Setra System Model 370 - Manual (Page 27)

If you are not at a known elevation, the gauge may be used in true
altimeter mode by obtaining an Altimeter Setting Indication (sea level
corrected pressure) from a local airport or weather station, and following
this procedure:
1. Press the CONV key until the engineering units of the known sea level
pressure appears in the display.
2. Enter the known sea level pressure and press ZERO. The difference
between the sea level pressure and the current pressure is displayed.
3. Press CONV until the desired altitude units of measure are displayed.
4. The gauge is now functioning as a corrected altimeter, and will display
true altitude as it is moved from one location to another.
CAUTION: In any true altimetry application, the accuracy of the measurement
is dependent on the quality of the correction made for local barometric pressure.
Distance from the reporting station and elapsed time from the measurement will
greatly affect the accuracy of the altitude measurement as barometric pressure
changes. Repeat the above procedure as often as new ASI data is available.
21