[PROJ] package 'geosphere' -- transforming spherical-based geographic coordinates to WGS84

[Douglas, David C]

April 17, 2020

Greetings,

I have used the destPointRhumb function in the package geosphere to generate a set of geographic coordinates.
If I understand correctly, the returned coordinates were derived on a sphere with defined radius r=6378137.

My desire is to transform the destPointRhumb coordinates to WGS84.
I expected the transformation would result in slightly different coordinates on the WGS84 ellipsoid.
My transformation attempt, however, did not change the coordinates at all -- which puzzles me.

Can someone show me what I'm doing wrong, or misunderstanding?

Thank you in advance.

David Douglas

###########################################################
require(tidyverse)
require(sf)
require(geosphere)
# request 5 coordinates along a rhumb line, 3000 km apart spanning south of equator to northern Greenland...
d <- (1:5 * 3000000)
gs <- d %>% geosphere::destPointRhumb(p = c(-110,-45), b = 20, r = 6378137) %>% tbl_df()
# apply CRS as sphere...
geospherePoints <- gs %>%  st_as_sf(coords = c("lon", "lat"), crs = "+proj=longlat +ellps=sphere +no_defs")
geospherePoints
# transform coordinates to WGS84 spheroid...
geospherePointsWGS84 <- geospherePoints %>% st_transform("+init=epsg:4326")
geospherePointsWGS84

# Result: the two sets of coordinates are qualitatively identical...  why?
geospherePointsWGS84 == geospherePoints
###########################################################

David C Douglas
USGS Alaska Science Center
250 Egan Drive
Juneau AK 99801
(907) 364-1576 office
ddouglas at usgs.gov

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20200417/d0963882/attachment.html>