[Proj] truble to translate coordinates from WGS84 to EPSG:31287

Wed Nov 3 08:30:44 EST 2010

Mikael,

If you merely "fiddle" the projection parameters (CM, lat0, scale, false 
coordinates, parallels maybe, etc.), the "direct projection" method might be 
regarded as a "dirty hack".  But the paper you cited two years ago:

http://www.fig.net/pub/fig2006/papers/ps05_03/ps05_03_04_engberg_lilje_0670.pdf

presents a methodical process for determining the parameters optimally (i.e. 
by least-squares constraint applied to the the survey data).  Seems rigorous 
and innovative to me.  And if the projection used is conformal, then the 
"direction projection" method is by definition conformal, too (i.e. 
preserves the angles of intersecting lines).  The 7-parameter transformation 
is also conformal, but multiple regression equations (MRE) and grid methods 
(NADCON, NTv2) are not.  A pertinent question would be, does "direct 
projection" fit the data better than a 7-parameter transformation?

Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

--------------------------------------------------
From: "Mikael Rittri" <Mikael.Rittri at carmenta.com>
Sent: Monday, November 01, 2010 9:06 AM
To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>
Subject: Re: [Proj] truble to translate coordinates from WGS84 to EPSG:31287

> Hello Markus,
>
> you wrote:
>> And on the the jhlabs site i found under "What is missing" the statement
>> "Coordinate system and geodetic datum conversion is missing."
>> (http://www.jhlabs.com/java/maps/proj/index.html)
>>
>> Doing the transformation with the jhlabs-lib would require some 
>> additional
>> coding for the datum transformation. Something i am not able to do with 
>> my
>> tiny knowlege in both fields, java and the math behind all that 
>> transformations.
>
> An alternative is to do the datum shift by an approximate "direct 
> projection"
> from WGS84/long-lat to MGI/Austria Lambert (EPSG:31287).  This is a method 
> where
> you would start by attaching the Austria Lambert projection to WGS84 - 
> that
> is, to the WRONG datum.  The results will not be correct, of course, so 
> then
> you fiddle with the projection parameters until the results are pretty 
> good.
>
> I think many people regard this method as a dirty hack, but the Swedish 
> Land
> Survey recommends it for a Swedish CRS, and I have to admit that I like 
> the
> method.  Out of curiousity, I tried it for MGI/Austria Lambert.  (Ideally, 
> my
> parameter fiddling should be derived from some original survey 
> observations
> for the MGI and WGS84 datums, but I used the 7-parameter datum shift 
> instead.
> This means that my "direct projection" is an approximation of an 
> approximation,
> so it can probably be improved, but not very much I think.)
>
> My result was pretty good. I finally arrived at the following 
> approximation
> of MGI/Austria Lambert:
>
>>proj +datum=WGS84 +proj=lcc +lat_1=46.0103424 +lat_2=48.988621 +lat_0=47.5 
>>+lon_0=13.33616275 +x_0=400268.785 +y_0=400057.553
>
> When I tested it on your example points, I got from FWTools 2.4.7:
>
> WGS84 long lat approx MGI/Austria Lambert, E N diff from geoland.at
> 14.2833092 48.3098392 470507.18  490503.02                0.02 m
> 16.371345  48.211996 625724.63  483601.73 0.07 m
> 9.747076  47.503042 130056.96  406637.16 0.73 m
>
> (I got the geoland.at results from your first letter,
> http://lists.maptools.org/pipermail/proj/2010-October/005454.html )
>
> Since the accuracy of your original 7-parameter datum shift was about 1.5 
> meters,
> the accuracy of my direct projection seems to about as good.
>
> Actually, I think my direct projection stays within 0.1 or 0.2 m 
> difference
> from the 7-parameter datum shift. I guess that geoland.at uses a more
> sophisticated datum shift, perhaps a grid file, which would explain the
> 0.73 m difference for the third example point.
>
> You can read an earlier thread about direction projection at the list 
> archives,
> http://lists.maptools.org/pipermail/proj/2008-December/thread.html
>
> Best regards,
> Mikael Rittri
> Carmenta AB
> Sweden
> http://www.carmenta.com
>
> -----Original Message-----
> From: proj-bounces at lists.maptools.org 
> [mailto:proj-bounces at lists.maptools.org] On Behalf Of Markus Hetzmannseder
> Sent: den 28 oktober 2010 15:50
> To: PROJ.4 and general Projections Discussions
> Subject: Re: [Proj] truble to translate coordinates from WGS84 to 
> EPSG:31287
>
> On Thu, 28 Oct 2010, Paul Kelly wrote:
>
>> On Thu, 28 Oct 2010, Markus Hetzmannseder wrote:
>>
>>
>> The proj command does a simple forward projection and ignores the
>> +towgs84 datum transformation parameters. The cs2cs command takes
>> these into account to give a more accurate result. If you leave off
>> the +towgs84 parameters from the cs2cs command line you will get the
>> same result as with the proj command line. It looks to me like the
>> JOSM plugin is not doing the datum transformation, only the projection
>> (I had a quick glance at
>> <http://svn.openstreetmap.org/applications/editors/josm/plugins/epsg31
>> 287/src/com/jhlabs/map/proj/LambertConformalConicProjection.java>)
>> to confirm this) - but that is certainly not easy to fix.
>
> Jeah, i know, i was coming to the same result myself too. The epsg31287 
> plugin is using the jhlabs java lib to do all the transformation. And on 
> the the jhlabs site i found under "What is missing" the statement 
> "Coordinate system and geodetic datum conversion is missing."
> (http://www.jhlabs.com/java/maps/proj/index.html)
>
> Doing the transformation with the jhlabs-lib would require some additional 
> coding for the datum transformation. Something i am not able to do with my 
> tiny knowlege in both fields, java and the math behind all that 
> transformations.
>
> Another way would be to rewrite the plugin with another calculation lib in 
> behind. Maybe something from http://geotools.org/ could help...
>
> At least we know now where that offset of the current epsg31287 plugin is 
> coming from. I hope it will help the author of the plugin a bit to find a 
> solution.
>
> Markus
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