[R-sig-Geo] ESRI Ascii Grid

[Roger Bivand]

On Tue, 13 Jan 2009, Brian R Miranda wrote:

> Hello,
>
> I have an Ascii grid that was created using the GridAscii command in
> ArcGrid.  The grid represents cell distance to railroads and has values
> ranging from 0 to 37,082.  When I bring this grid into R as a
> SpatialGridDataFrame, the maximum and/or the minimum values in the grid do
> not exactly match the original grid.  I have tried using both readGDAL and
> readAsciiGrid for this purpose.  I have confirmed in a text editor that
> the Ascii file does contain the maximum value of 37,082.  Below are the
> results I have seen.  Note the differences in the Max. value and Min.
> value (from readGDAL).

First, beware of the possible difference between the printed summary value 
and the actual value - printed summaries may round. What does range() of 
the variable say? The difference between the two functions is puzzling - 
could you make the data available (off list for example, zipped, 
preferably for download) to try to see what is going on?

Roger

>
>> g1 <- readAsciiGrid("G:/Oconto/WI_Fire/distrail_100m.asc")
>> summary(g1)
> Object of class SpatialGridDataFrame
> Coordinates:
>                min      max
> coords.x1 -263487.6 149512.4
> coords.x2  358821.1 666821.1
> Is projected: NA
> proj4string : [NA]
> Number of points: 2
> Grid attributes:
>  cellcentre.offset cellsize cells.dim
> 1         -263437.6      100      4130
> 2          358871.1      100      3080
> Data attributes:
>   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.    NA's
>      0    2878    6562    7896   11670   37080 6483714
>
>> g2 <- readGDAL("G:/Oconto/WI_Fire/distrail_100m.asc")
> G:/Oconto/WI_Fire/distrail_100m.asc has GDAL driver AAIGrid
> and has 3080 rows and 4130 columns
>> summary(g2)
> Object of class SpatialGridDataFrame
> Coordinates:
>        min      max
> x -263487.6 149512.4
> y  358821.1 666821.1
> Is projected: NA
> proj4string : [NA]
> Number of points: 2
> Grid attributes:
>  cellcentre.offset cellsize cells.dim
> x         -263437.6      100      4130
> y          358871.1      100      3080
> Data attributes:
>   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.    NA's
> -32770    2844    6516    7718   11600   32770 6483714
>
> The results from readAsciiGrid are pretty close (Max = 37,080) to the
> original grid, but readGDAL results in much different values.  I would
> expect both functions to create an object with exactly the same values as
> the Ascii grid.  Can anyone explain why this occurs, and if there is a
> different way to get my grid into R (I eventually need an 'im' to work
> with)?  I am new to spatial analysis using R, so if there is something
> obvious I am missing I apologize.  Thanks in advance for any help!
>
> Cheers,
> -Brian
>
>
> =======================
> Brian R. Miranda
> USDA Forest Service
> Northern Research Station
> brmiranda at fs.fed.us
>
> Institute for Applied Ecosystem Studies
> 5985 Highway K
> Rhinelander, WI  54501
> phone: 715-362-1186
> fax: 715-362-1166
>
>
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>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no