:Mercator projection in GeoTiff

classic Classic list List threaded Threaded
1 message Options
Reply | Threaded
Open this post in threaded view

: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?






Ron Russell

ICTS (Application Software Group 1)

UK Hydrographic Office

Telephone: +44(0)1823 337900 ext 3798

mailto: [hidden email]


The contents of this e-mail and any attachments are the property of the United Kingdom Hydrographic Office and are intended for the confidential use of the named recipient only. Its unauthorised use, disclosure, storage or copying is not permitted and may be unlawful. If you are not the intended recipient, please destroy all copies and inform the sender by return e-mail. This footnote also confirms that this e-mail message has been swept by Sophos Antivirus for the presence of computer viruses. You are however advised to carry out your own virus checks.


Geotiff mailing list
[hidden email]