[OSRS-PROJ] Re: Quick 'how-to' to on PROJ.4
Petri J. Riipinen
petri.riipinen at nic.fi
Sat Jul 28 08:57:29 EDT 2001
Precedence: bulk
Reply-To: osrs-proj at remotesensing.org
I'll just continue a bit.
From the new I found a table for GPS-users which shows these kind of
parameters
Finland Uniform
Long of origin: 27º 00.0 E
False easting: 3500000m
Scale: 1.0
False northing: 0m
Map Datum: Finland Hayford
dx: -78m
dy: -231m
dz: -97m
da: -251m
df: -0.1419270
Ok, I now use the dx,dy,dz in towgs84 and this gives me the accuracies
mentioned below. But how about 'da' and 'df'? Do they play a role in
getting even more accurate results? Can I incorporate them into
towgs84-parameter? I understood that it accepts either 3 or 7 parameters
and the above list shows 5 parameters. I guess that the rotation values are
0 as they are not mentionted, so the there is space for the 7th parameter,
would that be 'df'? How about 'da' then?
- Petri
-----------
Frank & others,
Well, you know what, it might be the datum after all... I found this one
Finnish web-page where it says like this:
------------------------
"The location of the center of Finland-Hayword ellipsoid differs from
WGS-84-ellipsoid center as follows:
X-axis direction 78 m
Y-axis direction 231 m
Z-axis direction 97 m.
Those figures have been published 1989-1998, after 1996 the published
numbers are:
X-axis direction 74.8 m
Y-axis direction 229.2 m
Z-axis direction 90.4 m"
--------------------------
So, might the problem be such that the the example map (that did in fact
come from Finnish Naional Land Survey-office) contains a wrong MapInfo-
ellipse code (-89,-98,-121). Maybe the number 28-ellipse isn't quite
correct when doing GPS-transformation from WGS84 to KKJ.
You see, I tried the above values instead of the ones you gave me and now I
get 0.5-0.9 m error in X-coordinate and 1.1 - 1.6 m error in Y-coordinate
when I compare the results with the few reference coordinates that you saw
earlier as well. I think that this is accurate enough for my needs.
Yes, I could code the WGS84->KKJ by hand, but I want to be prepared to
handle also other Scandinavian translations so I'd like to use PROJ.4 from
the beginning.
- Petri
At 21:49 27.7.2001 , Frank Warmerdam wrote:
>Precedence: bulk
>Reply-To: osrs-proj at remotesensing.org
>
>"Petri J. Riipinen" wrote:
> >
> > Precedence: bulk
> > Reply-To: osrs-proj at remotesensing.org
> >
> > Hello,
> >
> > Hmm... I might be out of my league now, but do you think that the
> > inaccuracy might come from the datum shift? I just learned that there are
> > 3- and 7-parameter shifts. Should a 7-parameter shift be used for a better
> > accuracy?
> >
> > At least in this page:
> > http://www.kms.dk/research/geodesy/reference/cotrans1_en.html there seems
> > to be 7 parameters for WGS84 - ED50 - transformation. Would this help?
> >
> > On this page: http://www.nls.fi/kartta/julkaisu/kkj.html there is some
> > (IMO) simple transformation formula. Do you think that this formula would
> > give me good enough results, if I can't get PROJ.4 to be accurate enough?
> > At the end of the page there is some example that might be a good reference
> > point when trying the transformations with cs2cs.
>
>Petri,
>
>The difference could be datum shift related, but it seems pretty significant
>for being the datum. You could of course build this transformation directly
>into your package by hand if you will only ever be interested in KKJ/WGS84
>conversion.
>
>Best regards,
>
>---------------------------------------+--------------------------------------
>I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
>light and sound - activate the windows | http://pobox.com/~warmerdam
>and watch the world go round - Rush | Geospatial Programmer for Rent
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<><><><><><><><>
Petri J. Riipinen
petri.riipinen at nic.fi
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