Scott
NTV2 datum grid file is normally used for NAD27 to NAD83 datum
conversion not the other way around.
However, if you want to go back to NAD27 you need to know as much as
possible about your NAD83 data.
Is it a recent photogrammetric compilation on NAD83 or a old NAD27
compilation converted to NAD83?
If it is a NAD27 to NAD83 conversion how was it converted?
Mapping institutes use different methods to achieve
datum changes for canadian data and it does not necessarily include any
datum changes. It could of been done using only an ellipsoid
change (Clarke 1866 to GRS80/WGS84) without datum consideration. Also
depending on the date of conversion to NAD83, the NTV2 grid might not
even of been completed and the older NTV1 methode could of been used...
What is the scale of the original mapping? I would say that smaller
scale then 1:50000 (e.g.1:250000) you don't necessarily need to consider
datum change.
A few products that can incorporate NTV2 datum method:
-Blue Marble Geographic Geographic Calculator/Transformer.
-The project command in ArcInfo (ArcCad) has a Datum change for Canada.
Obviously you need access to ArcInfo. From what I've tested it uses the
NTV2 grid.
-Mentor software also has a NTV2 datum option in their Tralaine
software. I recommend their site for more information on Datum change.
Bottom line, when you try to go "back" to an "old" mapping Datum you
need to be very very careful...
Hope this helps a bit.
Regards
Jacques Gelinas
--
G�osph�re G�omatique
http://www.travel-net.com/~geosphere
265 Freeman
Hull, QC
J8Z 2B1
tel. 819-773-4239
fax. 819-773-4263
Scott Snashall wrote:
Hello mappers,
With regards to the National Transformation convertor(NTV2) offered by
Geomatics Canada. Does anyone have any documentation on converting
mapinfo
files into files that the NTV2 can understand.
I am trying to convert a Nad83 file back to a Nad27 file.
Your help is appreciated
( :
Scott Snashall
Documentation specialist
Metronet/AT&T
403-705-6223
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
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