[Proj] Proj4js EPSG:4326 to EPSG:26786 (SP NAD27) 100+ feet Off

[Mikael Rittri]

PS:
Here is another transformation from NAD27 to NAD83(86), designed for Massachusetts:

   +towgs84=-38.67,117.84,209.59,0.0091,-0.9236,2.5411,-1.75953

which I think is better, but use it at your own risk. 
It is expressed as a Position Vector Transformation, as Proj.4 wants it. 

I got it from the SEVENPAR program, by entering six sample points that I had transformed with an online NADCON calculator. 
The max horizontal error for my six sample points was 37 cm (1.21 ft).
For your test point, the horizontal error is 16 cm (0.53 ft). I don't know the maximal error in the whole state. 

Regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

15 okt 2011 kl. 20:18 skrev "Mikael Rittri" <Mikael.Rittri at carmenta.com>:

> I think Proj4js supports a +towgs84 argument.
> If so, you can replace +datum=NAD27 by an appropiate +towgs84, the problem is just that the published 3- or 7-parameter datum shifts for NAD27 give poor accuracy. 
> The best one in EPSG is +towgs84=-9,161,179 intended for CONUS east of Mississippi river,
> but its accuracy is 11 m (one sigma).
> On your test point, it is wrong by 38.3 ft (11.7 m). 
> 
> So, I tried to construct a better 3-parameter datum shift for Massachussets. 
> Based on one point in Worcester, I got the result +towgs84=-5.16,143.64,208.44. 
> 
> When I then tried your example, I got 
> 
> 866077.79    231920.98
> 
> which is just 5.3 feet wrong. 
> 
> Of course, my three parameters are not the optimal +towgs84 values for Massachussets NAD27,
> and I don't know their accuracy in general, but I hope they can be of some use. 
> 
> Best regards,
> 
> Mikael Rittri
> Carmenta
> Sweden
> http://www.carmenta.com
> 
> 15 okt 2011 kl. 14:48 skrev "Scott DeHainaut" <scottdehainaut at comcast.net>:
> 
>> Frank - Thanks for the quick reply!  I was afraid of something like that...
>> are there any javascript or browser-accesible solutions that I could use?
>> 
>> Cheers,
>> 
>> -Scott DeHainaut
>> 
>> --
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>> Sent from the PROJ.4 mailing list archive at Nabble.com.
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