What is the central meridian of PTM Zone 5?

What is the central meridian of PTM Zone 5?

• A. 123°
• B. 125°
• C. 127°
• D. 126°

Answer: (B)

The central meridian is the longitude line that runs through the middle of a PTM zone and serves as the projection’s axis for that zone. PTM zones are six degrees wide, so the central meridian is simply the longitude halfway between the zone’s western and eastern edges. For Zone 5, that midpoint falls at 125° longitude. In other words, Zone 5 is designed so its center line is at 125°E, which is why 125° is the correct central meridian. If you check the idea by averaging the zone’s boundaries (for a 6° wide zone, boundaries would be 122°E and 128°E, giving (122 + 128)/2 = 125°E), you see why 125° makes sense. The other longitudes would place you off the zone’s center.

The central meridian is the longitude line that runs through the middle of a PTM zone and serves as the projection’s axis for that zone. PTM zones are six degrees wide, so the central meridian is simply the longitude halfway between the zone’s western and eastern edges. For Zone 5, that midpoint falls at 125° longitude. In other words, Zone 5 is designed so its center line is at 125°E, which is why 125° is the correct central meridian. If you check the idea by averaging the zone’s boundaries (for a 6° wide zone, boundaries would be 122°E and 128°E, giving (122 + 128)/2 = 125°E), you see why 125° makes sense. The other longitudes would place you off the zone’s center.