[R-sig-Geo] problem on converting the coordinates into interval [0, 1]?

[Roger Bivand]

Dear Zhijie Zhang,

On Sun, 23 Sep 2007, zhijie zhang wrote:

> Dear Roger,
>  I have a problem on converting the coordinates into interval [0,1], hoping
> that you can give me a hand.

I think that you need to explain why. If it is just because the 
coordinates "explode" because they are large, then simply scaling like 
this:

library(rgdal)
guichi <- readOGR(".", "guichi")
plot(guichi, axes=TRUE)
bbox(guichi)
proj4string(guichi)
t1 <- paste("+proj=tmerc +lat_0=0 +lon_0=117 +k=1.000000 +x_0=0",
   "+y_0=-3348000 +a=6378140 +b=6356755.288157528 +units=km")
t2 <- spTransform(guichi, CRS(t1))
plot(t2, axes=TRUE)
bbox(t2)
#          min      max
# r1 10.148541 79.88203
# r2  0.834946 61.90978

gets down to 70 by 80, which is not such a numerical risk (if need be, use 
+units=kmi to get nautical miles, which reduces the numbers even more). It 
has the key advantage that you can always get back to the true coordinates 
by transforming back.

In the internal code, you will see that many packages guard by using [-1, 
+1] on both dimensions. In your case and if you know that you need this, 
you would have to work out the scaling factor on the longer axis, so that 
points on the shorter axis would be scaled correctly. But if you want to 
keep the dimensions' aspect, you could just shift the origin and the units 
as I show above.

Hope this helps,

Roger

>  My data consist of points(cases), lines(rivers) and polygon (guichi city),
> and i want to change their coordinates into interval [0,1].  I have put the
> data in the attachment, so that you can use it.
> The following is the programs to read the data and one possible method for
> conversion:
>
> library(sp)
> library(foreign)
> library(mgcv)
> library(maptools)
>
> #read the polygon containing the studied points
> guichi <- readShapePoly("e:/guichi.shp")  #boundary polygons containing the
> points
> point_poly <-
> getPolygonCoordsSlot(getPolygonsPolygonsSlot(getSpPpolygonsSlot(guichi)[[1]])[[1]])
> #get the coordinates of guichi
> #plot(point_poly,xlab="x", ylab="y",type="l")
>
> #read the lines of two rivers in the guichi
> rivers <- readShapeLines("e:/qiupu.shp")  #change the coordinates??difficult
> #plot(rivers,add=T)
>
> #read the points of cases and controls
> case_control <- read.csv("e:/casecontrol.csv",sep=",", header=TRUE)
> #plot(case_control$x,case_control$y)
>
> *#Plot the whole figure*
> plot(point_poly,xlab="x", ylab="y",type="l")
> plot(rivers,add=T)
> points(case_control$x,case_control$y,add=T)
>
> ###################################################################################
> #one of the possible methods to convert the x/y coordinates into interval
> [0,1]
> *# But it seems there is a problem: it convert the x/y coordinates into
> interval [0,1], respectively.
> # In my opinion, they should be expand or  shrink according to the same
> minimum/maximum value*.
> st <- function(x)(x-min(x))/(max(x)-min(x))
> case_control[,c(8,9)] <- data.frame(lapply(case_control[,c(6,7)],st))
> ###################################################################################
> *Q1. The main problem is on the rivers, which has multiple lines. There are
> difficulties for conversion.
> Q2. when i convert the x/y coordinates, i'm not very sure whether i should
> use the same scale of conversion or not. *
> Could you please help us to solve it?
> Any suggestions/help are greatly appreciated.
>
>
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no