[STATSGRASS] R: east() north()

[Roger Bivand]

On Tue, 29 Apr 2003, javier garcia wrote:

> Hello;
> I'm learning to use R along with GRASS (Versions: R 1.6.2  GRASS 5.0.2   
> R-GRASS 0.2.8) 
> Trying to understand the use of 
> east() and north(), I've done:
> -------------------------------------------
> 
> > summary(east(G))
>    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
>  632200  647200  662100  662100  677000  692000
> > summary(north(G))
>    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
> 4164000 4172000 4181000 4181000 4190000 4199000
> > system("g.region -p")
> projection: 1 (UTM)
> zone:       30
> datum:      ** unknown (default: WGS84) **
> ellipsoid:  international
> north:      4198600
> south:      4163700
> west:       632200
> east:       692000
> nsres:      50
> ewres:      50
> rows:       698
> cols:       1196
> --------------------------------------------------------
> I can't see the meaning of north() and east().
> If east were the east of each cell in the grid, the result should be 
> something like:
> > summary(east(G))
>    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
>  632250  ...                                            692000


And indeed it (almost) is!! summary.default(), the function you are
calling in summary(east(G)), has a digits= argument. Increase the digits,
and you see it (in fact east() and north() return raster cell midpoints):

> summary(G)
Data from GRASS 5.0 LOCATION maas with 259 columns and 232 rows;
UTM, zone: 32
The west-east range is: 269870, 272460, and the south-north: 5650610, 5652930;
West-east cell sizes are 10 units, and south-north 10 units.
> system("g.region -p")
projection: 1 (UTM)
zone:       32
datum:      ** unknown (default: WGS84) **
ellipsoid:  wgs84
north:      5652930
south:      5650610
west:       269870
east:       272460
nsres:      10
ewres:      10
rows:       232
cols:       259
> summary(north(G))
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
5651000 5651000 5652000 5652000 5652000 5653000 
> summary(north(G), digits=7)
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
5650615 5651193 5651770 5651770 5652348 5652925 

so north() runs from south + nsres/2 to north - nsres/2.

This is a numerical representation thing - statisticians are often 
concerned with the "big picture", in which 5 meters in 5 million really do 
not matter, so the default number of digits *printed to the screen* is at 
least 3. For spatial position, cm can matter in millions of meters, so we 
need to set the digits= argument. The values in the variables are correct, 
its the values being displayed that are rounded.

Roger

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: Roger.Bivand at nhh.no