Raster data types in GDAL

DataType refers to the data type of the actual value in the image. The specific data type is defined in the gdalconst module.

First look at the specific usage and the results returned.

>>> from osgeo import gdalconst
>>> dir(gdalconst)
['CE_Debug', 'CE_Failure', 'CE_Fatal', 'CE_None', 'CE_Warning', ...

The beginning of those GDTs is the data type of the value. To view the data type of a band in an image, you only need to print the DataType property of this band.

>>> from osgeo import gdal
>>> rds = gdal.Open("/gdata/geotiff_file.tif")
>>> band = rds.GetRasterBand(1)
>>> band.DataType
1

The return result is integer. But what does this 1 mean?

The table lists the corresponding values of gdalconst and integers.

Table: Correspondence between Constant Quantity and Numeric Type in GDAL

According to the table, the value 1 indicates gdalconst.GDT_Byte. The data type here is corresponding to the type in NumPy.

Data access to data sets

The descriptive information of remote sensing images can be accessed by the methods described in the previous section, and some information such as acquisition time, processing time, spatial resolution and image size can be generally known. In order to process the remote sensing image, it is necessary to further access the data in the remote sensing image, that is, the gray value of the pixels in the image.

GDAL provides the following two functions to access image values:

• ReadRaster() reads image data (in binary form);

• ReadAsArray() reads image data (in the form of an array).

These are two very important functions. They can read the image data directly, and then calculate and analyze the raster data. These two functions have some parameters.

• xoff,yoff : Specify the position (in pixels) of the origin of the full image in the entire image to be read.

• xsize,ysize : Specifies the length and width (in pixels) of the rectangle to read a partial image.

• buf_xsize,buf_ysize : You can zoom after reading a part of the image. Then use these two parameters to define the final width and height of the scaled image, GDAL will process the data to this size.

• buf_type : can convert the type of data read (for example, the original data type is short, converted to byte).

• band_list : Adapt to multi-band conditions. You can specify which band to read.

Here’s a brief introduction to how to get the data in the GeoTIFF file.

>>> from osgeo import gdal
>>> dataset = gdal.Open("/gdata/lu75c.tif")
>>> from numpy import *
>>> dataset.ReadAsArray(2500,2500,3,3)
array([[12, 12, 12],
       [12, 12, 12],
       [12, 12, 12]], dtype=int16)
>>> array([[12, 12, 12],
... [12, 12, 12],
... [12, 12, 12]],dtype=int16)
array([[12, 12, 12],
       [12, 12, 12],
       [12, 12, 12]], dtype=int16)
>>> dataset.ReadRaster(2500,2500,3,3)
'\x0c\x00\x0c\x00\x0c\x00\x0c\x00\x0c\x00\x0c\x00\x0c\x00 ...

In this way, the image in the upper left corner of the image is located at \((3340,3780)\), and the width and height are \(10\) pixels are read out. The results returned by these two functions are different, where ReadAsArray() reads an array of NumPy of type int16 ; and ReadRaster() It is a binary type. ReadAsArray() For more explanation of the result type, see the table in the previous section.

Read data in band

>>> from osgeo import gdal
>>> from gdalconst import *
>>> dataset = gdal.Open("/gdata/geotiff_file.tif")
>>> band = dataset.GetRasterBand(1)
>>> band.ReadAsArray(100,100,5,5,10,10)
array([[48, 48, 54, 54, 53, 53, 51, 51, 45, 45],
       [48, 48, 54, 54, 53, 53, 51, 51, 45, 45],
       [32, 32, 40, 40, 45, 45, 46, 46, 44, 44],
       [32, 32, 40, 40, 45, 45, 46, 46, 44, 44],
       [22, 22, 34, 34, 43, 43, 58, 58, 57, 57],
       [22, 22, 34, 34, 43, 43, 58, 58, 57, 57],
       [39, 39, 52, 52, 56, 56, 56, 56, 49, 49],
       [39, 39, 52, 52, 56, 56, 56, 56, 49, 49],
       [59, 59, 61, 61, 49, 49, 42, 42, 39, 39],
       [59, 59, 61, 61, 49, 49, 42, 42, 39, 39]], dtype=uint8)

>>> band.XSize
10572
>>> band.YSize
9422
>>> band.ReadAsArray(95,100,5,5)
array([[ 52,  85,  81, 101,  77],
       [ 67,  86,  89, 105,  62],
       [ 69,  76,  86,  93,  38],
       [ 66,  68,  85,  76,  33],
       [ 60,  71,  89,  75,  47]], dtype=uint8)

ERROR 5: Access window out of range in RasterIO().  Requested
(95,100) of size 5x5 on raster of 100x100.