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

[Mikael Rittri]

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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