Patrick Giraudoux wrote:
Hi,
Thanks for all those information and the most valuable library you have developed.
If somebody is to develop something to write shapefiles from polygon coordinates within R (most welcome), I don't think that the attribute file (dbf) will be an important issue. If we have got an output with the index to link the dbf records to each shape of a shapefile, any text file can be handled in R or Excel to create a dbf. The real issue is the encoding of the *.shp and *.shx etc.. (and an index file for the dbf)
Most important is to consider that ESRI has stopped developping ArcView and AVENUE. They have moved to ArGIS and Visual Basic. In this new generation framework, importation is managed with ArcToolBox which, to my knowledge, is very limited considering importation of text objects (actually I even wonder if one can import something but attribute tables without ArcInfo). For instance, the way I found to solve the problem I met was to write a programme in R to wrap the polygon coordinates in a text file readable in GRASS (an opensource GIS), to import it into GRASS and to export the shapefile from GRASS. This shapefile was finally imported to ArcGIS. I would not insist on ESRI client policy. I just want to say that the more I know corporates the more I love opensource...
Anyway, there are many things that can be made in R which may deserve not only importation to R, but also exportation to many kinds of GIS. To write shapefiles from polygon, points or lines created in R would be most useful.
I don't know anything about shapefiles, but that's starting to sound like a "shapefile" graphics device (c.f. the gtkDevice, RSvgDevice packages).
Paul
Many thanks for your interest and support,
Patrick Giraudoux
Universit� de Franche-Comt� Laboratoire de Biologie environnementale EA3184 usc INRA F-25030 Besan�on Cedex
t�l.: +33 381 665 745 fax.: +33 381 665 797 http://lbe.univ-fcomte.fr
"Ce n'est pas en am�liorant la bougie que l'on a invent� l'�lectricit�", la recherche fondamentale est indispensable !
----- Original Message ----- From: <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Cc: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Sent: Thursday, February 26, 2004 7:11 PM
Subject: Re: [R] writing polygons/segments to shapefiles (.shp) or other ArcGIS compatible file
The main limitation of the shapefiles package that I put together is that it does not create shapefiles from R objects - rather it only writes shapefiles that have been read into R and manipulated within the constraints of the existing file structure. By this I mean that for example you can change the coordinates of points and write them back out. Or you can add a bunch of blank columns in the DBF outside of R and then fill them in with R. But I did not write any code to calculate byte offsets and such and that are needed when creating a shapefile from scratch.
So what I do when I want to create a new shapefile from within R is write out the format required by the ASCII Tool ArcView (Avenue) script. This script is available at: http://arcscripts.esri.com/details.asp?dbid=11442 The format is simple:
Works for space delimited ascii to point, polygon and polyline. The format for point ascii file is id, x, y (no comma for real data, space delimited). For polygon & polyline ascii files, the format is code (1 for start point, 2 for middle points, 3 for end point), x, y (no comma for real data, space delimited). Export shapefile to ascii file works for point, polyline and polygon shapefiles. The output format file is id, x, y. For polygon and polyline, the id is the sequence id of vertices.
Thus all you have to do is write a text file, install the script in ArcView and then use the ArcView extension to create a shapefile from the ASCII file. Unfortunately ASCII tool only works with the geography - you have to add the attributes later.
With all of that said, I would like to add to the shapefiles package the ability to write out shapefiles from scratch. Since the shapefiles format is rather unique, I think it would be best to use the maptools ShapeList/Map class format (or r-spatial's classes). For starters the package would just write out the geography (like the ASCII tool Avenue script). Later would be added the ability to write the dbf data out from the att.data element of the Map object.
Unfortunately I don't know when I will have the time to do this. If anyone else wants to do then please go ahead and I can help via email if needed. But adding the ability to write out shapefiles from scratch is on my list. Once I finish coding our travel demand model and we enter the application phase (probably in a few months) then will our demand to output shapefiles from R increase and I can justify spending the time to write the code.
I apologize that I have not paid much attention to the r-sig-geo discussion or Edzer's r-spatial project. If there is an effort underway to write out shapefiles then can someone please inform me of its status. If not, then maybe I can help. Thanks.
Benjamin Stabler Transportation Planning Analysis Unit Oregon Department of Transportation 555 13th Street NE, Suite 2 Salem, OR 97301 Ph: 503-986-4104
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-- Dr Paul Murrell Department of Statistics The University of Auckland Private Bag 92019 Auckland New Zealand 64 9 3737599 x85392 [EMAIL PROTECTED] http://www.stat.auckland.ac.nz/~paul/
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