readGDAL

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Read/write between GDAL grid maps and Spatial objects

The functions read or write GDAL grid maps. They will set the spatial reference system if available. GDALinfo reports the size and other parameters of the dataset. create2GDAL creates a GDAL data set from a SpatialGridDataFrame object, in particular to be able to save to GDAL driver formats that only permit copying rather than creation.

Keywords
spatial
Usage
readGDAL(fname, offset, region.dim, output.dim, band, p4s=NULL, ...,
 half.cell=c(0.5, 0.5), silent = FALSE)
asSGDF_GROD(x, offset, region.dim, output.dim, p4s=NULL, ...,
 half.cell=c(0.5,0.5))
writeGDAL(dataset, fname, drivername = "GTiff", type = "Float32",
 mvFlag = NA, options=NULL, copy_drivername = "GTiff", setStatistics=FALSE)
create2GDAL(dataset, drivername = "GTiff", type = "Float32", mvFlag = NA,
 options=NULL, fname = NULL, setStatistics=FALSE)
GDALinfo(fname, silent=FALSE, returnRAT=FALSE, returnCategoryNames=FALSE)
GDALSpatialRef(fname, silent=FALSE)
Arguments
fname
file name of grid map; in create2GDAL provides a way to pass through a file name with driver-required extension for sensitive drivers
x
A GDALReadOnlyDataset object
offset
Number of rows and columns from the origin (usually the upper left corner) to begin reading from; presently ordered (y,x) - this may change
region.dim
The number of rows and columns to read from the dataset; presently ordered (y,x) - this may change
output.dim
The number of rows and columns to return in the created object using GDAL's method to take care of image decimation / replication; presently ordered (y,x) - this may change
band
if missing, all bands are read
p4s
PROJ4 string defining CRS, if default (NULL), the value is read from the GDAL data set
half.cell
Used to adjust the intra-cell offset from corner to centre, usually as default, but may be set to c=(0,0) if needed; presently ordered (y,x) - this may change
silent
logical; if TRUE, comment and non-fatal CPL driver errors suppressed
...
arguments passed to either getRasterData, or getRasterTable, depending on rotation angles (see below); see the rgdal documentation for the available options (subsetting etc.)
dataset
object of class SpatialGridDataFrame-class or SpatialPixelsDataFrame-class
drivername, copy_drivername
GDAL driver name; if the chosen driver does not support dataset creation, an attempt is made to use the copy_drivername driver to create a dataset, and copyDatset to copy to the target driver
type
GDAL write data type, one of: Byte, Int16, Int32, Float32, Float64; UInt16, UInt32 are available but have not been tests
mvFlag
default NA, missing value flag for output file; the default value works for Int32, Float32, Float64, but suitable in-range value that fits the data type should be used for other data types, for example
options
driver-specific options to be passed to the GDAL driver; see Details below
setStatistics
default FALSE, if TRUE, attempt to set per-band statistics in the output file (driver-dependent)
returnRAT
default FALSE, if TRUE, return a list with a Raster Attribute Table or NULL for each band
returnCategoryNames
default FALSE, if TRUE, return a list with a character vector of CategoryNames or NULL for each band
Details

In writeGDAL, if types other than Int32, Float32, Float64 are used, the mvFlag argument should be used to set a no data value other than the default NA. Note that the flag only replaces NA values in the data being exported with the value of the argument - it does not mark data values equal to mvFlag as missing. The value is stored in the file being written in driver-specific ways, and may be used when the file is read. When the default mvFlag=NA is used, no NoDataValue is written to the file, and the input data is written as is.

Also in writeGDAL, the options argument may be used to pass a character vector of one or more options to the driver, for example options=INTERLEAVE=PIXEL, or options=c(INTERLEAVE=PIXEL, COMPRESS=DEFLATE). Typical cases are given in the examples below; it may also be necessary in some cases to escape quotation markes if included in the string passed to the driver.

Value

  • read.GDAL returns the data in the file as a Spatial object.

    Usually, GDAL maps will be north-south oriented, in which case the rgdal function getRasterData is used to read the data, and an object of class SpatialGridDataFrame-class is returned.

    Some map formats supported by GDAL are not north-south oriented grids. If this is the case, readGDAL returns the data as a set of point data, being of class SpatialPointsDataFrame-class. If the points are on a 45 or 90 degree rotated grid, you can try to enforce gridding later on by e.g. using gridded-methods(x)=TRUE.

Note

Non-fatal CPL errors may be displayed for some drivers, currently for the AIG ArcInfo 9.3 binary raster driver using GDAL >= 1.6.2; the data has been read correctly, but the contents of the info directory did not meet the specifications used to reverse engineer the driver used in GDAL (see http://trac.osgeo.org/gdal/ticket/3031)

Warning

Some raster files may have an erroneous positive y-axis resolution step, leading to the data being flipped on the y-axis. readGDAL will issue a warning: Y axis resolution positive, examine data for flipping, when the step is positive, but this need not mean that the data are flipped. Examine a display of the data compared with your knowledge of the file to determine whether this is the case (one known case is interpolation files created under Qgis up to February 2010 at least). To retreive the correct orientation, use flipVertical.

