On Tuesday 13 January 2009, Edzer Pebesma wrote: > This sounds very familiar; I ran into this about 6 years ago or so.
Could it be related to this ticket: http://trac.osgeo.org/gdal/ticket/2369 ??? should be fixed in a recent GDAL build. Cheers, Dylan > The asciigrid driver in gdal reads the first so 100K of the file to > figure out whether the data are integer or floating point. It then > searches for a dot in any of the numbers, indicating floating point > data. If it doesn't find a floating point (g2), it assumes integer and > proceeds with that. The funny min and max values equal the min and max > value of a signed two-bytes integer, +/- 2^15. The problem is that many > images come as integer data, and what do you want gdal to do: read your > 13 Mb (zipped!) first to figure out what it is, to start all over again? > This would make many other users very unhappy. > > I remember filing a bug long ago. > > How about putting a dot after your first value in the grid? > -- > Edzer > > Brian R Miranda wrote: > > Roger, > > > > Thanks for your help. I believe you are correct that the printed summary > > values have been rounded. I couldn't quite figure out how to extract the > > range specifically, but when I did a summary for the data.frame from each > > grid I got what I was expecting for the one created by readAsciiGrid > > (g1). However, the grid created by readGDAL (g2) still shows unexpected > > min and > > > > max values. See results below: > >> d1 <- g...@data > >> summary(d1) > > > > distrail_100m.asc > > Min. : 0 > > 1st Qu.: 2878 > > Median : 6562 > > Mean : 7896 > > 3rd Qu.: 11670 > > Max. : 37082 > > NA's :6483714 > > > >> d2 <- g...@data > >> summary(d2) > > > > band1 > > Min. : -32768 > > 1st Qu.: 2844 > > Median : 6516 > > Mean : 7718 > > 3rd Qu.: 11600 > > Max. : 32767 > > NA's :6483714 > > > > For my own purposes, I have found that I should continue using > > readAsciiGrid to bring in my grids. But I would be happy to share my > > dataset to help figure out why readGDAL has these strange results. My > > Ascii file is 13Mb zipped, probably too large to email, and I don't have > > an FTP site that can be accessed outside of the Forest Service. I can > > send the file to you off list using YouSendIt.com if you would like, or > > if you have an alternative suggestion for transferring the file, let me > > know. Thanks again for your assistance, and if you need any additional > > information from me, let me know. > > > > Cheers, > > -Brian > > > > ======================= > > Brian R. Miranda > > USDA Forest Service > > Northern Research Station > > brmira...@fs.fed.us > > > > Institute for Applied Ecosystem Studies > > 5985 Highway K > > Rhinelander, WI 54501 > > phone: 715-362-1186 > > fax: 715-362-1166 > > > > > > > > > > > > Roger Bivand <roger.biv...@nhh.no> > > 01/13/2009 02:49 PM > > Please respond to > > roger.biv...@nhh.no > > > > > > To > > Brian R Miranda <brmira...@fs.fed.us> > > cc > > r-sig-geo@stat.math.ethz.ch > > Subject > > Re: [R-sig-Geo] ESRI Ascii Grid > > > > 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 > >> brmira...@fs.fed.us > >> > >> Institute for Applied Ecosystem Studies > >> 5985 Highway K > >> Rhinelander, WI 54501 > >> phone: 715-362-1186 > >> fax: 715-362-1166 > >> > >> > >> [[alternative HTML version deleted]] > >> > >> _______________________________________________ > >> R-sig-Geo mailing list > >> R-sig-Geo@stat.math.ethz.ch > >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo