Felipe,
when using PROJ to transform a PROJ.4 string to WKT2, there's (as far as
I can think of) no loss of information. Loss of information can occur in
the other way round.
The main source of loss is that proj4 strings have a limited set of
known datums, so sometimes you'll just have a +ellps without any
+towgs84 or +nadgrids, which is underspecified.
When you've +ellps with +towgs84, datum transformations will be limited
to using WGS84 as a pivot, whereas if you had a WKT2 string with the
original datum name, PROJ could potentially avoid going through WGS84
when doing datum transformation.
Here's with +datum=WGS84, your PROJ.4 string is fully qualified. You can
remove the +ellps=WGS84 and +towgs84=0,0,0 which are completely
redundant here.
Even
Le 10/01/2022 à 21:06, Felipe Matas via gdal-dev a écrit :
Hi hi, I was looking for someplace to ask about this, and I was in
doubt is was here or in PROJ, but lets do a try.
Actually, probably I'm not the only one, I have some questions about
how WKT2 and PROJ4Strings are transformed.
From what I read, a PROJ4String don't have enough info to construct a
precise WKT2, so, actually how PROJ4String is deprecated my main
question is how to construct the right WKT2, there is a lot o data
stored in the old format, there is usually no more information, and
even the software/hardware that get the data in some way save all of
it with all the errors.
Actually, we can transform a old CRS to a WKT2:
|> st_crs("+type=crs +proj=utm +zone=19 +south +datum=WGS84 +units=m
+no_defs +ellps=WGS84 +towgs84=0,0,0" + ) Coordinate Reference System:
User input: +type=crs +proj=utm +zone=19 +south +datum=WGS84 +units=m
+no_defs +ellps=WGS84 +towgs84=0,0,0 wkt: BOUNDCRS[ SOURCECRS[
PROJCRS["unknown", BASEGEOGCRS["unknown", DATUM["World Geodetic System
1984", ELLIPSOID["WGS 84",6378137,298.257223563,
LENGTHUNIT["metre",1]], ID["EPSG",6326]], PRIMEM["Greenwich",0,
ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8901]]],
CONVERSION["UTM zone 19S", METHOD["Transverse Mercator",
ID["EPSG",9807]], PARAMETER["Latitude of natural origin",0,
ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8801]],
PARAMETER["Longitude of natural origin",-69,
ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8802]],
PARAMETER["Scale factor at natural origin",0.9996,
SCALEUNIT["unity",1], ID["EPSG",8805]], PARAMETER["False
easting",500000, LENGTHUNIT["metre",1], ID["EPSG",8806]],
PARAMETER["False northing",10000000, LENGTHUNIT["metre",1],
ID["EPSG",8807]], ID["EPSG",17019]], CS[Cartesian,2], AXIS["(E)",east,
ORDER[1], LENGTHUNIT["metre",1, ID["EPSG",9001]]], AXIS["(N)",north,
ORDER[2], LENGTHUNIT["metre",1, ID["EPSG",9001]]]]], TARGETCRS[
GEOGCRS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS
84",6378137,298.257223563, LENGTHUNIT["metre",1]]],
PRIMEM["Greenwich",0, ANGLEUNIT["degree",0.0174532925199433]],
CS[ellipsoidal,2], AXIS["latitude",north, ORDER[1],
ANGLEUNIT["degree",0.0174532925199433]], AXIS["longitude",east,
ORDER[2], ANGLEUNIT["degree",0.0174532925199433]], ID["EPSG",4326]]],
ABRIDGEDTRANSFORMATION["Transformation from unknown to WGS84",
METHOD["Geocentric translations (geog2D domain)", ID["EPSG",9603]],
PARAMETER["X-axis translation",0, ID["EPSG",8605]], PARAMETER["Y-axis
translation",0, ID["EPSG",8606]], PARAMETER["Z-axis translation",0,
ID["EPSG",8607]]]]|
So, in some way, GDAL is able to cover this "breach" of information
betwen PROJ4String and WKT2, but is not like the breach does not
exist, it was just handled in some way, so, the lack of info, also
means, there can be othe aproximations to that CRS.
Maybe I'm just confused, and mixing things the things I read, I'm just
trying to figure it out, what and how would be the right
transformation from proj4string to wkt2.
Thx.
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