[gdal-dev] Area from gdalinfo metadata on a .tif image

[Andrew C Aitchison]

On Thu, 26 Jul 2018, Tyler Burse wrote:
> I hope you all have had a great week so far. I am hoping you will be able
> to provide some help because I have not been able to find a way to get the
> area from the metadata returned by gdalinfo. What I get as the return of
> gdalinfo appears to be a non-georeferenced figure such as:
>
> C:\Temp>gdalinfo figure1.tif
> Driver: GTiff/GeoTIFF
> Files: figure1.tif
> Size is 244, 210
> Coordinate System is `'
> Metadata:
>  TIFFTAG_XRESOLUTION=96
>  TIFFTAG_YRESOLUTION=96
>  TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
> Image Structure Metadata:
>  INTERLEAVE=PIXEL
> Corner Coordinates:
> Upper Left  (    0.0,    0.0)
> Lower Left  (    0.0,  210.0)
> Upper Right (  244.0,    0.0)
> Lower Right (  244.0,  210.0)
> Center      (  122.0,  105.0)
> Band 1 Block=244x11 Type=Byte, ColorInterp=Red
> Band 2 Block=244x11 Type=Byte, ColorInterp=Green
> Band 3 Block=244x11 Type=Byte, ColorInterp=Blue
>
>
> Would you be able to help me with finding the best way to extract the area
> in kilometers squared from this info using GDAL?

The output includes
    Coordinate System is `'
so, formally, we do not know the relationship between the image and the
piece of the earth that it represents.

If the file specified the coordinate system,
then gdalinfo would report something like:
Coordinate System is:
PROJCS["OSGB 1936 / British National Grid",
     GEOGCS["OSGB 1936",
         DATUM["OSGB_1936",
             SPHEROID["Airy 1830",6377563.396,299.3249646,
                 AUTHORITY["EPSG","7001"]],
             TOWGS84[446.448,-125.157,542.06,0.15,0.247,0.842,-20.489],
             AUTHORITY["EPSG","6277"]],
         PRIMEM["Greenwich",0,
             AUTHORITY["EPSG","8901"]],
         UNIT["degree",0.0174532925199433,
             AUTHORITY["EPSG","9122"]],
         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"]],
     AXIS["Easting",EAST],
     AXIS["Northing",NORTH],
     AUTHORITY["EPSG","27700"]]
Origin = (-649749.999999999883585,1449750.000000000465661)
Pixel Size = (500.000000000000000,-500.000000000000171)
Metadata:
  ... 
Corner Coordinates:
Upper Left  ( -649750.000, 1449750.000) ( 21d55'22.41"W, 61d27'54.60"N)
Lower Left  ( -649750.000, -150250.000) ( 16d 0'12.44"W, 47d41'10.18"N)
Upper Right ( 1350250.000, 1449750.000) ( 16d 9' 2.77"E, 61d43'17.34"N)
Lower Right ( 1350250.000, -150250.000) ( 10d42'41.48"E, 47d50'21.53"N)

Note the lines:
     UNIT["metre",1,
         AUTHORITY["EPSG","9001"]],
- so this coordinate system and file is in metres (other commone coordinate systems use US feet or degrees as units).

This file represents a space on the ground which is
   1320250 - -649750 = 2000000m = 2000km east to west, and
   1449750 - -150250 = 1600000m = 1600km north to south,
so has an area of 3.2million km^2.

-- 
Andrew C. Aitchison					Cambridge, UK
 			andrew at aitchison.me.uk