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circular spatial polygon

4 messages · Kátia Emidio, Rolf Turner, Loïc Dutrieux +1 more

Original
Kátia Emidio
# 
Dear all,
My question is how to create a circular spatial polygon, with 8 arcs of 45
degrees, and radius measuring 15m. Having in the centre point the UTM
coordinates, zone 20S.

x=   *673593.21*
*y= **9365367.08*



*Thanks*
Rolf Turner
#
On 13/09/17 08:48, K?tia Emidio wrote:

            

      
      
      
    

        
"Circular polygon" is a contradiction in terms.  If a shape is a polygon 
then it is *not* a circle.  (Of course in real life we use polygons with 
large numbers of sides to *approximate* circles.  But 8 is not large!)

You can create an octagon with the required centre and radius using 
*spatstat* via:

     oct <- disc(radius=15,centre=c(673593.21,673593.21),npoly=8)

Does that provide (at least a start on) what you want?

cheers,

Rolf Turner

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Loïc Dutrieux
    

    

      
      #
    

  


  

      
This is also possible with rgeos

library(sp)
library(rgeos)

x <- 673593.21
y <- 673593.21

sp0 <- SpatialPoints(coords = data.frame(x=x, y=y),
                     proj4string = CRS('+proj=utm +zone=20 +south
+ellps=WGS84 +datum=WGS84 +units=m +no_defs'))
sp1 <- gBuffer(sp0, byid=TRUE, width = 15, quadsegs = 2)
plot(sp1)


Cheers,
Lo?c

        
On 12/09/2017 16:57, Rolf Turner wrote:

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Barry Rowlingson
    

    

      
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Your question is still a tiny bit imprecisely defined, since you haven't
said how you want this polygon to align to the axes. This function computes
the coordinates of an N-sided regular polygon centred at (x,y) of radius r,
of n sides, at angle theta:

ngon =
function(x,y,r,n,theta){
  phi = seq(theta, theta+2*pi, length=n+1)
  xc = x + r * sin(phi)
  yc = y + r * cos(phi)
  cbind(xc,yc)
}

Two possible solutions to your question are therefore:

 plot(ngon(673593.21, 9365367.08, 15, 8, 0), asp=1)
 plot(ngon(673593.21, 9365367.08, 15, 8, pi/8), asp=1)

although that only shows points, building some sort of spatial polygon
follows easily enough.

Creating points on a circle using trigonometry (sines and cosines) should
be in any introductory trigonometry book.

B



On Wed, Sep 13, 2017 at 2:30 AM, Lo?c Dutrieux <loic.dutrieux at conabio.gob.mx