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Posted by Chuck Taylor on August 26, 2004, 9:49 pm
Please log in for more thread options wrote:
>I'm looking for comments from 'the experts' about the following.
I'm not an expert, but I play one on Usenet. Perhaps Prof. Mugnier
will chime in and give you a better answer than mine.
>This page:
>http://www.internationalboundarycommission.org/coordinates/M49thp.txt
>lists the "NAD27 Official Values" for section M of the Canada/USA
>border, as published by the Internation Boundary Commission(IBC).
>
>Via email, I had asked the IBC to clarify in which variation of
>NAD27 those coordinates were expressed.
>
>The IBC replied that they didn't understand my question. I explained
>further, saying that from other wording in their email I took it
>to mean that they used NAD27 CONUS, but I wanted to confirm that,
>and suggested they update their coordinate lists to provide such
>information.
>
>In my email I had pointed out that there are regional variants of
>NAD27, such as NAD27 Alaska, NAD27 Canada, NAD27 Cuba,
>NAD27 Canada - Alberta and British Columbia, NAD27 CONUS, etc.
There's only one NAD27. But there are different techniques, and
different sets of parameters, for converting NAD27 coordinates to
another datum and vice-versa.
The names you mentioned above designate sets of transformation
parameters used with the 3-parameter Molodensky method to transform
points between NAD27 and WGS84. Molodensky isn't terribly accurate,
but it's the most widely used technique. Those names refer to
best-fit approximations for the regions they describe when using
Molodensky.
It doesn't help much that practitioners and agencies often refer to
these different sets of transformation parameters as datums, even
though they are *not* datums but *parameters that are input to a
transformation algorithm*. Even though the sets you listed are all
different, they all serve to relate *one* datum, NAD27, to the
reference datum, which in this case happens to be WGS84.
As an example of alternative techniques, Canada publishes NTv2, and
the U.S. publishes NADCON and CORPSCON, all of which transform points
between NAD27 and NAD83 with substantially higher accuracy than
Molodensky. These tools don't use the same parameters as Molodensky;
they use grid files and interpolation instead.
For a high-accuracy transform from NAD83 to WGS84 (or one of the
ITRFs) and vice-versa, there's HTDP. It uses yet another method.
See
<URL:http://home.hiwaay.net/~taylorc/bookshelf/math-science/geodesy/datum/transform/high-accuracy/>
(note the URL will probably wrap in your reader).
>The most recent email from the IBC stated:
>"The variations that you speak of, have nothing to do with how
>coordinates are published. That is just something that GPS and
>GIS system manufacturers came up with. The purpose was to make it
>easier for your handheld or gis software to transform from one
>datum to another or one coordinate system to another, but as far
>as I have seen no one publishes their NAD 27 coordinates the way
>you describe."
Not exactly. The U.S. Defense Mapping Agency (later NIMA, now NGA)
originated and published those regional parameters to work with the
Molodensky transformation and WGS84.
See <URL:http://earth-info.nga.mil/GandG/datums/dtp/index.html>. (NGA
is also guilty of confusing datums and transformation parameters.)
The IBC is right, however, in that there's only one way to express a
point's geographic coordinates in NAD27 (notwithstanding formatting
options like decimals vs minutes and seconds, of course).
>It was my 'understanding of things' that coordinates are useless
>without knowing in which datum they are expressed, either by
>an explicit statement of the datum, or by implication. I am
>aware that in many applications the difference between using
>one datum or another may not make any practical difference, but
>I'm talking about the theoretical difference.
>
>So, my question is, who is correct?
>Thanks
IBC used NAD27, of which there is only one. If you decide to
transform those points to another datum, you'll have to choose the
method you want to use. If you choose Molodensky to transform them to
WGS84, you'll want to choose the set of parameters that offers the
best compromise between accuracy and coverage of the region in which
you're interested.
Hope that helps.
--
Chuck Taylor
http://home.hiwaay.net/~taylorc/contact/
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