Skip to content

Raster package: colFromX behaviour

3 messages · Robert J. Hijmans, Jonathan

Original
Jonathan
# 
Hello group,

I am trying to get the column from point that exists the interfaces of two
cells and find that the results I'm getting are not consistent.  In the
below example I am getting the higher boundary for the cell for 0.1, 0.2,
and 0.4.  However, I get the lower boundary for 0.3.  

My current solution is to add a very small number to the coordinate if the
value falls on the interface but does anyone know a better workaround?

library(raster)
H <- raster(matrix(sample.int(100), ncol=10))

colFromX(H, 0.1)  # 2
colFromX(H, 0.2)  # 3
colFromX(H, 0.3)  # 3
colFromX(H, 0.4)  # 5

colFromX(H, 0.30000000000000001)  #3
colFromX(H, 0.3000000000000001)   #4

Cheers,
-Jonathan


Session Info 
R version 3.0.0 (2013-04-03)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United
States.1252   
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] raster_2.1-16 sp_1.0-8     

loaded via a namespace (and not attached):
[1] grid_3.0.0      lattice_0.20-15 tools_3.0.0   



--
View this message in context: http://r-sig-geo.2731867.n2.nabble.com/Raster-package-colFromX-behaviour-tp7583304.html
Sent from the R-sig-geo mailing list archive at Nabble.com.
1 day later
Jonathan
# 
Thank you Robert for your response,

I see it is a floating point issue so I've now come up with a solution
(modified your version slightly) that is functional for my use.  Basically
it rounds the calculated ratio to a certain number of digits and then
truncates the value.  I've attached it in case it is of use to someone.  I
am tracking points across the cells and needed values to be consistently on
one side or the other or else my points can become trapped in a particular
cell when an edge is reached.

colFromX <- function ( object, x, digits=10)	{
	if (inherits(x, 'Spatial')) { 
		x <- x at coords[,1] 
	}
	if (rotated(object)) {
		stop('this function is not supported for rotated rasters')
	}
	colnr <- trunc(round((x - xmin(object)) / xres(object), digits)) + 1
	colnr[ x == xmax(object) ] <- object at ncols
	colnr[ x < xmin(object) | x > xmax(object) ] <- NA
	return(as.vector(colnr))
}

Thank you again,
Jonathan



--
View this message in context: http://r-sig-geo.2731867.n2.nabble.com/Raster-package-colFromX-behaviour-tp7583304p7583314.html
Sent from the R-sig-geo mailing list archive at Nabble.com.