problem on converting the coordinates into interval [0, 1]?
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