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April 17, 2020</div>
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Greetings,</div>
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I have used the destPointRhumb function in the package geosphere to generate a set of geographic coordinates.</div>
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If I understand correctly, the returned coordinates were derived on a sphere with defined radius r=6378137.</div>
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My desire is to transform the destPointRhumb coordinates to WGS84.</div>
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I expected the transformation would result in slightly different coordinates on the WGS84 ellipsoid.</div>
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My transformation attempt, however, did not change the coordinates at all -- which puzzles me.</div>
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Can someone show me what I'm doing wrong, or misunderstanding?</div>
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Thank you in advance.</div>
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David Douglas</div>
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<span style="font-family:"Courier New",monospace">###########################################################</span></div>
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<span style="font-family:"Courier New",monospace">require(tidyverse)</span><span><br>
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<div><span style="font-family:"Courier New",monospace">require(sf)</span><br>
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<div><span style="font-family:"Courier New",monospace">require(geosphere)</span><br>
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<div><span style="font-family:"Courier New",monospace"># request 5 coordinates along a rhumb line, 3000 km apart spanning south of equator to northern Greenland...</span><br>
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<div><span style="font-family:"Courier New",monospace">d <- (1:5 * 3000000)</span><br>
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<div><span style="font-family:"Courier New",monospace">gs <- d %>% geosphere::destPointRhumb(p = c(-110,-45), b = 20, r = 6378137) %>% tbl_df()</span><br>
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<div><span style="font-family:"Courier New",monospace"># apply CRS as sphere...</span><br>
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<div><span style="font-family:"Courier New",monospace">geospherePoints <- gs %>% st_as_sf(coords = c("lon", "lat"), crs = "+proj=longlat +ellps=sphere +no_defs")</span><br>
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<div><span style="font-family:"Courier New",monospace">geospherePoints</span><br>
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<div><span style="font-family:"Courier New",monospace"># transform coordinates to WGS84 spheroid...</span><br>
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<div><span style="font-family:"Courier New",monospace">geospherePointsWGS84 <- geospherePoints %>% st_transform("+init=epsg:4326")</span><br>
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<div><span style="font-family:"Courier New",monospace">geospherePointsWGS84</span><br>
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<div><span style="font-family:"Courier New",monospace"># Result: the two sets of coordinates are qualitatively identical... why?</span><br>
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<div><span style="font-family:"Courier New",monospace">geospherePointsWGS84 == geospherePoints</span><br>
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<div style="margin:0px; font-size:10pt; font-family:Arial,Helvetica,sans-serif"><span style="font-family:"Courier New",monospace">###########################################################</span></div>
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<span style="font-size:10pt">David C Douglas</span></div>
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<span style="font-size:10pt">USGS Alaska Science Center</span></div>
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<span style="font-size:10pt">250 Egan Drive</span></div>
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<span style="font-size:10pt">Juneau AK 99801</span></div>
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<span style="font-size:10pt">(907) 364-1576 office</span></div>
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<span style="font-size:10pt">ddouglas@usgs.gov</span></div>
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