SETUP
ProMARK X
3-12
Access SETUP 9. Use the
RIGHT ARROW
to scroll through the available
datums until you find one that corresponds to your chart. Press
ENTER
.
NOTE
If you select British grid or Irish grid coordinates, the unit
automatically uses the OSGB or Eire datums, respectively; UPS
coordinates require WGS84. Other datums are not available
when any of these coordinate systems is chosen. (Refer to
Appendix 5
.)
If your map uses a datum that is not included in the choices listed on the
SETUP worksheet, you can use a user-entered datum. Refer to
Appendix
2
for the constants to convert WGS84 coordinates to the most common
local datums.
To enter a user-entered datum, press the
RIGHT ARROW
until an
undefined datum is displayed.
Press
ENTER
. If a value other than 0 is displayed, this user-entered
datum has been defined. Press
CLEAR
to delete the existing value or
select another user datum.
NOTE
Be sure to key in all leading zeros.
The unit will prompt you through the entry of values for
∆
a,
∆
f x 104,
∆
X,
∆
Y, and
∆
Z. (The unit displays “
∆
” as “DELTA.” Press the
RIGHT ARROW
to show a negative value. Press
ENTER
after each entry is complete to
continue. Press
ENTER
after keying in
∆
Z to save the datum.
SETUP 10 — Choosing an Altitude Reference.
The ProMARK X
calculates and stores altitude as height above the WGS84 ellipsoid, but
converts and expresses altitude in three ways; Mean Sea Level (MSL), Height
Above Geoid (HAG), and Height Above Ellipsoid (HAE) of the currently
selected datum. Which altitude reference you choose is determined by your
application.
Measurements of and on the earth are based on one or two models of the
earth. One model is the ellipsoid, which is a mathematically defined, regular
surface. Each datum uses the ellipsoid or arc of an ellipsoid that corresponds
most closely to the earth’s true shape in that area. Measurements of the
earth can be related to the ellipsoid vertically, as well as horizontally. These
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