coordinate system - Understanding GeoTIFF tags?

I'm trying to understand GeoTIFF info, but not getting far. For instance, using listgeo with one file I can get this output:

Geotiff_Information:
  Version: 1
  Key_Revision: 1.0
  Tagged_Information:
    ModelTiepointTag (2,3):
       0                0                0                
       617632.387       5169109.1        0                

    ModelPixelScaleTag (1,3):
       0.597164283      0.597164283      1                
  End_Of_Tags.
  Keyed_Information:
    GTModelTypeGeoKey (Short,1): ModelTypeProjected
    GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
    GeographicTypeGeoKey (Short,1): Unknown-4167
    GeogAngularUnitsGeoKey (Short,1): Angular_Degree
    GeogSemiMajorAxisGeoKey (Double,1): 6378137          
    GeogSemiMinorAxisGeoKey (Double,1): 6356752.31       

    GeogInvFlatteningGeoKey (Double,1): 298.257222       
    ProjectedCSTypeGeoKey (Short,1): Unknown-2134
    ProjLinearUnitsGeoKey (Short,1): Linear_Meter
  End_Of_Keys.
End_Of_Geotiff.

PCS = 2134 (name unknown)
GCS: 4167/(unknown)
Projection Linear Units: 9001/metre (1.000000m)


Corner Coordinates:
Upper Left    (  617632.387, 5169109.098)
Lower Left    (  617632.387, 5164958.209)
Upper Right   (  621797.608, 5169109.098)
Lower Right   (  621797.608, 5164958.209)
Center        (  619714.997, 5167033.654)

Now, I know that those corner coords are UTM, but does anything in the other tags tell me that?

Similarly, with a different projection, the same area gives me:

Geotiff_Information:

Version: 1
Key_Revision: 1.0
Tagged_Information:
   ModelTiepointTag (2,3):
      0                0                0                
      19198032.5       -5377596.2       0                
   ModelPixelScaleTag (1,3):
      0.597164283      0.597164283      1                
   End_Of_Tags.
  Keyed_Information:

    GTModelTypeGeoKey (Short,1): ModelTypeProjected
    GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
    GeographicTypeGeoKey (Short,1): GCS_WGS_84
    GeogAngularUnitsGeoKey (Short,1): Angular_Degree
    GeogSemiMajorAxisGeoKey (Double,1): 6378137          
    GeogSemiMinorAxisGeoKey (Double,1): 6356752.31       
    GeogInvFlatteningGeoKey (Double,1): 298.257224           
    ProjectedCSTypeGeoKey (Short,1): Unknown-3395    
    ProjCoordTransGeoKey (Short,1): CT_Mercator
    ProjLinearUnitsGeoKey (Short,1): Linear_Meter

    ProjStdParallel1GeoKey (Double,1): 0                
    ProjStdParallel2GeoKey (Double,1): 0                
    ProjNatOriginLongGeoKey (Double,1): 0                
    ProjNatOriginLatGeoKey (Double,1): 0                
    ProjFalseEastingGeoKey (Double,1): 0                
    ProjFalseNorthingGeoKey (Double,1): 0                
    ProjFalseOriginLongGeoKey (Double,1): 0                
    ProjFalseOriginLatGeoKey (Double,1): 0                
    ProjFalseOriginEastingGeoKey (Double,1): 0                
    ProjFalseOriginNorthingGeoKey (Double,1): 0                

    ProjCenterLongGeoKey (Double,1): 0                
    ProjCenterLatGeoKey (Double,1): 0                
    ProjCenterEastingGeoKey (Double,1): 0                
    ProjCenterNorthingGeoKey (Double,1): 0                
    ProjScaleAtNatOriginGeoKey (Double,1): 1                
    ProjAzimuthAngleGeoKey (Double,1): 0                
    ProjStraightVertPoleLongGeoKey (Double,1): 0                
  End_Of_Keys.
End_Of_Geotiff.


PCS = 3395 (name unknown)
Projection Method: CT_Mercator
   ProjNatOriginLatGeoKey: 0.000000 (  0d 0' 0.00"N)
   ProjNatOriginLongGeoKey: 0.000000 (  0d 0' 0.00"E)
   ProjScaleAtNatOriginGeoKey: 1.000000
   ProjFalseEastingGeoKey: 0.000000 m
   ProjFalseNorthingGeoKey: 0.000000 m
GCS: 4326/WGS 84
Datum: 6326/World Geodetic System 1984
Ellipsoid: 7030/WGS 84 (6378137.00,6356752.31)

Prime Meridian: 8901/Greenwich (0.000000/  0d 0' 0.00"E)
Projection Linear Units: 9001/metre (1.000000m)

Corner Coordinates:
Upper Left    (19198032.501,-5377596.196)
Lower Left    (19198032.501,-5383223.872)
Upper Right   (19203679.883,-5377596.196)
Lower Right   (19203679.883,-5383223.872)
Center        (19200856.192,-5380410.034)

The tags 'tell me' this is WGS84 - but what are the coords? I'm trying to reliably extract positional data, but can't because I don't understand the what the output is telling me?

Answer

Here's an extremely dense but useful explanation of the GeoTIFF specification: http://duff.ess.washington.edu/data/raster/drg/docs/geotiff.txt

The PCS tag is the EPSG number of the projection coordinate system. GCS is the EPSG number of the geographic coordinate system. So your first file is in UTM 59S, with linear unit of meters. The second file is World Mercator, also with linear unit of meters. The cell size of both is ~0.597m. GeogAngularUnitsGeoKey is the unit of the GCS, and ProjLinearUnitsGeoKey is the unit of the PCS.

Hopefully that will get you started.