[R-sig-Geo] Calculate the length of hail paths

[Frede Aakmann Tøgersen]

Hi

Why not check your results with calculators for great-circle distance like http://www.movable-type.co.uk/scripts/latlong.html. There the haversine formula is used. Using 

> res[[1]]
[[1]]
          [,1]    [,2]
[1,] -101.5000 32.2000
[2,] -101.4999 32.2001

> res[[2]]
[[1]]
          [,1]     [,2]
[1,] -89.68000 41.20000
[2,] -89.67987 41.20008

I get the distances 0.01457 km and 0.01405 km from that homepage.

Using the distHaversine() instead of distCosine() (see the above URL for the difference) then I get the distances 14.14557 and 14.21278 in meters. However the above homepage uses 6371 km as the radius of the earth whereas geosphere uses 6378137 m. There could be other subtleties having influence on the decimal precision. 

Yours sincerely / Med venlig hilsen


Frede Aakmann Tøgersen
Specialist, M.Sc., Ph.D.
Plant Performance & Modeling

Technology & Service Solutions
T +45 9730 5135
M +45 2547 6050
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> -----Original Message-----
> From: r-sig-geo-bounces at r-project.org [mailto:r-sig-geo-bounces at r-
> project.org] On Behalf Of St John Brown
> Sent: 27. oktober 2014 15:50
> To: r-sig-geo at r-project.org
> Subject: [R-sig-Geo] Calculate the length of hail paths
> 
> Hello,
> 
> I am trying to calculate the the length of the trajectory of historic hail storms
> in the United States. I have written the R code to do this but my results do
> not seem valid. The majority of my results are around 14 meters which does
> not seem correct.
> 
> My data comes from the NOAA in the form of a shape file (hail.zip) [1]. When
> I read the shape file I have an object of class SpatialLinesDataframe. The
> individual lines represent the paths of the historic hail storms.
> 
> Below I have created an example SpatialLines object with lines from the
> original data and my method for calculating the path length. As you can see
> the results are around 14 meters. Am I calculating the distances correctly?
> 
> I appreciate any help!
> 
> [1] http://www.spc.noaa.gov/gis/svrgis/
> 
> library(sp)
> library(geosphere)
> 
> #create example SpatialLines obj
> myLines1 = Lines(list(Line(matrix(c(-519049.1, -519039.1, -736427.4, -
> 736417.4), nrow=2, ncol=2))), ID="1")
> myLines2 = Lines(list(Line(matrix(c(527165, 527175, 261338.5, 261348.5),
> nrow=2, ncol=2))), ID="2")
> proj_str_lcc = CRS("+proj=lcc +lat_1=33 +lat_2=45 +lat_0=39 +lon_0=-96
> +x_0=0 +y_0=0 +datum=NAD83 +units=m +no_defs +ellps=GRS80
> +towgs84=0,0,0")
> mySpLines = SpatialLines(list(myLines1,myLines2), proj4string=proj_str_lcc)
> 
> #calculate distance of line paths
> proj.str.alb = CRS("+proj=longlat +datum=WGS84")
> mySpLines_alb = spTransform(mySpLines, CRS=proj.str.alb)
> 
> res = lapply(slot(mySpLines_alb, "lines"), function(x) lapply(slot(x,
> "Lines"),function(y) slot(y, "coords")))
> 
> f = function(i){
>   end_pnts = unlist(res[i],use.names = F)
>   p1 = end_pnts[c(1,3)]
>   p2 = end_pnts[c(2,4)]
>   return(distCosine(p1,p2)) #meters
> }
> 
> d = sapply(1:length(res), FUN=f)
> 
> > d
> [1] 14.14557 14.21278 #meters
> 
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