Interpreting GeoTiff pixel values using Python GDAL?

I was running python script that queries a GeoTiff file at the coordinates of a centroid of blocks and as a result I got a list of pixel values, which I have trouble interpreting. When I run gdalinfo to obtain basic information about my tiff I get the following (abridged output):

~$ gdalinfo arsenci020l.tif
Driver: GTiff/GeoTIFF
Files: arsenci020l.tif
       arsenci020l.tfw
Size is 10366, 7273
Coordinate System is:
PROJCS["Lambert Azimuthal Equal Area projection with arbitrary plane grid; projection center 

100.0 degrees W, 45.0 degrees N",
......(omitted)............
......(omitted)............
Band 1 Block=10366x1 Type=Byte, ColorInterp=Palette
Color Table (RGB with 256 entries)
0: 0,0,0,255
1: 255,255,255,255
2: 132,193,255,255
3: 204,204,204,255

My interpretation of Color Table given by gdalinfo was that every pixel value actually stands for 4 other entries.

The output of python script prints out 2,3,5, etc. How to interpret them? My first instinct was to look at the Color Table above given by gdalinfo, but I'm curious why it talks about RGB value and gives 4 numbers? (don't we represent RGB be in 3 numbers?) Is there an extra column?

Also, the information file about the GeoTiff has the following table:

  Attribute_Label: Arsenic concentration grid cell value.
  Attribute_Definition: 
    Each grid cell value is a color definition that represents an 
    estimate of the 75th percentile of arsenic concentration in 
    that region.  The concentrations were recorded in micrograms 
    per liter (ug/L; equivalent to parts per billion), and were 
    converted to colors according to the following table.  All 

    arsenic concentrations greater than 50 micrograms per liter 
    were recoded as 50 micrograms per liter.  There may be 
    intermediate colors in the image that are not included in this 
    table. 
    > Arsenic             Color        RGB values
    > Concentration         
    >--------------------------------------------------------------
    >  1 ug/L or less     dark green    50.67328 150.3464   0.67328
    >  3 ug/L             light green  152       251      152
    >  5 ug/L             yellow       255       255        0

    > 10 ug/L             orange       255       165        0
    > 50 ug/L or greater  red          255         0        0
    > Insufficient data   white        255       255      255
    > Non-US land         grey         204       204      204
    > Water               light blue   132       193      255

Here it uses 3 numbers to represent a color. Why then gdalinfo gives 4?

As a side note, how should I programmatically connect pixel value, RGB color and arsenic concentration somehow (without excluding any intermediate colors as this table did). Any advice on how to do that?

This is the first time I'm working with GeoTiff files.

Answer

The fourth value is alpha (i.e. RGBA), which you can ignore. The four value structure is expected.

You can read the colour tables into native lists/dicts with GDAL.

from osgeo import gdal
gdal.UseExceptions()

ds = gdal.Open(fname)
band = ds.GetRasterBand(1)
arr = band.ReadAsArray()
ct = band.GetColorTable()


# index value to RGB (ignore A)
i2rgb = [ct.GetColorEntry(i)[:3] for i in range(ct.GetCount())]

# RGB to index value (assumes RGBs are unique)
rgb2i = {rgb: i for i, rgb in enumerate(i2rgb)}

# Now look up an index, e.g., water is light blue
print(rgb2i[(132, 193, 255)])  # 2