[OSRS-PROJ] Older Stateplane Definitions

[Craig Bruce]

Frank Warmerdam <warmerdam at pobox.com> wrote:

> The ESRI WKT format has the false easting and northing in the same units
> as the projected coordinate system, which is why they are integral
> (numbers of feet).  Note that the OGC Coordinate Transformation
> specification states that these values are always in meters (unless
> otherwise indicated by a units field), so it is not very easy to have
> different uses of WKT coordinate system interoperate sometimes.

The Coordinate-Transformation spec (01-009) spec is unclear.  In Section
7.3.11 discussing the PARAMETER definition, the Coordinate-Transformation
spec (01-009) says, "The units of the parameter must be inferred from
its context.  If the parameter is inside a PROJCS, then its units will
match the units of the PROJCS."  So, the false easting and northing
are in the linear units given in the PROJCS definition.  For the older
state planes, this means feet.  Though, in this section on parameters,
the spec doesn't actually say anything about angular parameters, so it
could be that they should be the units from the GEOGCS definition.

But then, of course, the spec goes on to contradict itself in Sections
10.6.1.1 through 10.6.1.3, where it implies that meters and degrees must
always be used as the parameter values.  Which is right?  Who knows.
It also has no suggestions for canonical names of datums or most
projections or, formally, parameter names.  It's amazing that they can go
on for 117 pages and not address these most basic interoperability issues.

One unfortunate thing that the Coord-Trans spec says is in Section 7.3.14
about the PRIMEM definition, "If the PRIMEM clause occurs inside a GEOGCS,
then the longitude units will match those of the geographic coordinate
system."  This means that for the NTF-Paris PRIMEM defintion, it must be
given in Gradians the amount that the Paris PRIMEM is off from Greenwich.
I think that the previous common usage always gave this in degrees.

You indicated in a different message that almost every WKT 'tag' has a
UNIT parameter, but this isn't the case either.  The only relevant UNIT
parameters are the same two that were present before: inside of the GEOGCS
and PROJCS definitions.  However, the XML definitions do have more Units
definitions in them, on both the prime-meridian and ellipsoid definitions
at least, though it's questionable whether they're actually needed.

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Dr. Craig S. Bruce        | Tel: 819-771-8303 x205 |             CubeWerx Inc.
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