:Mercator_2sp

[Russell Ron]

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: [phone] ext 3798 
www.ukho.gov.uk
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[Frank Warmerdam]

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, [hidden email]
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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[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: [phone] 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:[hidden email]]
> Sent: 17 March 2009 14:10
> To: Russell Ron
> Cc: [hidden email]
> 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,
> [hidden email]
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for
Rent

**********************************************************************

 

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.

 

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[Laurence R Moore]

Complete equations are also in Snyder's USGS Professional Paper 1395,
available in several formats at http://pubs.er.usgs.gov/usgspubs/pp/pp1395
.  The Mercator is on pages 38-47 (hard copy page numbers; PDF pages are
47-56).  See the last section on page 47 (PDF 56) for the 2SP case.

-----------------------------
Larry Moore
USGS, Rolla MO
15S XC 1032 0144
[hidden email]


[hidden email] wrote on 03/17/2009 09:41:34 AM:

> to: geotiff
> 03/17/2009 09:51 AM
> From: "Russell Ron" <[hidden email]>
> To:<[hidden email]>
>
> 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: [phone] 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:[hidden email]]
> > Sent: 17 March 2009 14:10
> > To: Russell Ron
> > Cc: [hidden email]
> > 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,
> > [hidden email]
> > light and sound - activate the windows | http://pobox.com/~warmerdam
> > and watch the world go round - Rush    | Geospatial Programmer for
> Rent
>
> **********************************************************************
>
>
>
> 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]
> http://lists.maptools.org/mailman/listinfo/geotiff

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