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## 6.3.6 Cylindrical equidistant projection (-Jq -JQ)

This simple cylindrical projection is really a linear scaling of longitudes and latitudes. The most common form is the Plate Carrée projection, where the scaling of longitudes and latitudes is the same. All meridians and parallels are straight lines. The projection can be defined by:

- The central meridian [Middle of your map].
- Standard parallel [Equator].
- Scale in inch/degree or as 1:xxxxx (-Jq), or map width (-JQ).

The first two of these are optional and have defaults. When the standard parallel is defined, the central meridian must be supplied as well.

A world map centered on the dateline using this projection can be obtained by running the command:

    pscoast -Rg -JQ4.5i -B60f30g30 -Dc -A5000 -Gblack -P > GMT_equi_cyl.ps

Figure 6.21: World map using the Plate Carrée projection.

Different relative scalings of longitudes and latitudes can be obtained by selecting a standard parallel different from the equator. Some selections for standard parallels have practical properties as shown in Table 6.1.

Table 6.1: Standard parallels for some cylindrical equidistant projections.

Projection | Standard parallel
Grafarend and Niermann, minimum linear distortion | 61.7°
Ronald Miller Equirectangular | 50.5°
Ronald Miller, minimum continental distortion | 43.5°
Grafarend and Niermann | 42°
Ronald Miller, minimum overall distortion | 37.5°
Plate Carrée, Simple Cylindrical, Plain/Plane | 0°

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