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## 6.1.3 Lambert conic conformal projection (-Jl -JL)

This conic projection was designed by the Alsatian mathematician Johann Heinrich 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.

- Longitude and latitude of the projection center.
- Two standard parallels.
- 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°N and 45°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

    gmtset BASEMAP_TYPE FANCY PLOT_DEGREE_FORMAT ddd:mm:ssF GRID_CROSS_SIZE_PRIMARY 0.05i
    pscoast -R-130/-70/24/52 -Jl-100/35/33/45/1:50000000 -B10g5 -Dl -N1/thick -N2/thinner -A500 \
            -Glightgray -Wthinnest -P > GMT_lambert_conic.ps

Figure 6.3: Lambert conformal conic map projection

The choice for projection center does not affect the projection but it indicates which meridian (here 100°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°N and 47.5°N of 0.5-1%. Some areas, like Florida, experience scale errors of up to 2.5%.

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