Reprojections that involve different datums will require the following sequence:
1. If the input georeference units are a map projection (e.g. "UTM 11 S D-01") reproject the coordinates to the equivalent geographic coordinates with the same earth model (e.g. "LONG/LAT D-01").
2. Do the Datum shift from the geographic coordinates of the input datum to the geographic coordinates of the output datum.
"LONG/LAT D-03" "LONG/LAT D-01"
(NAD27, Canada) (NAD27, U.S.A.)
^ | ^ |
(a) | | (b) | |
| v | v
"LONG/LAT D-04" == "LONG/LAT D122" == "LONG/LAT D-02"
(NAD83, Canada) ( NAD83 ) (NAD83, U.S.A.)
^ |
(c) | |
| v
+----------> "LONG/LAT D000" -----------+
| +--------- ( WGS84 ) <--------+ |
| | | |
(d) | v (e) | v
"LONG/LAT D002" "LONG/LAT D001"
( Adindan ) ( WGS 72 )
Where:
(a) The datum shift is done using the Canadian NTv1 grid shift file
"grid.dac". If the "LONG/LAT" coordinates are outside of the area
the grid shift files cover, the datum shift will fail.(b) The datum shift is done using the U.S. NADCON grid shift files "conus.los" and "conus.las". If the "LONG/LAT" coordinates are outside of the area the grid shift files cover, the datum shift will fail.
(c) "LONG/LAT D-04" and "LOG/LAT D-02" coordinates are assumed to be equal to "LONG/LAT D122" coordinates. Conversions between NAD83 and WGS84 is done using a three term Molodensky transform.
(d) Conversions using datums "D002" to "D998" are done using a three term Molodensky transform.
(e) "LONG/LAT D001" is a special case, that uses formulas and parameters to transform between WGS72 and WGS84.
NOTE: A better alternative for WGS72 would be to use "D186" instead of "D001". "D186" uses a three term Molodensky transform. WGS72 in ARC/INFO corresponds to "D001" (NOT "D186").
The above diagram shows the process in general involves first converting from the input datum to WGS84 and then converting from WGS84 to the output datum. So WGS84 is central to most of the datum shifts involved.
3. If the output georeference units are a map projection (e.g. "UTM 11 S D000") reproject the coordinates from the geographic coordinates (e.g. "LONG/LAT D000") to the output map projection with the same earth model.
The choice of datum codes depends on what part of the world the coverage is. The following ellipsoids can be upgraded to the following datums:
- Areas within Canada, Bulk of Canada : 141 W to 44 W; 49 N to 84
N; Western Ontario : 95 W to 85 W; 48 N to 49 N Southern Ontario,
West: 85 W to 79 W; 42 N to 49 N; Southern Ontario, East: 79 W to
74 W; 42d30' N to 49 N; Southern Quebec, West: 74 W to 70 W; 45 N
to 49 N; Southern Quebec, East: 70 W to 68 W; 47 N to 49 N;
Atlantic area : 68 W to 44 W; 43 N to 49 N;
- "E000" (Clarke 1866) upgrades to "D-03" (NAD27 (Canada, NTv1))
- "E008" (GRS 1980) upgrades to "D-04" (NAD83 (Canada, NTv1))
- Areas outside Canada,
- "E000" (Clarke 1866) upgrades to "D-01" (NAD27 (USA, NADCON))
- "E008" (GRS 1980) upgrades to "D-02" (NAD83 (USA, NADCON))