Karl, Sue,
Although I'm not too familiar with the proj4 library, it looks to me
like the 'lat_ts' parameter represents the latitude of true scale.
Typically, for north or south polar stereographic projections, this
would be 90N or 90S. At the latitude of true scale the scale factor, k,
is equal to 1.
Depending upon your application domain, however, it may be desirable to
use some other small circle (parallel) as the latitude of true scale. In
which case the scale factor, k0, at the projection (=natural) origin
will be some other value. In the case of polar stereographic projections
it will be less than 1.
This leads to 3 typical scenarios:
1. The projection origin and latitude of true scale are located at the
pole (N or S) and the scale factor at this origin is equal to 1, i.e.
the nominal map scale is correct only at the pole.
2. The projection origin is at the pole and some scale factor, k0 < 1,
is assigned to this point. The latitude of true scale can then be
derived from standard equations. The nominal map scale is correct at all
points along this parallel of latitude.
3. The projection origin is at the pole and a latitude of true scale is
specified. The scale factor at the projection origin (or indeed anywhere
else) can then be derived from standard equations. Again, the nominal
map scale is correct at all points along the specified latitude of true
scale.
In CF the latitude of true scale is specified using the
"standard_parallel" attribute. I believe, therefore, that it is this
attribute that corresponds to your 'lat_ts' parameter.
If both the "scale_factor_at_projection_origin" and "standard_parallel"
attributes are specified then they should be mutually consistent
(although the CF specification doesn't explicitly say as such). I'm not
certain if there is a precedent for favouring one attribute over the
other. In some map projections it is usual to specify the standard
parallel/s (e.g. lambert conic conformal), while in others it is the
scale factor that is given (e.g. transverse mercator).
Hope this is of use.
Regards,
Phil
> Taylor wrote:Dear all,
>
> We received an email suggesting that a parameter was missing from the CF
> polar stereographic projection documentation. Can anyone clear this up?
>
> thanks,
> Karl
>
> Sue Dettling wrote:
> >
> > I am wondering if you are missing an arguement from the Polar
> stereographic projection documentaion
> >
> (http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.0/apf.html#id2976188)
> >
> > You list the following parameters for the projection:
> > 1) straight_vertical_longitude_from_pole
> > 2) latitude_of_projection_origin - Either +90. or -90.
> > 3) Either standard_parallel or scale_factor_at_projection_origin
> > 4) false_easting
> > 5) false northing
> >
> >
> > The proj4 reference
> http://www.remotesensing.org/geotiff/proj_list/polar_stereographic.html
> lists the following:
> >
> > +lat_ts=Latitude at natural origin
> > +lat_0=90
> > +lon_0=Longitude at natural origin
> > +k_0=Scale factor at natural origin (normally 1.0)
> > +x_0=False Easting
> > +y_0=False Northing
> >
> >
> > In reading both pages it seems the corresponence of args is:
> > +lat_ts == ???
> > +lat_0 == (2) latitude_of_projection_origin
> > +lon_0 == (1) straight_vertical_longitude_from_pole
> > +k_0 == (3) scale_factor_at_projection_origin
> > +x_0=False Easting == (4) false_easting
> > +y_0=False Northing == (5) false northing
> >
> > Is this an error in the documentation that there doesnt seem to be a
> map parameter for lat_ts?
> >
> > Thanks, your response is greatly appreciated,
> >
> > Sue
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