Parallels map
The polynomial model was derived from 3825 maps-each with a different spatial extent and computationally determined standard parallels that minimize the mean scale distortion index. The spatial extent is defined by the length of the mapped central meridian segment, the central latitude of the displayed area, and the width-to-height ratio of the map. The model defines the standard parallels with polynomial expressions based on the spatial extent of the mapped area. This article presents a polynomial model that quickly provides the standard parallels for the three most common conic map projections: the Albers equal-area, the Lambert conformal, and the equidistant conic projection. These methods are computationally expensive and cannot be used for real-time web mapping and GIS applications where the projection is adjusted automatically to the displayed area.
![parallels map parallels map](https://geology.com/world/cia/map-of-africa.gif)
There also exist more sophisticated methods that determine standard parallels such that distortion in the mapped area is minimized. These rules of thumb are simple to apply, but do not result in maps with minimum distortion. Rules of thumb exist for placing the standard parallels based on the width-to-height ratio of the map. In order to minimize the distortion of the mapped area, the two standard parallels of conic projections need to be selected carefully. Conic projections are appropriate for these cases because they show the mapped area with less distortion than other projections.
![parallels map parallels map](https://i.ytimg.com/vi/pZ8a1aadfBc/maxresdefault.jpg)
Conic map projections are appropriate for mapping regions at medium and large scales with east–west extents at intermediate latitudes.