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

[Barry Rowlingson]

I downloaded the hail and tornado shapefile It looks like hailstorms in the
original data files don't really move, except maybe one 10m grid square.
This first one from 2013 goes nowhere:

Slot "coords":
         [,1]      [,2]
[1,] 185758.3 -246307.6
[2,] 185758.3 -246307.6

I reckon theyve tried to use the same format as the tornado data, which can
have quite large differences between start and end point, but hailstorms
are such a short intense event that the motion isn't important relative to
the size.

So I don't think there's essentially any "track" information in the data,
and it should be considered point events.

Barry




On Mon, Oct 27, 2014 at 2:49 PM, St John Brown <st_john_brown at yahoo.com>
wrote:

> 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
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>

	[[alternative HTML version deleted]]