See Also

image, asciigrid

Aliases
  • readGDAL
  • asSGDF_GROD
  • writeGDAL
  • create2GDAL
  • GDALinfo
  • print.GDALobj
  • GDALSpatialRef
Examples
library(grid)
GDALinfo(system.file("external/test.ag", package="sp")[1])
x <- readGDAL(system.file("external/test.ag", package="sp")[1])
class(x)
image(x)
summary(x)
x@data[[1]][x@data[[1]] > 10000] <- NA
summary(x)
image(x)

x <- readGDAL(system.file("external/simple.ag", package="sp")[1])
class(x)
image(x)
summary(x)
x <- readGDAL(system.file("pictures/big_int_arc_file.asc", package="rgdal")[1])
summary(x)
cat("if the range is not 10000, 77590, your GDAL does not detect big
")
cat("integers for this driver
")
y = readGDAL(system.file("pictures/Rlogo.jpg", package = "rgdal")[1], band=1)
summary(y)
y = readGDAL(system.file("pictures/Rlogo.jpg", package = "rgdal")[1])
summary(y)
spplot(y, zcol=1:3, names.attr=c("red","green","blue"), 
	col.regions=grey(0:100/100),
	main="example of three-layer (RGB) raster image", as.table=TRUE)
data(meuse.grid)
gridded(meuse.grid) = ~x+y
proj4string(meuse.grid) = CRS("+init=epsg:28992")
fn <- tempfile()
writeGDAL(meuse.grid["dist"], fn)
GDALinfo(fn)
writeGDAL(meuse.grid["dist"], fn, setStatistics=TRUE)
GDALinfo(fn)
mg2 <- readGDAL(fn)
proj4string(mg2)
SP27GTIF <- readGDAL(system.file("pictures/SP27GTIF.TIF", 
package = "rgdal")[1], output.dim=c(100,100))
summary(SP27GTIF)
image(SP27GTIF, col=grey(1:99/100))

GDALinfo(system.file("pictures/cea.tif", package = "rgdal")[1])
GDALSpatialRef(system.file("pictures/cea.tif", package = "rgdal")[1])
cea <- readGDAL(system.file("pictures/cea.tif", package = "rgdal")[1], 
output.dim=c(100,100))
summary(cea)
image(cea, col=grey(1:99/100))
fn <- system.file("pictures/erdas_spnad83.tif", package = "rgdal")[1]
erdas_spnad83 <- readGDAL(fn, offset=c(50, 100), region.dim=c(400, 400), 
output.dim=c(100,100))
summary(erdas_spnad83)
image(erdas_spnad83, col=grey(1:99/100))
erdas_spnad83a <- readGDAL(fn, offset=c(50, 100), region.dim=c(400, 400))
bbox(erdas_spnad83)
bbox(erdas_spnad83a)
gridparameters(erdas_spnad83)
gridparameters(erdas_spnad83a)
tf <- tempfile()
writeGDAL(erdas_spnad83, tf, drivername="GTiff", type="Byte", options=NULL)
all.equal(erdas_spnad83, readGDAL(tf))
writeGDAL(erdas_spnad83, tf, drivername="GTiff", type="Byte", 
options="INTERLEAVE=PIXEL")
all.equal(erdas_spnad83, readGDAL(tf))
writeGDAL(erdas_spnad83, tf, drivername="GTiff", type="Byte",
options=c("INTERLEAVE=PIXEL", "COMPRESS=DEFLATE"))
all.equal(erdas_spnad83, readGDAL(tf))

x <- GDAL.open(system.file("pictures/erdas_spnad83.tif", package = "rgdal")[1])
erdas_spnad83 <- asSGDF_GROD(x, output.dim=c(100,100))
GDAL.close(x)
summary(erdas_spnad83)
image(erdas_spnad83, col=grey(1:99/100))

tf <- tempfile()
xx <- create2GDAL(erdas_spnad83, type="Byte")
xxx <- copyDataset(xx, driver="PNG")
saveDataset(xxx, tf)
GDAL.close(xx)
GDAL.close(xxx)
GDALinfo(tf)

tf2 <- tempfile()
writeGDAL(erdas_spnad83, tf2, drivername="PNG", type="Byte")
GDALinfo(tf2)

GT <- GridTopology(c(0.5, 0.5), c(1, 1), c(10, 10))
set.seed(1)
SGDF <- SpatialGridDataFrame(GT, data=data.frame(z=runif(100)))
opar <- par(mfrow=c(2,2), mar=c(1,1,4,1))
image(SGDF, "z", col=colorRampPalette(c("blue", "yellow"))(20))
title(main="input values")
pfunc <- colorRamp(c("blue","yellow"))
RGB <- pfunc(SGDF$z)
SGDF$red <- RGB[,1]
SGDF$green <- RGB[,2]
SGDF$blue <- RGB[,3]
image(SGDF, red="red", green="green", blue="blue")
title(main="input RGB")
tf <- tempfile()
writeGDAL(SGDF[c("red", "green", "blue")], tf, type="Byte", drivername="PNG")
t1 <- readGDAL(tf)
image(t1, red=1, green=2, blue=3)
title(main="output PNG RGB")
par(opar)
Documentation reproduced from package rgdal, version 0.7-7, License: GPL (>= 2)

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