 60 ]
> Easting (x):   289632.50
> Northing (y):  -3310514.64
> Meridian scale (h) : 1.03942808  ( 3.943 % error )
> Parallel scale (k) : 1.03942808  ( 3.943 % error )
> Areal scale (s):     1.08041073  ( 8.041 % error )
> Angular distortion (w): 0.000
> Meridian/Parallel angle: 90.00000
> Convergence : 5d [ 5.00000000 ]
> Max-min (Tissot axis a-b) scale error: 1.03943 1.03943
> 
> The key values for Northern Greenland are:
> 
> echo -40 82 | proj -VS ${PROJSTR}
> ...
> ...
> Meridian scale (h) : 0.97460025  ( -2.54 % error )
> Parallel scale (k) : 0.97460025  ( -2.54 % error )
> Areal scale (s):     0.94984565  ( -5.015 % error )
> 
> My question is how to interpret the Areal scale error
> 
> I think that in this projection N. Greenland is over-represented by 8% area
> and any areal feature there should be divided by ~1.0804 or linear features
> divided by ~1.0394 to bring them to true scale. Similarly, in S. Greenland I
> should adjust a areal feature by dividing by 0.94984565. Is this correct?
> 
> Thanks,
> 
>    -k.
>