Does ComputeBandStats take nodata into account?

问题

I am trying to compute the stats for an image which is only partly covered by data. I would like to know if ComputeBandStats ignores the pixels with the same value as the files nodata.

Here is my code:

inIMG = gdal.Open(infile)

# getting stats for the first 3 bands
# Using ComputeBandStats insted of stats array has min, max, mean and sd values
print "Computing band statistics"
bandas = [inIMG.GetRasterBand(b+1) for b in range(3)]
minMax = [b.ComputeRasterMinMax() for b in bandas]
meanSD = [b.ComputeBandStats(1) for b in bandas]
print minMax
print meanSD

For the image without the nodata attribute the output is:

Computing band statistics
[(0.0, 26046.0), (0.0, 24439.0), (0.0, 22856.0)]
[(762.9534697777777, 647.9056493556284), (767.642869, 516.0531530834181), (818.0449643333334, 511.5360132592902)]

For the image with nodata = 0 the output is:

Computing band statistics
[(121.0, 26046.0), (202.0, 24439.0), (79.0, 22856.0)]
[(762.9534697777777, 647.9056493556284), (767.642869, 516.0531530834181), (818.0449643333334, 511.5360132592902)]

The min and max values have changed such that 0 is no longer min, which makes sense, because in the second version it is nodata and therefore not regarded by ComputeRasterMinMax(). However, the mean and standard deviation has not changed.

Does this mean that ComputeBandStats doesn't disregard the nodata values?
Is there any way to force ComputeBandStats to disregard the nodata values?

回答1:

Setting the NoData value has no effect on the data itself. You can try it this way:

# First image, all valid data
data = numpy.random.randint(1,10,(10,10))
driver = gdal.GetDriverByName('GTIFF')
ds = driver.Create("stats1.tif", 10, 10, 1, gdal.GDT_Byte)
ds.GetRasterBand(1).WriteArray(data)
print ds.GetRasterBand(1).ComputeBandStats(1)
print ds.GetRasterBand(1).ComputeStatistics(False)
ds = None

# Second image, values of "1" set to no data
driver = gdal.GetDriverByName('GTIFF')
ds = driver.Create("stats2.tif", 10, 10, 1, gdal.GDT_Byte)
ds.GetRasterBand(1).SetNoDataValue(1)
ds.GetRasterBand(1).WriteArray(data)
print ds.GetRasterBand(1).ComputeBandStats(1)
print ds.GetRasterBand(1).ComputeStatistics(False)
ds = None

Note that the stats returned by ComputeBandStats are unchanged, but that those returned by ComputeStatistics are:

>>> (4.97, 2.451346568725035)
>>> [1.0, 9.0, 4.970000000000001, 2.4513465687250346]

>>> (4.97, 2.451346568725035)
>>> [2.0, 9.0, 5.411111111111111, 2.1750833672117]

You can confirm manually that the stats are correct:

numpy.mean(data)
numpy.mean(data[data != 1])
numpy.std(data)
numpy.std(data[data != 1])

>>> 4.9699999999999998
>>> 5.4111111111111114
>>> 2.4513465687250346
>>> 2.1750833672117

来源：https://stackoverflow.com/questions/37857254/does-computebandstats-take-nodata-into-account