Defining A User Map Datum - Magellan MobileMapper - GPS Software User Manual

The latest release of
receiver software allows
you to define the full
set of 9 parameters
defining a map datum.
After pressing MENU
and selecting
Setup>Map
Datum>Primary/Sec-
ondary and "USER" in
the prompted list, you
now have access to two
different screens on
which these 9 parame-
ters are presented:
1st screen:
Delta A (meters)
Delta F (X 10,000)
Delta X (meters)
Delta Y (meters)
Delta Z (meters).
2nd screen (select
Next> at the bottom of
the 1st screen to
access this screen):
Rotation X (sec)
Rotation Y (sec)
Rotation Z (sec)
Scale factor (ppm).
80

Defining a User Map Datum

Whether you are in the office using MobileMapper Office, or
in the field working with your receiver, the system lets you cre-
ate the user map datum you need.
However you will not use the same methods in the receiver
and in MobileMaper Office to define the semi-major axis and
flattening for your user datum.
In MobileMapper Office, you will use the conventional way of
defining a user datum, i.e. by entering the semi-major axis (a),
the inverse flattening (1/f) and the other 7 parameters.
The method used in the receiver is different as it is based on
the use of parameters ∆a (Delta A)and ∆f (Delta F) –known as
the Molodensky parameters– instead of a and 1/f. The other 7
seven parameters are exactly the same as those in MobileMap-
per Office.
∆a and ∆f are defined as follows:
∆
a (m) = a(WGS 84) - a(Local Datum)
∆
f = f(WGS 84) - f(Local Datum)
Note that the flattening (f), instead of the inverse flattening
(1/f), is used in the expression of ∆f. So we have:
1
∆f
---------------------------
=
1
(
-- - WGS84
f
Because the resulting ∆f is a very small quantity, it is multi-
plied by 10 000 to make it easier to handle. (The resulting
value is closer to 1.) It is this value that you have to enter in
the Delta F field on the MobileMapper Pro handheld screen.
1
---------------------- -
–
1
) ( )
-- - Local
f