Thanks Even. I also reached the article in the Wikipedia, but I couldn't find the meaning of "m" in "m(phi0) Then I went to PROJ code. In the simplified version it was kind of clear that they were equivalent, but got very confused with the notation, and didn't know what is pj_mlfn (thanks for the tip). I downloaded "map projections a working manual" from USGS, and the formulas are a bit different, that confused me even more (I hope they are equivalent ;) That pushed me to ask here.
Cheers. .___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ... .... ._ .__ Entre dos pensamientos racionales hay infinitos pensamientos irracionales. On Fri, 13 Jan 2023 at 16:13, Even Rouault <[email protected]> wrote: > Hi Javier, > > interesting. Quick numerical experimentation seems to confirm that they > are "equivalent", as well as looking at the drawing of how the projection > is built. > > And looking at the maths of the approximate ellipsoidal forward method in > src/projections/tmerc.cpp, one can see: > > xy.y = P->k0 * (.... - Q->ml0 + ....... ) > > (the false northing value is added in generic PROJ code) > > where > > Q->ml0 = pj_mlfn(P->phi0, sin(P->phi0), cos(P->phi0), Q->en); > > where pj_mlfn() computes the meridional distance (formula in src/mlfn.cpp) > > So - P->k0 * Q->ml0 should be the value of the false northing equivalent > to the effect of phi0. (I believe :-)) > > Also confirmed by the formula N=N0 +y(lam,phi) - k0 * m(phi0) of > https://en.wikipedia.org/wiki/Transverse_Mercator_projection#Coordinates,_grids,_eastings_and_northings > > Even > Le 13/01/2023 à 15:28, Javier Jimenez Shaw a écrit : > > Hi > > 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? > > Thanks. > Javier > .___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ... .... ._ .__ > Entre dos pensamientos racionales > hay infinitos pensamientos irracionales. > > > _______________________________________________ > PROJ mailing > [email protected]https://lists.osgeo.org/mailman/listinfo/proj > > -- http://www.spatialys.com > My software is free, but my time generally not. > >
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