[Qgis-user] NAD27 to NAD83

[Tyler Veinot]

Hi All;
This question was up not long ago and I have been speaking with Steve King
on it and made some headway but he thought I should repost to the forum. I
am going to try and be more clear; I apologize for not being completely
competition in communications lol.
Here in PEI I have old data and new data I need to map. The working CRS is
EPSG 2954 a 3D realization of NAD83 in a Stereographic Projection for PEI;
PEI Stereographic Projection NAD83 CSRS. In the olden days we used NAD27
but there were 2 realizations of it NAD27 Stereographic Imperial and NAD27
Sterographic Metric. Because the province had definitions of these 2 CRS I
was able to make them easily in QGIS, so I have 2 custom coordinate systems
one NAD27 Imperial and the other Metric. We also have ATS77 Metrics which I
come across every so often but generally have a handle on. There is an old
XP based software used to transform coordinates from NAD27 Imp/Met to
NAD83CSRS but it only works on XP and we have completely removed those
computers from our network; so I have been trying to setup a transformation
in QGIS so that I wont need the older software.

I have a point with xy coordinates in both NAD27 Imp and NAD83 CSRS, what I
am trying to do is make a CRS and Transformation that will display both
coordinates in the same spot in their respective CRS.

So far I managed to get into the software and obtain the GSB file it uses
for the transformation, and apply that to my CRS;
+proj=sterea +lat_0=47.25 +lon_0=-63.0 +k=0.999912 +x_0=152400 +y_0=213360
+ellps=clrk66 +units=ft +nadgrids=mar2783.GSB
Which turned out to be the same has;
+proj=sterea +lat_0=47.25 +lon_0=-63.0 +k=0.999912 +x_0=152400 +y_0=213360
+ellps=clrk66 +units=ft +nadgrids=NTV2_0.gsb
But the when use it one the same coordinates transformed by the software
they are consistently about 23cm away from each other. We determined that
it is because PROJ4 assumes gridshifts are always from X datum to WGS84,
but NAD83 uses GRS80 so my question is, is there a way around this?
Should I use a NAD27 to WGS84 gridshift and let QGIS reporject it to NAD83
CSRS? Or is QGIS running on the notion that the 2 are coincidental?
Thoughts?
Tyler
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