[R] adjusting the map of France to 1830

[Michael Friendly]

I'm doing some analyses of historical data from France in 1830 on 'moral 
statistics' that I'd like to
show on a map.  I've done most of my analyses in SAS, but a few things 
would work better in R.
To do this, I have to adjust the modern map,

library(maps)
map('france')

to adjust for changes in departments (86 in 1830, to 97 now).  I've read 
the documentation
for the maps and maptools package, but there seems to be no functions to 
allow this, and
I can't find information on the exact structure of map datasets, but I 
understand them to
be delimited lists of polygon coordinates.

In SAS, all maps have (one or more) ID variables representing the 
geographical region,
and there is also a proc gremove that can remove internal boundaries 
inside the polygons
for regions with the same ID.  Is there some way I can do this in R?

Here's what I did in SAS:

*-- Fix the map of France to conform to Guerry:
    - adjust the 97 current departments to correspond to the 86 in 1830
    - delete those not part of France then
;

data gfrtemp;
    set maps.france;
    /* Corse was one dept - merge these to one area, new ID */
    if id in (201, 202)    then dept=200;

    /* Seine et Oise (78) was cut into
    Essonne (91), Val d'Oise (95) and Yvelines (78) */
    else if id in (91, 95)    then dept=78;

    /* Seine (75) now split into
    Hauts-de-Seine (92), Seine-Saint-Denis (93) et Val-de-Marne (94)*/
    else if id in (92, 93, 94)    then dept=75;
   
     /* departments not part of France in 1830 */
    else if id in (
        6,     /* Alpes-Maritimes */
        73,74, /* Savoie, Haute-Savoie */
        90)    /* Territore-de-Belfort */
        then delete;
    else                       dept=id;
    run;
   
*-- remove internal boundaries based on merged DEPT;
proc sort data=gfrtemp;
    by dept;

proc gremove data=gfrtemp out=gfrance;
   by dept;
   id id;
   run;



-- 
Michael Friendly     Email: friendly at yorku.ca 
Professor, Psychology Dept.
York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street    http://www.math.yorku.ca/SCS/friendly.html
Toronto, ONT  M3J 1P3 CANADA