[PROJ] Syntax to change projection from UTM to TM

[Ulises Ibarra]

Thanks Greg and Javier
I'm putting below what I wanted to do according to the syntax I used.
But I want to clarify my purpose again: Change UTM projection coordinates
to a terrain projection, to use with topography, that is, with a scale
factor of 1.

El sáb, 7 dic 2024 a las 2:04, Javier Jimenez Shaw (<j1 at jimenezshaw.com>)
escribió:

>
>
> On Sat, 7 Dec 2024, 01:10 Ulises Ibarra via PROJ, <proj at lists.osgeo.org>
> wrote:
>
>>
>> El vie, 6 dic 2024 a las 17:59, Greg Troxel via PROJ (<
>> proj at lists.osgeo.org>) escribió:
>>
>>> Ulises Ibarra via PROJ <proj at lists.osgeo.org> writes:
>>>
>>> > echo 480037.249 2143783.696 2271.294 | cct +proj=pipeline +step +inv
>>> > +proj=utm +zone=14 +ellps=GRS80 +k=0.9996439573 +step +proj=tmerc
>>> > +lat_0=19.388187333 +lon_0=-99.190762728 +x_0=479969.231
>>> +y_0=2143791.499
>>> > +ellps=GRS80 +k=1
>>>
>>> I am far from an expert here, but "UTM zone 14" defines everything about
>>> the projection, so I don't understand why you are needing to specify the
>>> k.  Also sort of the ellipse, but people use UTM with both NAD83 and
>>> WGS84, and you are using it with ITRF2008.
>>>
>>
>> * I need to specify in detail because I think I have no idea what I'm
>> doing :P
>>
> If you explain all the inputs in detail, including those k and why you
> expect that output, I can try to guess it.
>

Now, I think I'm not understanding well how PROJ syntax works, but I'll
explain what I want to do step by step:

A) Enter a coordinate (East, North, Elevation) and send it to the cct
command to reproject it:
echo 480037.249 2143783.696 2271.294 | cct

B) Define a "pipeline" projection
+proj=pipeline

C) I really don't know why to use "+inv" but if I don't use it, it just
produces an error.
+step +inv

D) Tell PROJ the characteristics of the source cartographic projection:
+proj=utm (projection=utm)
+zone=14 (zone 14 _North (I see that I did not specify that it should be in
the Northern hemisphere))
+ellipse=GRS80 (I assumed that if the GNSS processing software gave me the
results of the ground control points in the ITRF2008 Datum, I should use
the GRS80 ellipse)
+k=0.9996439573 (I also tell PROJ the scale factor that the GNSS processing
software told me that these points have)

E) I try to tell PROJ the characteristics of the target cartographic
projection:
+step
+proj=tmerc (the target cartographic projection)
+lat_0=19.388187333 +lon_0=-99.190762728 (the latitude and longitude of the
point through which I want the meridian to pass where the transverse
cylinder would make tangency)
+x_0=479969.231 +y_0=2143791.499 (the coordinates of the point that would
be the false origin of the local coordinate system)
+ellps=GRS80 (I tell PROJ to use the ellipsoid that the ITRF2008 Datum uses)
+k=1 (and finally, that the scale factor is equal to 1, because I cannot
tell it, or perhaps I do not know how to tell it, that the radius of the
transverse cylinder is tangent to the point I have been talking about, the
point that coincides with the longitude lon_0)

Now that I am reading a little more, I am wondering if the syntax of PROJ,
or of the pipelines, is to tell it the source and the target, and that
instead of that it would be necessary to tell it exactly what operations to
do.

The PROJ pipeline I think is correct, except for the values themselves (k,
> false easting and northing)
>
>> .
>> * So I understand that the k is not necessary to use in the command. Is
>> this correct?
>> * I don't understand what you said about there being people who use both
>> NAD83 and WGS84 and I am using  ITRF2008, could you explain it better?
>> Maybe I shouldn't specify the ellipsoid?
>>
>>>
>>> https://www.oc.nps.edu/oc2902w/maps/utmups.pdf
>>>
>>> may be useful
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