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Kristian Evers Danish Agency for Data supply and Efficiency

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Presentation on theme: "Kristian Evers Danish Agency for Data supply and Efficiency"— Presentation transcript:

1 The future of coordinate transformation in geospatial open source software
Kristian Evers Danish Agency for Data supply and Efficiency I am a geophysicist at the national mapping authority in Denmark. Work in the geodetic office. Amongst other things i work on reference frames and coordinate transformations. Special focus has been on PROJ lately, which we are planning on using as our main transformation software in Denmark.

2 The earth is dynamic. Continents drift, the sea level and the earth’s crust rise and unstable soil layers collapse. This is handled in many different ways, which is why coordinate transformation is a tough problem to solve.

3 static The earth is dynamic. Continents drift, the sea level and the earth’s crust rise and unstable soil layers collapse. stay where they are for ever never ever change BUT, as always in physics, if we simplify the problem enough, eventually it becomes simple. This trick is used in QGIS and PROJ today. stay stable

4 Explain the components.
EPSG Guidance note 7-1

5 Earth-centered, Earth-fixed Co-rotating with the Eurasian plate
Details on datums. The connection between the coordinate system and the real world. And what makes transformation difficult. It’s the box in the middle that we’re interested in today. Earth-centered, Earth-fixed Co-rotating with the Eurasian plate Rigid continental plate Only defined in Europe Models the world in 1989 Earth-centered, Earth-fixed Co-rotating with the Earth Elastic continents move freely Globally defined Models the world today EPSG Guidance note 7-1

6 Standard method of coordinate transformation today
CRS A Inverse projection Inverse grid shift Geodetic -> cartesian 7 parameter Helmert WGS84 The recipe PROJ.4 and thus QGIS follows 7 parameter Helmert Cartesian -> geodetic Grid shift Projection CRS B

7 Standard method of coordinate transformation today
CRS A Inverse projection Inverse grid shift Geodetic -> cartesian 7 parameter Helmert WGS84 Hub datum 7 parameter Helmert Cartesian -> geodetic Grid shift Projection CRS B

8 Standard method of coordinate transformation today
Inverse projection Inverse grid shift 7 parameter Helmert Cartesian -> geodetic Projection ED50 / UTM 32N ETRS89 / WGS84 Geodetic cartesian Grid shift +proj=utm +zone=32 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m Hub datum Example of datum shift ED50 -> ETRS89 +proj=utm +zone=32 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m

9 Okay, so one size doesn’t fit all
Okay, so one size doesn’t fit all. There’s a bunch of problems with this approach. Here’s some of them.

10 (if you care about high accuracy data)
The problem with WGS84 (if you care about high accuracy data) WGS84 is ill-defined (it’s not, but the way we use it is…) 6 different versions (realizations) exists - which one is never specified The different versions can differ up to 1 m Most regional datums are said to be equivalent to WGS84 They are not! WGS84 moves away from ETRS89 with ~2.5 cm per year (~70 cm out of sync today) Using WGS84 as a hub datum introduces uncertainties to your data In principle you add ~2m of uncertainty to your cm accuracy data when transforming with WGS84 as a hub datum Many coordinate systems are not defined in terms of WGS84 example on next slide Sorry for the wall of text…

11 System 45 -> ETRS89 / UTM 33 N
Polynomial mapping Inverse UTM projection on the Hayford ellipsoid To geodetic coordinates To cartesian coordiantes Helmert shift System45 TC32 ED50 ETRS89 UTM projection on the GRS80 ellipsoid ETRS89 / UTM 33N

12 The earth IS dynamic

13 Iceland Iceland is also working on a dynamic datum. The country is moving in two directions and theres a lot of both seismic and vulcanic activity. Not very suitable for a conventional static datum.

14 Dynamic datums Coordinates are not ”attached” to the Earth. Geometrically defined by satellite systems, not physical objects on the ground. Coordinates change with time. The same point on earth will have a different coordinate every time you measure it. Timestamps are a necesary component in a coordinate. But coordinates will stay accurate at mm to cm level! Urgh, another wall of text…

15 Introducing Transformation Pipelines in PROJ
A flexible framework that allows for complex transformations Transformations are constructed as a set of daisy-chained basic building blocks Not limited to spatial transformations – pipelines are fully time-aware More than two geodetic techniques available 14-parameter shift Velocity and deformation models Molodensky transform Polynomial mappings Affine transformation Exposed in a new and more coherent API for PROJ Last one, I promise!

16 Transformation Pipeline example
proj=pipeline proj=horner proj=utm inv ellps=intl zone=32 proj=cart inv ellps=intl proj=cart proj=helmert System45 TC32 ED50 ETRS89 proj=utm ellps=GRS80 zone=33 ETRS89 / UTM 33N

17 Transformation Pipeline example
+proj=pipeline +step +init=./s45b.pol:s45b_tc32 +step +proj=utm inv +ellps=intl +zone=32 +step +proj=cart ellps=intl +step +proj=helmert +ellps=GRS80 +x= y= z= +rx= ry= rz= s= +step +proj=cart +inv +ellps=GRS80 +step +proj=utm +ellps=GRS80 +zone=33 proj=pipeline proj=horner proj=utm inv ellps=intl zone=32 proj=cart inv ellps=intl proj=cart proj=helmert System45 TC32 ED50 ETRS89 proj=utm ellps=GRS80 zone=33 ETRS89 / UTM 33N

18 Thanks for your attention!
Questions?


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