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

7 messages · zhijie zhang, Roger Bivand

Original
zhijie zhang
# 
Dear Roger,
  I have a problem on converting the coordinates into interval [0,1], hoping
that you can give me a hand.
  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
# 
Dear Zhijie Zhang,
On Sun, 23 Sep 2007, zhijie zhang wrote:

            

      
      
      
    

        
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

            

      
      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        
      

    

    

      
      

        


  

        

      Roger Bivand
    

    

      
      #
    

  


  

      
On Mon, 24 Sep 2007, zhijie zhang wrote:

        

            

      
      
      
    

        
Exactly - with the original data, the units *are* general, because they 
are in meters. Provided the other data is also using the same metric, this
also permits a direct interpretation of bandwidth, possibly with a 
physical or biological basis. In the same sense, you can also get a 
relative bandwidth from (bw / max(diff(bbox(<obj>)))), and multiply it out 
again for a different context.

            

      
      
      
    

        
Just loops of lapply(), not worse:

lns <- slot(rivers, "lines")

new_lns <- lapply(lns, function(x) {
   Lns <- slot(x, "Lines")
   new_Lns <- lapply(Lns, function(y) {
     new_crds <- mess_up(slot(y, "coords"))
     Line(new_crds)
   })
   Lines(new_Lns, ID=slot(x, "ID")
})
new_rivers <- SpatialLines(new_lns)

for your mess_up() - note, untried. Also note that the projection will not 
be defined.

Roger

            

      
      
      
    

        

  
    

      
      
    

  


  


      

    

    
1 day later

    

      
      

        
      

    

    
3 days later

    

      
      

        


  

        

      Roger Bivand
    

    

      
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Dear Zhijie Zhang,

I'm working on an approach to this. I was asked (also on Tuesday) whether 
it is possible to disguise map position to increase confidentiality for 
patient locations. This is a valid reason to elide spatial position, so 
I'll try to see what can be done, and it will include your case.

I will send a draft version for you to try when I have something that 
seems to work.

Best wishes,

Roger

        
On Tue, 25 Sep 2007, zhijie zhang wrote:

        

            

      
      
      
    

        

  
    

      
      
    

  


  


      

    

    
2 days later