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What is the meaning of the Pixel Size field in the gdalinfo output on a TIFF image?

I have 2 images, one is a PAN image (size 400MB) while one is a DEM (size 25MB). Both are in TIFF format. When I run the gdalinfo command on these 2, I get the following:

For the PAN image:

Pixel Size = (0.600000023841858,-0.600000023841858)

For the DEM:

Pixel Size = (0.000277777777778,-0.000277777777778)

I also get the same values (with one extra value) when I run another command, exiftool, on these 2 images:

For the PAN image:

Pixel Scale : 0.600000023841858 0.600000023841858 0

For the DEM:

Pixel Scale : 0.000277777777778 0.000277777777778 0

I can see that the output is same, except that gdalinfo puts a minus sign before second value, and exiftool gives 3 values where the 3rd value is 0.

What should I infer from these values? Which image has a better spatial resolution? I know its the PAN image that has a better resolution, but how do I understand that from these values?

I say this because since 0.00027 < 0.6, so one might think that the DEM has a better resolution. Just like when we say that an image has a spatial resolution of 0.25 meter while other has 0.3 meter resolution, it means the 0.25 meter one (the lower one) has better resolution. Or maybe I am understanding it wrong if I'm looking at the value of the number in this case.

Whats the unit of these numbers? Is it meters? If not, then what? Which gdalinfo field could tell me the unit? Right now I get this metadata field in gdalinfo which has the word 'unit' in it:

TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)

How to I explain someone that in this context, bigger number means better resolution, not the other way?

Some additional info that I feel might be helpful:

The metadata field AREA_OR_POINT is as follows:

For the PAN: - POINT

For the DEM: - AREA

2

Pixel size is (as it sounds) the size of a pixel in the units of the raster's projection. So there is nothing you can infer from just the number, 0.00027 < 0.6 might mean that the first raster is higher resolution or it may mean one is measured in feet and the other in metres (or degrees or furlongs).

Here is the gdalinfo output for a raster I have:

Driver: GTiff/GeoTIFF
Files: /data/os-data/rasters/ras250_gb/data/NC.tif
Size is 4000, 4000
Coordinate System is:
PROJCS["OSGB 1936 / British National Grid",
    GEOGCS["OSGB 1936",
        DATUM["OSGB_1936",
            SPHEROID["Airy 1830",6377563.396,299.3249612664951,
                AUTHORITY["EPSG","7001"]],
            AUTHORITY["EPSG","6277"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4277"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",49],
    PARAMETER["central_meridian",-2],
    PARAMETER["scale_factor",0.9996012717],
    PARAMETER["false_easting",400000],
    PARAMETER["false_northing",-100000],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","27700"]]
Origin = (200000.000000000000000,1000000.000000000000000)
Pixel Size = (25.000000000000000,-25.000000000000000)

In this case my Pixel Size is 25 metres, compared to this one:

Driver: GTiff/GeoTIFF
Files: /data/natural_earth/HYP_HR_SR_OB_DR/HYP_HR_SR_OB_DR.tif
Size is 21600, 10800
Coordinate System is:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433],
    AUTHORITY["EPSG","4326"]]
Origin = (-180.000000000000000,90.000000000000014)
Pixel Size = (0.016666666666670,-0.016666666666670)

Where the pixels are 0.016 of a degree in size but are much lower resolution.

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  • I didn't notice the UNIT field in the Coordinate System section. I have the same 2 cases - the 0.6 value is in meters while the 0.00027 value is in degrees. Now I understand it. – Nancy Dec 23 '19 at 7:09

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