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I was looking for an exhaustive reference on instructions supported by PROJ string, but couldn't find anything yet. Only this page in PROJ library docs - I guess that's all there is.

I have quite a straightforward goal in mind - convert certain lat/lng to specific UTM (lets say ESPG:32618 or +proj=utm +zone=18 +datum=WGS84 +units=m +no_defs) and then offset easting and northing by a certain amount. But I want destination to be an actual custom coordinate system. So how do I derive it from that UTM CRS, given above mentioned operations? Should be something trivial, right? How do I do that?

I tried to set false easting and false northing, like this:

+proj=utm +zone=18 +datum=WGS84 +units=m +x_0=-5000 +y_0=-5000 +no_defs

But doesn't seem to have any effect in PostGIS and continues to ST_Transform to mere UTM 18N.

1 Answer 1

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In the Universal Transverse Mercator system, most of the parameters are implied and fixed. utm does not have a x_0 parameter, so the PROJ library ignores it.

You have to use the more generic Transverse Mercator projection, write out all the implied parameters, and then change those:

+proj=tmerc +datum=WGS84 +lon_0=-75 +k_0=0.9996 +x_0=500000 +y_0=0
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  • Where do I take those implied params from? Can I see somehow what params are used to generate this or that UTM zone PROJ string?
    – jayarjo
    Nov 14, 2020 at 14:37
  • Aw... wait, they are written in that table. Is it really just that easy? Huh.
    – jayarjo
    Nov 14, 2020 at 14:41
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    In general, you can look up the parameters in the projection's WKT.
    – CL.
    Nov 14, 2020 at 14:44

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