Users will encounter this message
only when converting coordinates from NAD27 to NAD83 (or vice versa) and which,
geographically, reside in Canada. As discussed in the last issue of the Casual
Cartographer, when converting from NAD27 to NAD83 (or vice versa) Tralaine and other
Mentor Software products expect to use a grid based data file(s) to obtain shift
information. For U.S. geography, these data files (i.e. the .LAS and .LOS files) are in
the public domain and Mentor Software distributes copies of these with all products. The
file which covers Canadian geography is not in the public domain and its use must be
licensed from Geomatics Canada:
Geomatics Canada charges a non-trivial license fee, and the file is quite large, ~14MB.
Distribution is via CD-ROM. Simply copy the NTV2_0.GSB file into the Tralaine or
TralaineDLL data directory, or Hawkeye’s installation directory, and these products
will automatically pick up on the presence of the data file and use its contents whenever
appropriate.
Users who obtain this data file should be aware of an important point. The US data
files (i.e. .LAS and .LOS) and the Canadian data file overlap to a certain degree in terms
of the geography covered. In the overlap regions the two systems will not produce
the same results exactly. The automatic file selection feature built into Mentor
Software’s products will automatically select the Canadian data file. This is true as
the data grid in the Canadian file is denser than that in the US counterparts. Thus,
Tralaine, et al, assume that the file with the denser grid will produce the more accurate
results.
Since your conversion project was, most likely, supposed to be done yesterday, there is
a less preferable alternative you can use in an emergency situation. Note that: 1)Mentor
Software’s coordinate conversion products include support for multiple regression
datum transformations; 2)the US Defense Mapping Agency (DMA) has published a multiple
regression formula for Canada, and 3)Mentor Software’s coordinate conversion products
include this multiple regression definition in its dictionaries. This definition is named
NAD27-CN. Using the Coordinate System Editor, you can reference your coordinate system to
this datum, instead of the more general NAD27. In so doing you will be instructing
Tralaine, et al, to use the multiple regression formula instead of the grid data file
based conversion technique it normally expects to use.
Be careful here. The grid data file based conversion will, generally, produce results
within a few centimeters of the true result. The multiple regression technique described
above will, in general, provide results accurate to 4 or 5 meters. Thus, there is a
significant accuracy penalty for this work around solution. However, since the difference
between NAD27 and NAD83 can be as high as hundreds of meters, a solution which gets you
within 4 or 5 meters might be acceptable in an emergency.
