[R-sig-Geo] Derivation of intermediate points (great circle geometry) in the geosphere package

[Robert J. Hijmans]

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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