[Geotiff] :Mercator_2sp

Russell Ron Ron.Russell at UKHO.gov.uk
Tue Mar 17 04:06:40 EST 2009

Good Morning,
Can anyone give any guidance as to the keys that should be used when specifying the Mercator_2sp projection? What we used to do, and was apparently accepted by ArcGis 9.2 no longer works with ArcGis 9.3. It seems impossible to satisfy all software!
My current 'best attempt' is (for example)
      ProjCoordTransGeoKey (Short,1): CT_Mercator
      ProjLinearUnitsGeoKey (Short,1): Linear_Meter
      ProjNatOriginLongGeoKey (Double,1): xxxx                
      ProjNatOriginLatGeoKey (Double,1): yyyy                
      ProjFalseEastingGeoKey (Double,1): 0                
      ProjFalseNorthingGeoKey (Double,1): 0                
      ProjScaleAtNatOriginGeoKey (Double,1): 1                
      ProjStdParallel1GeoKey (Double,1): yyyy
Where yyyy is what I would call 'Latitude of true scale'. 
This does not seem a true definition of Mercator where I think the Latitude origin is always the equator!
However these keys seem to do what we want with most software that we use (but not GDAL!)
Ron Russell
IT Specialist - Digital Mapping & Charting
United Kingdom Hydrographic Office

Tel: +44 (0) 1823 337900 ext 3798 


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