[Proj] Transformation pipelines paper

[Kristian Evers]

All,

Thomas Knudsen and I have written a paper about the Transformation Pipelines
framework that have been introduced to PROJ.4 recently. We have submitted it
to the FIG Working Week 2017 which is held in Helsinki next week. I'll be
presenting the paper in the "Geodesy and Surveying Applications II" session on
Thursday June 1st. Please stop by and say hello if you are there.

The paper can be found at
http://www.fig.net/resources/proceedings/fig_proceedings/fig2017/papers/iss6b/ISS6B_evers_knudsen_9156.pdf
I have pasted in the abstract below.

In section 4, about transformation pipelines, we show case a few examples of
how transformation pipelines can be used. The examples all feature the new
and much improved implementation of the Helmert transform. Keeping the
lively discussion about the Helmert transform in March in mind I figure that the
paper will be of great interest to those involved in the discussion.

Happy reading,
Kristian

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TRANSFORMATION PIPELINES IN PROJ.4

For more than 2 decades, PROJ.4 has been the globally leading map projection
library for open source geospatial software. While focusing on mathematically
well-defined 2D projections from geographical to planar coordinates, PROJ.4
has nevertheless, since its introduction in the 1980s, provided limited
support for more general geodetic datum transformations, and has gradually
introduced a higher degree of support for 3D coordinate data and reference
systems.

The support has, however, been implemented over a long period of time, as
need became evident and opportunity was found, by a number of different
people, with different needs. Hence, the PROJ.4 3D support has not been the
result of neither deep geodetic, nor careful code architectural considerations.
This has resulted in a library that supports only a subset of commonly
occurring geodetic transformations. To be more specific: It supports any datum
shift that can be completed by a combination of two Helmert shifts and a
non-linear planar correction derived from interpolation in a correction grid.
While this is sufficient for most small scale mapping activities, it is not at
all sufficient for operational geodetic use, nor for many of the rapidly
emerging high accuracy geospatial applications in agriculture, construction and
transportation. To improve this situation, we have introduced a new framework
for implementation of geodetic transformations, which will appear in the next
release of the PROJ.4 library.

Before describing the details, let us first remark that most cases of geodetic
transformations can be expressed as a series of elementary operations, the
output of one operation being the input of the next. E.g. when going from UTM
zone 32, datum ED50, to UTM zone 32, datum ETRS89, one must, in the simplest
case, go through 5 steps:

1. Back-project the UTM coordinates to geographic coordinates
2. Convert the geographic coordinates to 3D cartesian geocentric coordinates
3. Apply a Helmert transformation from ED50 to ETRS89
4. Convert back from cartesian to geographic coordinates
5. Finally project the geographic coordinates to UTM zone 32 planar coordinates.

The homology between these steps and a Unix shell style pipeline is evident.
With this as its main architectural inspiration, the primary feature of our
implementation is a pipeline driver, that takes as its user supplied arguments,
a series of elementary operations, which it strings together in order to
implement the full transformation needed. Also, we have added a number of
elementary geodetic operations, including Helmert transformations, general
high order polynomial shifts and the Molodensky transformation. In anticipation
of upcoming support for full time-varying transformations, we also introduce a
4D spatiotemporal data type, and a programming interface (API) for handling this.

With these improvements in place, we assert that PROJ.4 is now well on its way
from being a mostly-map projection library, to becoming an
almost-generic-geodetic-transformation library.

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