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Karl, Sue,<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
This leads to 3 typical scenarios:<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
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).<BR>
<BR>
Hope this is of use.<BR>
<BR>
Regards,<BR>
<BR>
Phil<BR>
<BR>
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<FONT COLOR="#000000"> </FONT>
<FONT COLOR="#000000">We received an email suggesting that a parameter was missing from the CF </FONT>
<FONT COLOR="#000000">polar stereographic projection documentation. Can anyone clear this up?</FONT>
<FONT COLOR="#000000">thanks,</FONT>
<FONT COLOR="#000000">Karl</FONT>
<FONT COLOR="#000000">Sue Dettling wrote:</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > I am wondering if you are missing an arguement from the Polar </FONT>
<FONT COLOR="#000000">stereographic projection documentaion</FONT>
<FONT COLOR="#000000"> > </FONT>
<FONT COLOR="#000000">(<A HREF="http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.0/apf.html#id2976188">http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.0/apf.html#id2976188</A>)</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > You list the following parameters for the projection:</FONT>
<FONT COLOR="#000000"> > 1) straight_vertical_longitude_from_pole</FONT>
<FONT COLOR="#000000"> > 2) latitude_of_projection_origin - Either +90. or -90.</FONT>
<FONT COLOR="#000000"> > 3) Either standard_parallel or scale_factor_at_projection_origin</FONT>
<FONT COLOR="#000000"> > 4) false_easting</FONT>
<FONT COLOR="#000000"> > 5) false northing</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > The proj4 reference </FONT>
<FONT COLOR="#000000"><A HREF="http://www.remotesensing.org/geotiff/proj_list/polar_stereographic.html">http://www.remotesensing.org/geotiff/proj_list/polar_stereographic.html</A> </FONT>
<FONT COLOR="#000000">lists the following:</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > +lat_ts=Latitude at natural origin</FONT>
<FONT COLOR="#000000"> > +lat_0=90</FONT>
<FONT COLOR="#000000"> > +lon_0=Longitude at natural origin</FONT>
<FONT COLOR="#000000"> > +k_0=Scale factor at natural origin (normally 1.0)</FONT>
<FONT COLOR="#000000"> > +x_0=False Easting</FONT>
<FONT COLOR="#000000"> > +y_0=False Northing</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > In reading both pages it seems the corresponence of args is:</FONT>
<FONT COLOR="#000000"> > +lat_ts == ???</FONT>
<FONT COLOR="#000000"> > +lat_0 == (2) latitude_of_projection_origin</FONT>
<FONT COLOR="#000000"> > +lon_0 == (1) straight_vertical_longitude_from_pole</FONT>
<FONT COLOR="#000000"> > +k_0 == (3) scale_factor_at_projection_origin</FONT>
<FONT COLOR="#000000"> > +x_0=False Easting == (4) false_easting</FONT>
<FONT COLOR="#000000"> > +y_0=False Northing == (5) false northing</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > Is this an error in the documentation that there doesnt seem to be a </FONT>
<FONT COLOR="#000000"> map parameter for lat_ts?</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > Thanks, your response is greatly appreciated,</FONT>
<FONT COLOR="#000000"> ></FONT>
<FONT COLOR="#000000"> > Sue</FONT>
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</PRE>
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