Trimble Outdoors AcutimeTM2000 Synchronization Kit 0x8E-14, Set New Datum, Command Packet 8E-14

A Trimble Standard Interface Protocol

0x8E-14

Command Packet 8E-14

Set New Datum

This packet allows the user to change the default datum from WGS-84 to one of 180 selected datums or to a user-entered custom datum. The datum is a set of 5 parameters which describe an ellipsoid to convert the GPS receiver's internal coordinate system of XYZ ECEF into Latitude, Longitude and Altitude (LLA). This will affect all calculations of LLA in packets 4A and 84 and 8F-AC and 8F-0B. The receiver responds with packet 8F-14.

The user may want to change the datum to match coordinates with some other system (usually a map). Most maps are marked with the datum used and in the US the most popular datum for maps is NAD-27. The user may also want to use a datum that is more optimized for the local shape of the earth in that area. However, these optimized datums are truly "local" and will provide very different results when used outside of the area for which they were intended. WGS-84 is an excellent general ellipsoid valid around the world.

To change to one of the internally held datums the packet must contain exactly 2 bytes representing the integer value of the index of the datum desired. For a list of the datums and their indexes, see Table A-101.

Table A-71 Command Packet 8E-14

Byte #	Type	Value
0	Superpacket ID	0x14
1-2	INTEGER	Datum index

Acutime 2000 Synchronization Kit User Guide A-87