[Proj] Transformation pipelines - your opinion?

support at mnspoint.com support at mnspoint.com
Mon Oct 24 01:50:40 EST 2016


I 100 % agree with Nicolas to keep all parts that are not required to be
inside Proj.4 outside it. Later it is much easier to read and handle
such parts as required. You should just add a layer above Proj4 which
could be used if your code required and omitted if more simple
functionality is only required. As Nicolas wrote: 

"BUT I'm more incline to keep only cartographic projection inside proj4
and to implement pipeline and other coordinate transformation outside
proj4 in another library/librairies." 

The same separate library idea should be ok with the syntax scanner idea
for Proj.4 which is not yet implemented since it does not require to be
inside Proj.4 at all. Since syntax scanning for errors is totally an
independent step.. and can as well be omitted if not required. 

You can also do what ever you wish if you rename you end product
something else than Proj.4 .. to keep it consistent (compare "libproj"
for an example of such projects). 

Regards: Janne. 


Thomas Knudsen kirjoitti 2016-10-21 15:55:

> Nicolas, I understand, and respect, your opinion, but i disagree for a number of reasons. Let me start with the simplest, and the one I think will probably also be the most convincing counterargument for your major concern ("...people who are only interrested in cartographic transformation functions of proj4 will have to pay for something they don't want..."): 
> If you take an architectural look at the proj.4 library, it consists of (currently) 146 different projections, each adding in the range of 2-10 kilobytes to the library footprint. 
> The pipeline functionality is typical in that respect: it is organized as a (small) number of additional projections (yes really: the pipeline driver itself is, architectually speaking, just another projection). And, as you can see in the (edited) compilation result below, the current weight of the pipeline package is only approximately twice the weight of the probably most used projection (the Engsager extended transverse mercator, etmerc): 
> $ gcc -I. -W -Wall -Wextra -pedantic -O2 -c PJ_pipeline.c PJ_horner.c PJ_cart.c PJ_helmert.c proj_etmerc.c 
> $ dir *.o 
> 2.655 horner.o 
> 3.576 PJ_cart.o 
> 6.465 PJ_helmert.o 
> 8.382 PJ_pipeline.o 
> 10.733 proj_etmerc.o 
> Let us assume that the final pipeline functionality may end up weighing 3 times that, it is still only 6 times the weight of etmerc, and all in all less than 10% of the total weight of the library on my test platform (gcc under Windows 7). 
> For these, say, 60 kB, you buy an infrastructure for implementing a large number of transformations through use of external parameter files, rather than by letting them add to the direct weight of the library, by implementing them as hard coded projections. 
> My colleague Kristian Evers and I are in the process of adding proj.4 support for a number of Danish, Faroese and Greenlandic systems, each of which (due to a very large number of parameters) will weigh a substantial fraction of 60 kB if not implemented using pipelines and external parameter files. 
> Hence, I believe that even if you have no use for the pipeline functionality, it will cost you only a modest amount of additional weight - much less weight than the addition of, say, 10 extra hard coded projections 
> Now, add to this, that the conceptual simplicity of a projection library in comparison to the complexity of a full geodetic framework, really is an illusion: While in theory projections are simple (in the sense "mathematically well defined") transformations from angular to linear coordinates, in reality they are not at all simple, if you want them to relate to any kind of real world. 
> Evidently, if you only need to relate a latitude/longitude pair in a given horizontal datum, to a set of projected coordinates in the same horizontal datum, a projection library is all you need. 
> But that is very seldom the case, as also indicated by the fact that NAD27-to-NAD83 transformation was part of the proj bundle right from the start, and the fact that sponsors during the years have found value in sponsoring Frank Warmerdam's work on implementing first horizontal datum shifts, later on also vertical. 
> The latter also hints at the fact that today, the vast majority of coordinate-capture is done by GPS/GNSS, and hence natively 3D, although not necessarily referred to a system that makes much sense, except for low accuracy work. 
> To preserve the high geometrical accuracy from the global system, when transforming to a regional reference system (e.g. ETRS89) and vertical datum (e.g. NAP/EVRS), you need access to more fundamental geodetic functionality (although not much more than already included in the pipeline package), before you can finally transform your latitude, longitude, and elevation data into traditional map coordinates, using the projection functionality already existing in proj.4. 
> When moving away from the stable parts of tectonic plates, you also need support for intra plate deformation models. This is the case for large parts of Scandinavia, which are heavily influenced by post-glacial uplift. Using the pipeline metaphor, this can be expressed in fairly simple terms, making it accessible to end users through direct support from their domain specific software, using proj.4 for georeferencing. 
> From airborne LiDAR mapping, we have learned the lesson, that provision of high accuracy geodata results in unexpected and innovative uses, leading to request for even higher accuracy. In other words: accuracy is addictive. And without proper geodetic handling of transformations, you will never consistently approach an accuracy much better than metre level (consistent with the difference between global and plate fixed coordinates). 
> Geodetic transformations are typically not algorithmically complex, compared to what is already included in proj.4 - we just need a platform for handling the metadata and stringing together elementary transformations. As already hinted at above, the overall weight of the functionality will be only a fraction of the existing library. 
> Form the geodetic side, I see proj.4 with the pipeline extension as the only viable road to a successful dissemination of the functionality required. And from the proj.4 side, I see extended geodetic functionality as the only viable road for continued relevance in an increasingly always-connected 3D society. 
> /Thomas 
> 2016-10-13 10:41 GMT+02:00 NDavid <ericnico.david at gmail.com>:
>> Sorry for the (very) late reply,
>> I've read the discussion about this pipelines feature at github and your
>> proj4 page and
>> I've definitely see such coordinate transformation pipeline as very useful.
>> BUT I'm more incline to keep only cartographic projection inside proj4 and
>> to implement
>> pipeline and other coordinate transformation outside proj4 in another
>> library/librairies.
>> And perhaps with a more C++ style than ANSI C.
>> Some potential coordinate transform that, I think, could fit into a pipeline
>> are
>> - cartographic projection (of course)
>> - datum conversion with planar gridshift or use of geoid.
>> - unit transform
>> - cartesian <-> polar <-> cylindric coordinate
>> - temporal transform (gps time to utc time ? )
>> - trajectory georeferencing (for point cloud, mobile mapping camera)
>> - conversion between sensor coordinate system and platform/IMU
>> - ..
>> Some of this coordinate transformation need additional data (trajectory
>> files, grid files,
>> datum/ellispoid dictionnary) and if implement inside proj4 that could lead
>> to insert
>> into proj4 some dependencies or making it a bigger library.
>> And so people who are only interrested in cartographic transformation
>> functions of
>> proj4 will have to pay for something they don't want.
>> I know that pj_transform/cs2cs/pj_apply_gridshift are not only about
>> cartographic
>> projection but also about datum transform. I understand to keep such
>> function
>> inside proj4 for historical and compatibility reasons but I don't think this
>> is
>> their "right" place.
>> just my 2 cents about this pipeline proposal.
>> Nicolas
>> --
>> View this message in context: http://osgeo-org.1560.x6.nabble.com/Transformation-pipelines-your-opinion-tp5269960p5290527.html [1]
>> Sent from the PROJ.4 mailing list archive at Nabble.com.
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