Handy GPS - Custom datum
This page allows you to edit the parameters for the custom datum, which can be selected on the drop-down datum list on the main page of the app
At the top is the name of the datum. This can be anything you like, but should be short as it needs to appear in the drop-down list on the main page.
The first two datum parameters are the ellipsoid semi-major axis (a), and inverse flattening (1/f)*.
The next seven parameters are the datum shift, or "toWGS84", parameters. If you only have three datum shift params (DX,DY,DZ), you can zero the remaining
four parameters and they will not be used. However if you have the full 7 parameters (DX,DY,DZ,RX,RY,RZ,S) then you can enter these and a more accurate
datum transformation will be performed. An abridged Molodensky transformation is used if only three parameters are provided, otherwise a Bursa-Wolf transform is used.
Following the datum parameters are several projection parameters which may also be configured. These are the false easting (FE), false northing (FN), scale factor (k), Lat Origin, Override Lon of Origin, and Lon Origin. The normal values of the first three parameters for the northern hemisphere are FE=500000,FN=0,k=0.9996 and for the southern hemisphere are FE=500000,FN=10000000,k=0.9996.
The normal values for the other parameters are lat origin=0.0, override lon of origin = off, lon origin = 0.0. Note that if override lon of origin is off, the lon origin value is ignored.
After this is the units factor. This is the value that when multiplied by the easting or northing, will convert them to metres. Normally this is set to 1.0, since Eastings and Northings are usually entered in and displayed in metres. But you can set this value to 0.3048006 if you want the Easting and Northing in feet instead.
Finally, there is an option to use affine parameters. These parameters are useful for defining the local grid coordinate systems that are often used for large projects like mine sites
Examples:
- SAD69 (South American 1969): a=6378160, 1/f=298.25, dx=-57.0, dy=1.0, dz=-41.0, FE=500000, FN=10000000, k=0.9996
- NAD83 (7-parameter transform, 2009 values): a=6378137, 1/f=298.257222101, dx=1.004, dy=-1.910, dz=-0.515, rx=0.02672, ry=0.00034, rz=0.01099, s=-0.0015, FE=500000, FN=0, k=0.9996
- ED50: a=6378388, 1/f=297, dx=-87, dy=-98, dz=-121, FE=500000, FN=0, k=0.9996
- British National Grid (OSGB36):: a=6377563.40, 1/f=299.325, dx=375, dy=-111, dz=431, FE=400000, FN=-100000, k=0.999601, lat origin = 49.0, override lon of origin = on, lon origin = -2.0
- Bogota Observatory (1975):: a=6378388, 1/f=297, dx=307, dy=304, dz=-318, FE=500000, FN=0, k=0.9996
- NZTM2000:: a=6378137, 1/f=298.257222, FE=1600000, FN=10000000, k=0.9996, lat origin = 0, override lon of origin=on, lon origin = 173
- Qatar National Grid: a=6378388, 1/f=297, dx=-119.425, dy=-303.659, dz=-11.0006, rx=-1.1643, ry=-0.174458, rz=-1.09626, s=3.65706, FE=200000, FN=300000, k=0.99999, lat origin = 24.45, override lon of origin = on, lon origin = 51.2166666667
- GDA 2020: a=6378137, 1/f=298.257222101, dx=-0.06511, dy=0.01087, dz=0.04019, rx=0.0394924, ry=0.0327221, rz=0.0328979, s=0.009994, FE=500000, FN=10000000, k=0.9996
Units
The customizable datum and projection parameters have the following units:
- a: metres
- 1/f: no units
- dx: metres
- dy: metres
- dz: metres
- rx: seconds
- ry: seconds
- rz: seconds
- s: parts per million
- FE: metres (usually 500000)
- FN: metres (usually 10000000 for southern and 0 for northern hemisphere)
- k: no units
- Lat Origin: degrees
- Lon Origin: degrees
*If you want to compute 1/f and you know the semi-major axis (a) and semi-minor axis (b) of the ellipsoid, the formula is 1/f = a/(a-b).
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