Difference of Meridional Parts or Meridional Difference.
The Difference of meridional parts or Meridional difference (Δϕc): It’s the distance measured on the meridian between two parallels of a Mercator projection. Or, the difference between the meridional parts of any two given parallels.
To find out the difference of meridional parts between two parallels, we determine the meridional parts corresponding to the two parallels and make their difference.
This difference is found by subtraction, if the two parallels are on the same side of the Equator, and by addition if on opposite hemispheres.
The meridional difference is expressed in equatorial miles [Em].
Mercator mile (Mm): It is the difference of meridional parts corresponding to one minute of difference of latitude.
The Mercator mile does not have a constant value and increases with latitude.
The Mercator mile represents the size of a minute of a meridian at any given latitude, and is used to determine the latitude of a point and to measure the distances on the map.
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