Datums

PCI Geomatics supports horizontal datums. Vertical datums are not supported.

Reprojections that involve different datums require the following sequence:

If the input georeference units are a map projection (for example, UTM 11 S D-01), reproject the coordinates to the equivalent geographic coordinates with the same Earth model (for example, LONG/LAT D-01).
Perform a datum shift from the geographic coordinates of the input datum to the geographic coordinates of the output datum.
```
                       "LONG/LAT D-01"
                       (NAD27, U.S.A.)

                              ^ |
                          (a) | |
                              | v

                        "LONG/LAT D122" 
                        (    NAD83    )

                               ^ |
                           (b) | |
                               | v

            +----------> "LONG/LAT D000" -----------+
            | +--------- (    WGS84    ) <--------+ |
            | |                                   | |
        (c) | v                               (d) | v

    "LONG/LAT D002"                       "LONG/LAT D001"
    (   Adindan   )                       (    WGS 72   )
    
```
where:

(a) The datum shift uses the U.S. NADCON conus.los and conus.las grid shift files. If the longitude and latitude coordinates are outside the grid shift files cover area, no shift is applied.

(b) Conversions between NAD83 (D122) and WGS84 (D000) use a three-term Molodensky transformation.

(c) Conversions between datums WGD84 (D000) and Adindan (D002) use a three-term Molodensky transformation.

(d) D001 is a special case that uses formulas and parameters to transform between WGS72 and WGS84.

The diagram above shows the general process in first converting the input datum to WGS84 and then converting from WGS84 to the output datum. WGS84 is central to most of the involved datum shifts.

If the output georeference units are a map projection (for example, UTM 11 S D000) reproject the coordinates from the geographic coordinates (for example, LONG/LAT D000) to the output map projection with the same Earth model.

The choice of datum codes depends on the part of the world covered. The following ellipsoids can be upgraded to the following datums:

For areas within Canada:

Area	Longitude / Latitude
Bulk of Canada	141 W to 44 W and 49 N to 84N
Western Ontario	95 W to 85 W and 48 N to 49 N
Southern Ontario, West	85 W to 79 W and 42 N to 49 N
Southern Ontario, East	79 W to 74 W and 42d30' N to 49 N
Southern Quebec, West	74 W to 70 W and 45 N to 49 N
Southern Quebec, East	70 W to 68 W and 47 N to 49 N
Atlantic area	68 W to 44 W and 43 N to 49 N

E000 (Clarke 1866) upgrades to D-03 (NAD27 (Canada, NTv1)
Areas outside of Canada:
- E000 (Clarke 1866) upgrades to D-01 (NAD27 (USA,NADCON)
- E008 (GRS 1980) upgrades to D122 (NAD83)
E019 (Normal Sphere) upgrades to D800
E012 (WGS 84) upgrades to D000
E005 (WGS 72) upgrades to D001