[PROJ] Fwd: proj list issue
Even Rouault
even.rouault at spatialys.com
Wed Apr 17 09:24:53 PDT 2019
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.
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