USEREXPDATUM

Set custom expanded datum

Platform:

OEM719, OEM729, OEM7500, OEM7600, OEM7700, OEM7720, PwrPak7, CPT7, CPT7700, SMART7, SMART2

This command permits entry of customized ellipsoid and datum transformation parameters. The command DATUM USER must be sent for the parameters set by USERDATUM to take effect.

Only a single set of USEREXPDATUM or USERDATUM entered parameters is valid at one time. Newer parameters entered by either command will overwrite previously entered parameters.

The transformation used by this command is the 7 parameter Helmert transformation, with the parameters provided being for the transformation from the user datum to WGS84:

Where:

X, Y, Z are the coordinates in the user datum

X_s, Y_s, Z_s are the coordinates in WGS84

T_x, T_y, T_z are the translation parameters corrected for translation rate (metres)

R_x, R_y, R_z are the rotation parameters corrected for rotation rate (radians)

s is the scale difference corrected for scale rate (unitless)

A given parameter P is corrected for rate:

P(t)=P(t_r)+ Ṕ̇(t_r) (t-t^r)

Where:

t_r is the reference epoch of the parameters

t is the current epoch

P(t_r) is the parameter at the reference epoch

Ṕ̇̇(t_r) is the rate of the parameter at the reference epoch

The convention used by this command differs from that used by the DATUMTRANSFORMATION command, and from parameter conventions seen typically in transformation sources. Care should be taken that the parameter convention matches that in the equation above.

See the DATUM command for important guidance and details on datum operation.

Message ID: 783

Abbreviated ASCII Syntax:

USEREXPDATUM semi_major_axis flattening dx dy dz rx ry rz scale xvel yvel zvel xrvel yrvel zrvel scalevel epoch

Factory Default:

USEREXPDATUM 6378137.0 298.25722356280 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

ASCII Example:

USEREXPDATUM 6378137.000 298.25722356280 0.000000000 0.000000000 0.000000000 0.00000000 0.000000000 0.000000000 0.000000000 0.000000000 0.000000000 0.000000000 0.0000 0.000000000 0.000000000 0.000000000 0.000000000

Use the USEREXPDATUM command in a survey to fix the position with values from another known datum so that the GPS calculated positions are reported in the known datum rather than WGS84. For example, it is useful for places like Australia, where the continent is moving several centimetres a year relative to WGS84. With USEREXPDATUM, you can also input the velocity of the movement to account for drift over the years.

Field	Field Type	ASCII Value	Binary Value	Description	Format	Binary Bytes	Binary Offset
1	Command header	-	-	USEREXPDATUM header This field contains the command name for abbreviated ASCII or the message header for ASCII or Binary.	-	H	0
2	semi_major_axis	6300000.0 - 6400000.0 m		Datum semi-major axis (a) (metres)	Double	8	H
3	flattening	290.0 - 305.0		Reciprocal Flattening, 1/f = a/(a-b)	Double	8	H+8
4	dx	± 2000.0 m		Translation values (metres)	Double	8	H+16
5	dy	± 2000.0 m			Double	8	H+24
6	dz	± 2000.0 m			Double	8	H+32
7	rx	± 10.0 radians		Datum rotation angles A positive sign is for counter clockwise rotation and a negative sign is for clockwise rotation	Double	8	H+40
8	ry	± 10.0 radians			Double	8	H+48
9	rz	± 10.0 radians			Double	8	H+56
10	scale	± 10.0 ppm		Scale value is the difference in ppm	Double	8	H+64
11	xvel	± 2000.0 m/yr		Velocity vector along X‑axis	Double	8	H+72
12	yvel	± 2000.0 m/yr		Velocity vector along Y‑axis	Double	8	H+80
13	zvel	± 2000.0 m/yr		Velocity vector along Z‑axis	Double	8	H+88
14	xrvel	± 10.0 radians/yr		Change in the rotation about X over time	Double	8	H+96
15	yrvel	± 10.0 radians/yr		Change in the rotation about Y over time	Double	8	H+104
16	zrvel	± 10.0 radians/yr		Change in the rotation about Z over time	Double	8	H+112
17	scalevel	± 10.0 ppm/yr		Change in scale from WGS84 over time	Double	8	H+120
18	epoch	0.0 year		Reference epoch of parameters (decimal year) Example: 2011.00 = Jan 1, 2011 2011.19 = Mar 11, 2011	Double	8	H+128