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Jean-Yves:
You have to be careful!
A Datum is NOT a Spheroid! A Spheroid will be
associated with a Datum, e.g.: NAD-83 is a Geocentric Datum (Earth Centered
Earth Fixed-ECEF), were the Geodetic Reference System of 1980 (GRS-80) Spheroid
is used. With the "Old" NAD-27 Datum, the Clarke 1866 Spheroid was used and the
Datum origin, were the Astronomical derived Coordinates of Station Meades
Ranch are the Origin Coordinates of the NAD-27 Datum, with a Clarke 1866
Spheroidal Height of 0.0 m.
Best regards,
Leo J. Romeyn
Geodetic Positioning Service Inc.
Satellite Imaging Corporation
11802 Marrs Drive
Houston, TX 77065-1202
Tel: 713.932.6375
Fax: 281.890.7051
----- Original Message -----
Sent: Tuesday, August 21, 2001 04:01
PM
Subject: Re: reprojection precision
Hello Ken,
Of course your coordinate will be different
from one datum to another (only Nad83 and WGS 84 are basically the same).
Datum is a spheroid to represent best the earth for the region in which you
are. If the spheroid parameters change, the "earth also changes".
Read
page 440 of the 6.0 User Guide (chap 28). you can also read Chap
27.
Regards,
Jean-Yves
At 11:50 2001-11-21 -0800, you
wrote:
G'day
all
I just reprojected a batch
of GeoTiffs (USGS DOQQs) from Nad83 to Nad27 using the Batch reprojection
wizard from ERM. I have noticed that the same point on the photo now longer
has the same Lat and Long in the 2 projections (the UTM coordinates for the
same points are different, but that is mean't to be the case I believe - ?).
I used a bilinear resampling method for the reprojection, but the same thing
happened if I used the nearest neighbour method.
The point in Nad83 has UTM coordinates (zone11) 561392.93E
/ 4544597.60N; 41:3:7.94N / 116:16:10.06W
The same point in Nad27 is 561314.00E / 4544799.00N; 41:3:7.62N /
116:16:13.37W
Is this sort of
variation in Lat and Long normal?
Thanks in advance
Cheers
Ken Hickey
------------------------------------------------------------------
Dr
Kenneth Hickey
Mineral Deposit Research Unit (MDRU)
Department of
Earth and Ocean Sciences
University of British Columbia
6339 Stores
Rd, Vancouver, BC V6T 1Z4, Canada
phone +1-604-822-3765; Fax
+1-604-822-6088
email: [EMAIL PROTECTED]
.
Jean-Yves Lauture
Geomatics
Representative
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