Derivation of intermediate points (great circle geometry) in the geosphere package
HI Tom, Can you try the latests version that is on R-Forge? I have fixed and improved a couple of things in it, and am about to upload this version to CRAN. Robert On Wed, Apr 14, 2010 at 11:57 PM, Tomislav Hengl
<hengl at spatial-analyst.net> wrote:
I have started using the geosphere package kindly provided by Robert Hijmans [http://cran.r-project.org/web/packages/geosphere/]. My intention is to derive a global map of the air routes density using the open access flight routes [http://openflights.org/data.html]. I have discovered some strange results when trying to derive the intermediate points on a path: gcIntermediate(p1, p2, n) *this uses the formulas described in [http://williams.best.vwh.net/avform.htm#Intermediate] I discovered that, depending on the location of points considering quadrants, the current formula will (systematically?) shift the coordinates to some new quadrant, which is obviously a bug:
library(geosphere) gcIntermediate(c(0,0), c(10,10), 5) ?# this one is OK
? ? ? ? ?lon ? ? ?lat [1,] 1.803343 1.830237 [2,] 3.422141 3.468623 [3,] 4.961631 5.019001 [4,] 6.508449 6.565756 [5,] 8.150886 8.192447
gcIntermediate(c(-10,10), c(10,10), 5) ?# this one needs to be fixed: ifelse(lon>0, lon+180, 180-lon); -1*lat
? ? ? ? ? lon ? ? ? lat [1,] ?173.6232 -10.08957 [2,] ?176.8920 -10.13646 [3,] ?180.0000 -10.15108 [4,] -176.8920 -10.13646 [5,] -173.6232 -10.08957
gcIntermediate(c(-20,-10), c(5,10), 5) ?# this is OK
? ? ? ? ? ? lon ? ? ? lat [1,] -15.4984322 -6.467336 [2,] -11.4054190 -3.175910 [3,] ?-7.5000000 ?0.000000 [4,] ?-3.5945810 ?3.175910 [5,] ? 0.4984322 ?6.467336
gcIntermediate(c(-20,-25), c(10,5), 5) # this needs to be fixed:
?for(lon2<lon1) {ifelse(lon>0, 180-lon, 180+lon)}; -1*lat
? ? ? ? ? lon ? ? ? ?lat [1,] ?165.7968 20.0356468 [2,] ?170.9455 15.1760517 [3,] ?175.7253 10.3444590 [4,] -179.6410 ?5.4446120 [5,] -174.9524 ?0.3684525 Is somebody really good in geosphere geometry? BR & thanx T. Hengl http://home.medewerker.uva.nl/t.hengl/
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