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Can I calculate the area of a polygon?

3 messages · Andreas Kretschmer,,,, (Ted Harding)

Original
Andreas Kretschmer,,,
# 
Hello,

I'm verry new in R.

My Problem:

I have a PostgreSQL-Server with installed support for the R-Language.
And now I have a table with a Polygon like this:

test_db=# select * from geo where id=1;
 id |             koerper
----+---------------------------------
  1 | ((0,0),(0,2),(2,2),(3,3),(5,0))
(1 row)

Now, I need to know the area of this object. Please, can you tell me, if
it possible to use the installed plr - Language zu calculate the area?

I have some experience to draw diagramms from Postgres via the
R-language, but I'm a absolute novice to use R to solve problems like
this.


Regards, Andreas
(Ted Harding)
#
On 19-Dec-04 Andreas Kretschmer,,, wrote:
Welcome!
Not sure what you mean by "the installed plr - Language", but for
calculating areas og polygons you may find the following function
useful.
Example based on the coordinates ((0,0),(0,2),(2,2),(3,3),(5,0))
you give above.

Let X be the matrix of the coordinates:

  > X
       [,1] [,2]
  [1,]    0    0
  [2,]    0    2
  [3,]    2    2
  [4,]    3    3
  [5,]    5    0


Function "area":

  area<-function(X){
    X<-rbind(X,X[1,])
    x<-X[,1]; y<-X[,2]; lx<-length(x)
    -sum((x[2:lx]-x[1:lx-1])*(y[2:lx]+y[1:lx-1]))/2
  }

  > area(X)
  [1] -9.5

Note that this function generates a "signed area": positive
if the bounding contour is "counter-clockwise" in the order
of the vertices as given in X, negative if clockwise.

The principle of the function is that it first closes the
polygon by repeating the first point, and then (in effect)
calculates the contour integral

  Int -y*dx

round the contour of the polygon (equivalent to Int x*dy).

Hoping this helps,
Ted.


--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 094 0861  [NB: New number!]
Date: 19-Dec-04                                       Time: 13:40:32
------------------------------ XFMail ------------------------------
Andreas Kretschmer,,,
# 
am  Sun, dem 19.12.2004, um 13:40:32 -0000 mailte Ted Harding folgendes:
The Database-System PostgreSQL is ready to extend with external
features. As example: pl/r is a procedural language. So can I share data
between the database and R.

,----[  lynx --dump  plr-overview.html  ]
| Chapter 1. Overview
|
|    PL/R is a loadable procedural language that enables you to write
|    PostgreSQL functions and triggers in the [6]R programming language.
|    PL/R offers most (if not all) of the capabilities a function writer
|    has in the R language.
`----
Danke, Merci, thank you.
Yes.


Regards, Andreas