[Proj] GeoTiff <-> Proj.4
support.mn at elisanet.fi
support.mn at elisanet.fi
Thu Oct 2 01:29:15 EDT 2008
"Gerald I. Evenden" <geraldi.evenden at gmail.com> kirjoitti:
> A question: is the "Proj" part of the above list referring to the proj we all
> know and love?
These names are from the GeoTiff definition. Version 1.8.2 year 2000 is
probably the most recent. It defines the geoinformation tags for TIF files.
> > Angulars:
> > ProjStdParallelGeoKey 3078 in GeogAngularUnit -> lat_1
> > ProjStdParallel2GeoKey 3079 in GeogAngularUnit -> lat_2
> > ProjOriginLongGeoKey 3080 in GeogAngularUnit -> lon_0
> > ProjOriginLatGeoKey 3081 in GeogAngularUnit -> lat_0
> > ProjFalseOriginLongGeoKey 3084 in GeogAngularUnit -> ?
> > ProjFalseOriginLatGeoKey 3085 in GeogAngularUnit -> ?
> > ProjCenterLongGeoKey 3088 in GeogAngularUnit -> ?
> > ProjCenterLatGeoKey 3089 in GeogAngularUnit -> ?
Do you have any idea what is the difference betwen "Origin"
"FalseOrigin" or "Center", or are they all the same?
> > Linears:
> > ProjFalseEastingGeoKey 3082 in ProjLinearUnits -> x_0
> > ProjFalseNorthingGeoKey 3083 in ProjLinearUnits -> y_0
> > ProjFalseOriginEastingGeoKey 3086 in ProjLinearUnits -> ? (if used, maybe also x_0)
> > ProjFalseOriginNorthingGeoKey 3087 in ProjLinearUnits -> ? (if used, maybe also y_0)
> > ProjCenterEastingGeoKey 3090 in ProjLinearUnits -> ? (if used, maybe also x_0)
> > ProjCenterNorthingGeoKey 3091 in ProjLinearUnits -> ? (if used, maybe also y_0)
Here are almost the same names: "False" "FalseOrigin" "Center". They must be
all the same??
> > Ratios:
> > ProjScaleAtOriginGeoKey 3092 in ratio -> k_0
> > ProjScaleAtCenterGeoKey 3093 in ratio -> ? (maybe k_0)
> > Others:
> > ProjAzimuthAngleGeoKey 3094 in GeogAzimuthUnit -> alpha
> > ProjStraightVertPoleLongGeoKey 3095 in GeogAngularUnit -> ?
> > ProjRectifiedGridAngleGeoKey 3096 in GeogAngularUnit -> ?
> Yes, I agree: "?"
Why they couldn't just say "put all angulars in degrees and linears
in meters" and convert if needed, what ever you might have internally.
The life would be much easier...
> As an aside, the problems of linear units in proj, proj4, libproj4 is probably
> forgotten so I will make a reminder: units are established by the unit that
> is used to define the Earth's radius, R=, or semi-major axis, a=. If R=1
> then the units are an Earth with a radius=1 so an easting of 100,000 would be
> a ridiculous value. If a is in US surveyor's feet then easting and northing
> must be in US surveyor's feet. If one uses the internal ellipsoid figure
> table, ellps=, then because all a= entries in the table are in meters the
> user must refer to the other linear unit items in meters.
> BTW: using lproj without specifying a radius or semi-major axis will cause a
> default R=1.
> The bottom line is that the user is responsible for both defining the linear
> units as well as ensuring parameter unit consistency.
Yes, the only problem is that they want to import files from other systems
without any "problems" and I am standing in a swamp here...
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