[PROJ] Latitude of natural origin in Transverse Mercator

[Javier Jimenez Shaw]

Hi Greg

The latitude at origin is defined in the EPSG
https://epsg.org/coord-operation-method_9807/Transverse-Mercator.html
It is used in some systems, like in Argentina (with latitude -90) and as
Charles mentions in other answer, in "OSGB36 / British National Grid" (with
latitude 49)

Cheers,
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
Entre dos pensamientos racionales
hay infinitos pensamientos irracionales.



On Fri, 13 Jan 2023 at 16:00, Greg Troxel <gdt at lexort.com> wrote:

> Javier Jimenez Shaw <j1 at jimenezshaw.com> writes:
>
> > I have a question about Transverse Mercator.
> > What is the impact of the parameter "Latitude of natural origin",
> compared
> > with the "False Northing"? (apart from the fact that one is in degrees
> and
> > the other in meters)
> > Is there any difference on using a Latitude of natural origin of, let's
> > say, 40 degrees, or compute the equivalent False Northing?
>
> Snyder's Album [1] says that TM uses the equator.  I had never heard of
> latitude of natural origin.  Snyder's Manual [2] seems to align with
> equator only but I may have missed it.
>
> It seems like there is some more complicated definition of TM.
> ESRI [3] lists but does not explain latitude of natural origin.
>
> I would guess that latitude of natural origin means that some other
> latitute is used as the place in lat/lon that maps to 0 (before adding
> the false northing).  But, it's an ellipsoidal projection and I can't
> convince myself that this is equivalent to a different false northing.
>
> To me, it seems wrong or at best confusing to interchange these
> parameters even if it is equivalent, as one seems to be about how the
> projection is done and the other is just about moving numbers away from
> 0 so they won't be negative.
>
> I am curious how you were led to latitude of natural origin.
>
>
> [1] https://www.usgs.gov/publications/album-map-projections
> [2] https://pubs.usgs.gov/pp/1395/report.pdf
> [3]
> https://desktop.arcgis.com/en/arcmap/latest/map/projections/transverse-mercator.htm
>
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