Jason,
You wrote:
>> I've just encountered an odd problem. I'm trying to convert UTM coords
>> between NAD27 and WGS84 datums for 150K points in Canada. I did an easy
>> CentroidX&Y(obj) command with the map window in the WGS84 projection I
>> wanted and updated new columns with the values, which appeared to work.
>> The
>> problem came when I verified the result using Geographic Calculator - the
>> values were off by quite a few meters (up to 50). Is this an acceptable
>> error when dealing with transformations or are the two using different
>> projection parameters? I know they both use the Molodensky method of
>> transformation (which has it's own degree of
>> (in)accuracy), but I'm not sure if they are both using the same NAD27 - is
>> it Canada Mean or Canada MRE?
>> Another loaded question - does anyone know off hand what the dX,dY and dZ
>> parameters MI uses for the conversion?
The Geographic Calculator can use the Molodensky, Bursa/Wolfe, MRE, or Canadian
National Transformation for NAD27 to NAD83 (WGS84) conversion within Canada. We
recommend the Canadian National Transformation (essentially equivalent to the NADCON
method here in the States). This method is not currently supported by MapInfo. The
bottom line is that you can select amongst several NAD27-Canada datum transformations
from within the Geographic Calculator.
We have done a little digging inside TAB files (using our FormatX TAB Reader control)
and have determined that MapInfo uses the following parameters for the
["Longitude / Latitude (NAD 27 for Canada)", 1, 66] datum transformation:
DX = -10.0 m
DY = 158 m
DZ = 187 m
Ellipsoid = Clarke 1866
Be sure to verify which method you are using within both applications. When comparing
results, always compare apples to apples. Our tech support folks
([EMAIL PROTECTED] or 207-582-8722) would be glad to help you out.
Thanks,
Jeff Cole
[EMAIL PROTECTED]
Blue Marble Geographics
"The Geographic Software Component Company."
www.bluemarblegeo.com
261 Water Street, Gardiner, Maine 04345 USA
Voice: 207-582-6747 Fax: 207-582-7001
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