[Proj] Proj4 Datum Grids

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Hello, 

yes, that is exactly the idea in Proj .. if the datum is not Proj
predefined then you should use the towgs84 statement. You can either use
the "towgs84=.." statement or "datum=..." and the conversion mechanism
is the same, assuming that you use the same parameters in your towgs84
statement as in the library definition. You should also use the correct
ellipse always ... and preferably always define it. 

Here are the usual library predefinitions: 

Datum ID - Ellipse - Definition/comments 

WGS84 - WGS84 - towgs84=0,0,0
GGRS87 - GRS80 - towgs84=-199.87,74.79,246.62 / Greek Geodetic Ref Sys
1987
NAD83 - GRS80 - towgs84=0,0,0 / North American Datum 1983
NAD27 - clrk66 - nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat / NAD
1927
potsdam - bessel - towgs84=606.0,23.0,413.0 / Potsdam Rauenberg 1950
DHDN
carthage - clark80 - towgs84=-263.0,6.0,431.0 / Carthage 1934 Tunisia
hermannskogel - bessel - towgs84=653.0,-212.0,449.0 / Hermannskogel
ire65 - mod_airy -
towgs84=482.530,-130.596,564.557,-1.042,-0.214,-0.631,8.15 / Ireland
1965 

see also: 

https://en.wikipedia.org/wiki/Helmert_transformation 

regards: Janne. 

Dan Crosby kirjoitti 2018-03-20 00:15:

> Hi all, 
> 
> ....  
> 
> Am I correct in thinking that, provided the relevant ellipse is defined in Proj, I can perform any datum shift with the appropriate +towgs84 entry? 
> 
> Dan 
> 
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