Zone Descriptions (ZD)

ZONE DESCRIPTIONS (ZD)

The first step in time conversions is determining the zone description. Theoretically, the

zone description could be found by dividing the local longitude by 15, since each zone is

15 wide, but problems arise because the zone boundaries have been modified (for greater

convenience) along geographical and political boundaries. Cities and other populated

areas are not split between two time zones. In some countries that overlap two or three

zones, one zone is used throughout. Also, zone descriptions are influenced by daylight

savings time.

Figure 4.2-12 Enroute Supplement

The most common source for Zone Descriptions is the IFR Enroute Supplement. The ZD is

found by looking up the departure or arrival airport and locating the ZD after the latitude

and longitude coordinates in the first paragraph. For Sherman Field the ZD is -6 except in

daylight savings time when it is -5. (Figure 4.2-12) An additional source for ZD is the TPC

that covers the area of interest. For this navigation course, the zone description will always

be given to you.

CONVERSION FORMULAS

Once the zone description has been determined, it can be applied to local mean time to

obtain GMT. Using the ZD from the enroute supplement, the formula is:

GMT (Z) = LMT - ZD

4.6-30