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

Xs, Ys, Zs are the coordinates in WGS84

Tx, Ty, Tz are the translation parameters corrected for translation rate (metres)

Rx, Ry, Rz 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(tr)+ ̇(tr) (t-tr)

Where:

tr is the reference epoch of the parameters

t is the current epoch

P(tr) is the parameter at the reference epoch

̇̇(tr) 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 (see page 1) or Binary (see page 1).

-

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