[GIS] In a GeoTiff, what unit are extents in

extentsgdalgeotiff-tiffunits

I was looking at the GDAL metadata of a USGS GeoTiff quad (o48118e8) with the GdalToTilesWin program. It dumped the following info:

Name o48118e8.tif
Size X/Y 5315/6780
ResolutionX/Y 2.4384/2.4384
Extent
North 5388204.0171
West 350547.3487
South 5371671.6651
East 363507.4447
It is not Geodesic WGS84(EPSG:4326)
PROJCS[
    "NAD27 / UTM zone 11N",
    GEOGCS[
        "NAD27",
        DATUM["North_American_Datum_1927",
        SPHEROID[
            "Clarke 1866",
            6378206.4,
            294.9786982139006,
            AUTHORITY["EPSG","7008"]
        ],
        AUTHORITY["EPSG","6267"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4267"]
    ],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-117],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",0],
    UNIT[
        "metre",
        1,
        AUTHORITY["EPSG","9001"]
    ],
    AUTHORITY["EPSG","26711"]
]

Could someone explain how to understand extent? For instance, does North 5388204.0171 mean 5388204.0171 meters north of the NAD27?

Or maybe the Unit.Metre has something to do with it?

How do these 'extents' convert to lat/long (if at all?)

Also, out of curiosity, I presume the SizeX/Y is pixels, but what is the ResolutionX/Y?

Best Answer

The units of the extent will inherit from the units of the projection of the data, so to convert the extent to lat/long, you would need to reproject your data to a geographic projection (usually WGS1984) that uses degrees as units. Your data appears to have been projected to NAD27 / UTM zone 11N, which has units of meters. The extent refers to the x and y values in the projection of the furthest N, S, E, and W points (or ymin, ymax, xmin, xmax) in the layer, forming a box around it. So, visually:

            5388204.0171
          |----------------|     and,       350547.3487  |                | 363507.4447
          |----------------|
            5371671.6651

would define the spatial extent of the file. Sometimes the extent is expressed in terms of the coordinates of the corners of the box around the layer. In your case, the upper left corner would be: (350547.3487, 5388204.0171) and so on.

Also, the resolution refers to the x and y length, in units of your projection, of each pixel in the raster and the size is the number of rows and columns (number of pixels in the x and y directions)

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