Roger, The "factors" elements refer to a projection, not to a CRS (the CRS includes reference frame information), and not to a transformation (which describes a path between two different CRS).
So you should only call proj_factors on a PJ object obtained from a plain classic proj(ection) string, i.e: p = proj_create(0, "proj=utm zone=33 ellps=GRS80"); Den tors. 24. okt. 2024 kl. 14.11 skrev Roger Oberholtzer via PROJ < proj@lists.osgeo.org>: > Hi all, > > I have been using proj_factors in a couple CRS to get the > median_convergence value. I need this when rotating a point that has > been projected into the CRS. > > In my usual setup, I define something like the following (WGS84 (from > a GPS receiver) to Sweref99): > > proj_normalize_for_visualization(0, > proj_create_crs_to_crs( 0, > "EPSG:4326", > "EPSG:3006", > 0)))) > return(ISERROR); > > This works fine. I get the points I expect. But as I will need to > rotate that point using data from the GPS (heading is true north), I > need to know how that differs from the projected point's north. So I > use proj_factors. As it turns out, I cannot use the definition from > above to do this. The median_convergence value is wrong. So what I > have been doing (for EPSG:3006 - it is CRS-specific) is defining > something like this: > > proj_create(0, "+proj=utm +zone=33 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 > +units=m +no_defs"); > > You will note that this is the implementation of EPSG:3006, but > without +type=crs. Why without that parameter? Time. When including > it, the proj_factors call takes around 60 times longer. This is > extremely significant when generating, say, point clouds with hundreds > of million points. A processing time of 15 minutes becomes 15 hours. > The values provided are exactly the same. But one takes significantly > longer to provide them. > > I see this effect in more than EPSG:3006. For example EPSG:5677. I > suspect that it is generally the case. > > What I would like to do is something like this: > > proj_normalize_for_visualization(0, > proj_create_crs_to_crs( 0, > "EPSG:4326", > "EPSG:3006", > 0)))) > return(ISERROR); > > proj_create(0, "EPSG:3006"); > > > Where the first is used to project the point, and the second is used > to call proj_factors for that point. > > This would allow a general solution, which is what I am after. So I > either need to call something other than proj_create, or I need to see > how to remove +type=crs in this context. > > Does this make sense? > > -- > Roger Oberholtzer > _______________________________________________ > PROJ mailing list > PROJ@lists.osgeo.org > https://lists.osgeo.org/mailman/listinfo/proj >
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