Verify and uncomment Swiss datum parameters

[neteler]

12/23/07 12:17:20 changed by pkelly

Re: the commented-out 7-parameter transform

I originally put this in and the source is now at ​http://www.asprs.org/resources/grids/08-2001-swiss.pdf

As far as I can remember the two reasons I left it commented were:

1) The parameters given are for CH1903+ which is apparently different from CH1903 - I have no idea how much or if the differences are big enough to be relevant.

2) I didn't have any test points to verify the parameters - in particular we always need to check the sign of the 3 rotation parameters (the 4th, 5th and 6th numbers out of the 7) as the American/European conventions are different.

If somebody can verify these two points then we can uncomment the parameters.

Paul

[neteler]

I received explanations from Switzerland (in deutscher Sprache). Available on demand.

Markus

[swisstopoum]

CH1903 and CH1903+ are basically the same system (only defined a little bit differently). The problem is only the realisation of the system (CH1903/LV03 by triangulation ; CH1903+/LV95 by GPS). The differences between the 2 systems are in the order of 1.5 m in maximum. Today we don't recommend the use of the 7 parameter transform any more. So please - whenever possible - don't use them. For the transformation from CH1903+ to ETRS89 we use officially the following 3 parameters: dX=674.374 m, dY=15.056 m; dZ=405.346 m.

If necessary, you can model the differences between CH1903+ and CH1903 with a distortion grid in the NTv2 format. This grid can be downloaded here: ​http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/lv03-lv95/chenyx06/distortion_grids.html

More information on the Swiss system and projection can be found here: ​http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/sys/refsys.parsysrelated1.2487.downloadList.78978.DownloadFile.tmp/swissprojectionen.pdf

In this document you find all the necessary formulas and parameters - as well as some examples. I revised this document in the meantime - but could not put it on the web yet. It contains some more explanations and more complete examples. Look for the revision date of the document: It should be 'october 2008'. If you need it right now, please contact me. Urs

[msieczka]

Replying to swisstopoum:

CH1903 and CH1903+ are basically the same system (only defined a little bit differently).

The differences between the 2 systems are in the order of 1.5 m in maximum.

Quite a difference.

Today we don't recommend the use of the 7 parameter transform any more. So please - whenever possible - don't use them. For the transformation from CH1903+ to ETRS89 we use officially the following 3 parameters: dX=674.374 m, dY=15.056 m; dZ=405.346 m.

Strange. Is 3 parameter transformation more accurate than the 7 one?

More information on the Swiss system and projection can be found here: [​http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey /sys/refsys.parsysrelated1.2487.downloadList.78978.DownloadFile.tmp/swissprojectionen.pdf]

"Not found". Please pass a valid link.

[pkelly]

Many thanks to swisstopo. The information is really helpful and when I have time to digest it I will update GRASS's handling of the Swiss datums appropriately.

Probably we can include the NTv2 grid in GRASS so it is irrelevant whether some areas have less accuracy with the 3-parameter rather than the 7-parameter transform. And for users who only require approx. metre-level accuracy, treating CH1903 as the same as CH1903+ may be adequate. But at least finally the difference between them is clear (I think!): CH1903 is the old classically triangulated datum in widespread use throughout Switzerland. CH1903+ is a redefinition of this datum, derived from GPS rather than classical triangulation. I suspect in many areas there is little difference between the too - perhaps the fact that there are so many big mountains in Switzerland caused some distortion within the old CH1903, and that has been removed in CH1903+? I'm just speculating though. It sounds like a similar situation to when the Ordnance Survey of Great Britain redefined their OSGB36 to be referenced to GPS - I don't have a figure for the biggest difference between a point in the two definitions of that same datum though. Their website is also quite detailed like the Swiss one - perhaps the information would be there if we went looking for it.

@Maciek re: missing link, try ​http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/sys/refsys.parsysrelated1.2487.downloadList.82881.DownloadFile.tmp/swissprojectionen.pdf

[swisstopoum]

Replying to msieczka:

Strange. Is 3 parameter transformation more accurate than the 7 one?

Answer: A 7-parameter transformation is always more accurate than a 3-parameter solution. But in the case of Switzerland it really does not improve the situtation significantly. The residuals remain in the order of 1.5 meters if you apply the 7-parameter transform. So, it only would complicate the calculation (sign and units of the rotations) without too many benfits. If you want to go near the 1cm-level, the NTv2 grid (or any other method of modelling local distortions) is the right approach.

[swisstopoum]

Just some comments to this: a) NTv2: I really think it would be fine to support NTv2. There are some countries who apply this approach (it was developped in Canada and then adopted in Australia, France, Germany,...). More and more commercial GIS developpers start to support this option. b) Distortions of triangulation networks: This is not a specific Swiss problem. Ok, the mountains play some role, but you have similar (or larger) distortions in all the traditional networks throughout the world. So, if you get (or give) a 7-parameter transformation set for such a national network, it is always only a rough approximation, which hardly gives results of better than 1 metre.

[hamish]

Replying to swisstopoum:

Just some comments to this: a) NTv2: I really think it would be fine to support NTv2. There are some countries who apply this approach (it was developped in Canada and then adopted in Australia, France, Germany,...). More and more commercial GIS developpers start to support this option.

