5.2.2 Lambert Conic Conformal Projection (-–Jl -JL)

This conic projection was designed by Lambert (1772) and has been used extensively for mapping of regions with predominantly east-west orientation, just like the Albers projection. Unlike the Albers projection, Lambert's conformal projection is not equal-area. The parallels are arcs of circles with a common origin, and meridians are the equally spaced radii of these circles. As with Albers projection, it is only the two standard parallels that are distortion-free. To select this projection in GMT you must provide the same information as for the Albers projection, i.e.

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Longitude and latitude of the projection center

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Two standard parallels

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Map scale in inch/degree or 1:xxxxx notation (-Jl), or map width (-JL)

The Lambert conformal projection has been used for basemaps for all the 48 contiguous States with the two fixed standard parallels 33$^{o}$N and 45$^{o}$N. We will generate a map of the continental USA using these parameters. Note that with all the projections you have the option of selecting a rectangular border rather than one defined by meridians and parallels. Here, we choose the regular WESN region, a FANCY basemap frame, and use degrees west for longitudes. The generating commands used were

#!/bin/sh
#    $Id: GMT_lambert_conic.sh,v 1.1 2001/03/21 04:10:21 pwessel Exp $
#

gmtset BASEMAP_TYPE FANCY DEGREE_FORMAT 3 GRID_CROSS_SIZE 0.05i
pscoast -R-130/-70/24/52 -Jl-100/35/33/45/1:50000000 -B10g5 -Dl -N1/1p -N2/0.5p -A500 -G200 \
   -W0.25p -P > GMT_lambert_conic.ps
gmtset GRID_CROSS_SIZE 0

Figure 5.7: Lambert conformal conic map projection

The choice for projection center does not affect the projection but it indicates which meridian (here 100$^{o}$W) will be vertical on the map. The standard parallels were originally selected by Adams to provide a maximum scale error between latitudes 30.5$^{o}$N and 47.5$^{o}$N of 0.5-1%. Some areas, like Florida, experience scale errors of up to 2.5%.