# Random Outstanding Issues

Various outstanding issues with these projections, EPSG, GeoTIFF, PROJ.4 and OpenGIS in no particular order. This is also not a comprehensive list of outstanding issues.

## Stereographic

1. Are Stereographic, Polar Stereographic, and Oblique Stereographic really all the same projection?
2. Is the latitute of natural origin for Polar Stereographic required to be 90 or -90? Shouldn't this be mentioned?
3. Shouldn't all the sterographic projections include a scale at the origin?
4. Why does the EPSG tables have a scale for Oblique Stereographic, but the (old) GeoTIFF definition doesn't?
5. Why do the GeoTIFF parameters have such variable names for the different stereographic projections? PS uses StraightVertPoleLong and NatOriginLat, OS uses NatOrigin, and Stereographic uses ProjCenterLat/Long.
6. In PJ_stere.c for PROJ.4 in function INVERSE(e_inverse) the calculation of phi_l should likely test for a rho of zero (at the origin when xy.x and xy.y are zero. Verify doing an inverse translation of position (0,0) with pci_eg/sg.tif.

         if( rho == 0.0 )
           phi_l = asin(cosphi * P->sinX1);
         else
           phi_l = asin(cosphi * P->sinX1 + (xy.y * sinphi * P->cosX1 / rho));

7. The PCI ps.tif file has a ProjNatOriginLatGeoKey. In PCI's PROJ.HLP this number is called the latitude of true scale, but it carried through to PROJ.4 as the +lat_0 value, not as +lat_ts (which it should perhaps be). The Intergraph stereo_np.tif file has two lattitudes. One is the ProjNatOriginLatGeoKey and the other is ProjCenterLatGeoKey. I imagine that the ProjScaleAtNatOriginGeoKey and ProjNatOriginLatGeoKey are really supposed to be equivelent to giving a latitude of true scale that isn't the north pole (necessarily) if the scale is 1.0.

   The current formulation includes a scale at the origin, but suggests that the origin must be either the north or south pole. Perhaps this is mixing up use of ProjCenterLatGeoKey to mark the pole with the use of an origin to establish the position of true scale (or alternatively to just give the scale at the origin). Is the solution to provide two formulations of polar stereographic, or to expect translators to boil their internal forumulations down to a pole indicator, and scale at the pole?

## ObliqueMercator

The EPSG definition of Oblique Mercator includes an angle from rectified to skewed grid. This isn't included for GeoTIFF, and doesn't appear to be supported by any system I have seen. Is this equivelent to what Peter Laskowski calls pre or post transformation rotation (that could be applied to most projections)?

## Polyconic

Should Polyconic use NatOriginLat/Long, or ProjCenterLat/Long? Different vendors do different things. I have indicated NatOrigin in the Polyconic definition, but this is at odds with previous thoughts on geotiff-l, and the implementations of some companies.

## Azimuthal Equidistant

The PCI and PROJ.4 results for reprojection don't match very closely with the PCI ae.tif sample:

    PCI:
    Upper Left  Corner             :    117d38'28.22" W  Lon   33d54'13.08" N  Lat

    gdalinfo:
    Origin (long/lat) = (-117.641,33.904) (117d38'26.91"W,  33d54'14.34"N)

This amounts to approximately sixty meters of error in about 20000 meters. I don't know if this is a fundamental difference between GCTP and PROJ.4 or what. This should be reviewed at some point.

## Miller Cylindrical

The PCI (GCTP) and PROJ.4 results for this projection are also not matching very closely. It appears to be a problem similar to the above.

    PCI:
     Upper Left  Corner             :    117d38'28.21" W  Lon   33d54'13.08" N  Lat

    gdalinfo:
     Origin (long/lat) = (-117.641,33.8682) (117d38'27.53"W,  33d52'5.42"N)

## Robinson

The PCI (GCTP) and PROJ.4 results for this projection are also not matching very closely. It appears to be a problem similar to the above. The problem might relate to the distance from the origin but this is commonly used as a world projection, so I am surprised at the huge difference (6 degrees in ns direction!)

    PCI:
     Upper Left  Corner             :    117d38'28.21" W  Lon   33d54'13.08" N  Lat

    gdalinfo:
      Origin (long/lat) = (-117.672,39.2717) (117d40'20.81"W,  39d16'18.19"N)

I gather from the GCTP and PROJ.4 documentation that Robinson is only available as a spherical projection, and yet above was used with Clark 1866, which is not spherical. I would suggest that the problem might relate to different handling of non-spherical worlds, but this shouldn't account for the large difference. In general shouldn't we be keeping track of spherical projections and warning of use of non-spherical ellipsoids?

## Equirectangular

The PCI (GCTP) and PROJ.4 results for this projection are also not matching very closely. It appears to be a problem similar to the above.

    PCI:
     Upper Left  Corner             :    117d38'28.21" W  Lon   33d54'13.08" N  Lat

    gdalinfo:
     Origin (long/lat) = (-117.641,33.8653) (117d38'27.53"W,  33d51'55.12"N)

## Gnomonic

The PCI (GCTP) and PROJ.4 results for this projection are also not matching very closely. It appears to be a problem similar to the above.

    PCI:
     Upper Left  Corner             :    117d38'28.22" W  Lon   33d54'13.08" N  Lat

    gdalinfo:
     Origin (long/lat) = (-117.641,33.9035) (117d38'27.54"W,  33d54'12.51"N)

## Lambert Azimuthal Equal Area

The PCI (GCTP) and PROJ.4 results for this projection are also not matching very closely. It appears to be a problem similar to the above.

    PCI:
     Upper Left  Corner             :    117d38'28.22" W  Lon   33d54'13.08" N  Lat

    gdalinfo:
     Origin (long/lat) = (-117.641,33.904) (117d38'26.91"W,  33d54'14.34"N)

A more detailed examination suggests that GCTP always uses a radius of 6370997 (E019 - Normal Sphere) for spherical projections. For at least LAEA the PROJ.4 and GCTP results were the same if the PROJ.4 radius was overridden to be 6370997 - I would need to verify that this is true of other problems above as well.

## Orthographic

The PCI (GCTP) and PROJ.4 results for this projection are also not matching very closely. It appears to be a problem similar to the above.

    PCI:
     Upper Left  Corner             :    117d38'28.22" W  Lon   33d54'13.08" N  Lat

    gdalinfo:
    Origin (long/lat) = (-117.641,33.9035) (117d38'27.54"W,  33d54'12.51"N)