[Geotiff] :RE: :Mercator_2sp

[Russell Ron]

Frank,

Calculating the scale factor at the equator given 'latitude of true
scale' is no problem - see

http://www.remotesensing.org/geotiff/proj_list/mercator_2sp.html

however several pieces of viewing software refuse to accept a floating
point value for the Scale at Natural origin (in particular ArcGis). I
can't remember whether GDAL did or not (I must admit it is at the end of
my list!). ListGeo and Proj4 are happy with the keys I have suggested.

Before dealing with Geotiffs, I don't think I had ever come across any
real Mercator projection that did not use the Mercator_2sp model!

It is all a bit frustrating.

I will see if I can cope with raising a ticket.

Thanks,

(BTW, thanks for your effort in supporting libgeotiff and also proj4)

Ron Russell

IT Specialist - Digital Mapping & Charting

United Kingdom Hydrographic Office

Tel: +44 (0) 1823 337900 ext 3798 

www.ukho.gov.uk

The United Kingdom Hydrographic Office

P please consider the environment before printing this email 


> -----Original Message-----
> From: Frank Warmerdam [mailto:warmerdam at pobox.com]
> Sent: 17 March 2009 14:10
> To: Russell Ron
> Cc: geotiff at lists.maptools.org
> Subject: Re: [Geotiff] :Mercator_2sp
> 
> Russell Ron wrote:
> > 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,
> 
> There is only one Mercator formulation in GDAL (and in the libgeotiff
> "normalize" code), and it assumes it is provided a scale at the origin
> which is normally on the equator.
> 
> If you have a standard parallel at which the scale is 1, I guess it is
> expected that you will somehow use it to compute the scale at the
origin
> and provide that instead.
> 
> The GDAL/libgeotiff handling of Mercator is pretty much defined by
> 
>    http://www.remotesensing.org/geotiff/proj_list/mercator_1sp.html
> 
> I must confess I don't know a good way of computing the scale at the
> equator if given a latitude of true scale.  I suppose you could
establish
> two points at that latitude that are truly one meter apart (using some
> non-mercator projection with this as a latitude of true scale) and
then
> see how far those points are apart in mercator to establish the
scaling
> needed to return it to a distance of one meter in mercator.
> 
> I would be open to adding proper support for Mercator_2SP if someone
> can provide me with some guidance.  It is annoying that there is only
> one CT_Mercator in libgeotiff, so it is going to be very messy to
> normalize this projection in libgeotiff into a single set of
parameters.
> 
> A ticket would be helpful to track this.
> 
> Best regards,
> --
>
---------------------------------------+--------------------------------
--
> ----
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for
Rent

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