[Geotiff] :Mercator projection in GeoTiff

[Russell Ron]

Good Afternoon,

 

How can I define a Mercator_2SP projection in Geotiff?

 

For many years (as far as I can tell) we have produced a header with the

ProjCoordTransGeoKey set to CT_MERCATOR and both the
ProjScaleAtNatOriginGeoKey tag (set to 1.0)

and the ProjStdParallel1GeoKey tag (set to the required Latitude) as
parameters.

 

ListGeo records both these tags in the header, but in its interpretation
section records it as a CT_Mercator projection with a
ProjScaleAtNatOriginGeoKey of 1.000000. (No mention of
ProjStdParallel1GeoKey).

 

We have successfully displayed this data correctly georeferenced (as we
see it) in several viewers (CARIS HPD and Geomatica Freeview for
example), but version 9.1.3 of ArcMap seems to ignore the standard
parallel, and displays the data using Scale at Natural Origin.

 

I can see that the definition of the coordinate system is ambiguous and
I have tried to amend the GEOTIFF header (using geotifcp) to remove the
ProjScaleAtNatOriginGeoKey from the header. ListGeo now records the tags
as I would wish, but still interprets it as a Mercator_1SP projection
with a scale factor of 1.000000.

 

Is this expected behaviour?  I can obviously calculate a corresponding
scale factor for the Latitude of true scale, but this seems an unnatural
way of defining a Mercator projection.

 

Can anybody point me in the right direction?

 

Thanks,

 

 

 

Ron Russell

ICTS (Application Software Group 1)

UK Hydrographic Office

Telephone: +44(0)1823 337900 ext 3798

mailto: ron.russell at ukho.gov.uk

 


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