GRASS does and has supported grid transforms for many years. You just need the grid file for your country. grid files are in $GISBASE/etc/nad/ and /usr/share/proj/.

Answer: A 7-parameter transformation is always more accurate than a 3-parameter solution.

I dare you to say that on the PROJ4 mailing list.

Different 3,7 term parameter sets will be more or less valid depending on where in the country (SRS) you are. If anyone is interested, further reading here: ​http://grass.osgeo.org/wiki/Gis_Concepts#Geodesy_and_Cartography

Hamish

[swisstopoum]

Replying to hamish:

Answer: A 7-parameter transformation is always more accurate than a 3-parameter solution.

I dare you to say that on the PROJ4 mailing list.

Ok, my mistake. It is only true if you determine the parameters out of the same data. If you use different data sets you can get any result.

[neteler]

Here is the datum (again):

towgs84=674.374,15.056,405.346,0,0,0,0

see ​http://lists.maptools.org/pipermail/proj/2009-July/004806.html

It would be good to get that fixed before final 6.4.0.

Markus (on the road)

[pkelly]

Now fixed in r38374, r38375 and r38376. The differences in parameters are actually very minor from those already in GRASS so I don't know if it will result in any shift in reprojections (that isn't already within the margin of error). It's just a pity we can't use the gridshift file due to the current structure of PROJ.4 - gridshift files can only be used if they transform to WGS84 directly.

[hamish]

Replying to pkelly:

It's just a pity we can't use the gridshift file due to the current structure of PROJ.4 - gridshift files can only be used if they transform to WGS84 directly.

complete ignorance at my end: is there any practical difference between ETRS89 and WGS84? e.g. is ETR89 a continental plate locked datum, or is the difference just the flattening term of the ellipsoid, or ...?

Hamish

[pkelly]

My understanding: ETRS89 is fixed to the European/Eurasian plate. It was the same as WGS84 in 1989 and since then has been drifting by a centimetre or two per year. But the ellipsoid is the same (well, given the GRS80/WGS84 difference which is miniscule). Presumably it's quite easy to compensate for the drift between ETRS89 and WGS84 when interpreting GPS co-ordinates (do some GPS receivers do this? I have no idea), and then after that it is simple to apply a localised country datum transformation grid or parameters, which don't change over time.

The interesting thing about CH1903+ is that it is only compatible with ETRS89 in the sense that a simple rotation brings it into alignment (3-parameter transform) and there is negligible distortion. But to convert from CH1903 to ETRS89 you first need to apply the gridshift transform to remove the localised surveying distortions and get to CH1903+, and then apply the rotation separately. However the structure of PROJ.4 only allows for the concept of a gridshift file converting to WGS84.

[hamish]

Replying to pkelly:

ok, I was a bit confused when updating the upcoming release notes if to call it an update to CH1903 or CH1903+. From comment:2 I took the new 3-param terms in r38376 to belong to CH1903+.

(wiki:Release/6.4.0-News)

so will the 3-param terms drift at the same rate as the plate?

Hamish

[pkelly]

ok, I was a bit confused when updating the upcoming release notes if to call it an update to CH1903 or CH1903+. From comment:2 I took the new 3-param terms in r38376 to belong to CH1903+.

Yes I was confused too, which is why I asked on the PROJ.4 list for more clarification. My understanding now is that the 3-parameter transformation is an (almost) exact transformation from CH1903+ to ETRS89 and has the same accuracy over all of Switzerland. Since CH1903 and CH1903+ are very close and only ever differ from each other by a couple of metres (the difference varies across Switzerland), it is reasonable to consider CH1903 as an approximation of CH1903+ and hence the parameters are also valid for CH1903 to ETRS89 (with an accuracy of a couple of metres). Indeed as the difference between ETRS89 and WGS84 (a few tens of centimetres) is much smaller than the accuracy of the parameters as applied to CH1903, they can be equally considered an accurate transformation from CH1903 to WGS84.

so will the 3-param terms drift at the same rate as the plate?

As mentioned above for CH1903, for the foreseeable future the difference between WGS84 and ETRS89 due to drifting of the plate is much smaller than the error in the 3-parameter transformation. So for CH1903 the accuracy of the 3-parameter transformation (when used to transform to WGS84) can be considered constant over time. (Obviously applied to ETRS89 the accuracy is actually constant over time.)

When the 3-parameter transform is applied to the CH1903+ to WGS84 transformation you are right that the accuracy will gradually degrade over time (a few centimetres per year). Again, applied to the CH1903+ to ETRS89 transformation it is constant.

Now I'm just waiting for someone to tell me I've missed an important detail and got this all wrong! ;) I think we need a new library in the free software world that separates the datum from the projection handles all this stuff properly for best accuracy.

GRASS has never had an entry in the datum.table for CH1903+, only CH1903. All I did in the SVN commit was to correct the 3-parameter transform for CH1903 (and it was a very small change). I guess we could add CH1903+ in with the same parameter set, but I wasn't sure if + signs would be allowed in the datum name and that could take a while to check so didn't want to rush into making that change. Ideally we would support CH1903 and CH1903+ separately, using the gridshift file for CH1903, but the way PROJ.4 currently works we can't support that...