Keywords: GMT_Docs

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## 6.1.2 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.

  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
    gmtset GRID_CROSS_SIZE_PRIMARY 0

Figure 6.2: 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%.

Next: 6.1.3 Equidistant Conic Projection Up: 6.1 Conic Projections Previous: 6.1.1 Albers Conic Equal-Area Contents Index