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The procedures PLOT, OPLOT, PLOTS, XYOUTS, and CONTOUR can be used to display and annotate geographical data on maps created by the routines MAP_SET, MAP_GRID, and MAP_CONTINENTS. The MAP_IMAGE procedure can be used to warp regularly-gridded images to map projections.
The following simple example creates a CONTOUR plot over a Mollweide map projection and then over a polar stereographic projection. The resulting map is shown below.
; Make a 10 degree latitude/longitude grid covering the Earth: lat = REPLICATE(10., 37) # FINDGEN(19) - 90. lon = FINDGEN(37) # REPLICATE(10, 19) ; Convert lat and lon to Cartesian coordinates: X = COS(!DTOR * lon) * COS(!DTOR * lat) Y = SIN(!DTOR * lon) * COS(!DTOR * lat) Z = SIN(!DTOR * lat) ; Create the function to be plotted, set it equal ; to the distance squared from (1,1,1): F = (X-1.)^2 + (Y-1.)^2 + (Z-1.)^2 MAP_SET, /MOLLWEIDE, 0, 0, /ISOTROPIC, $ /HORIZON, /GRID, /CONTINENTS, $ TITLE='Mollweide Contour' CONTOUR, F, lon, lat, NLEVELS=7, $ /OVERPLOT, /DOWNHILL, /FOLLOW ; Fill the contours over the northern hemisphere and ; display in a polar sterographic projection: MAP_SET, /STEREO, 90, 0, $ /ISOTROPIC, /HORIZON, E_HORIZON={FILL:1}, $ TITLE='Stereographic Contour' ; Display points in the northern hemisphere only: CONTOUR, F(*,10:*), lon(*,10:*), lat(*,10:*), $ /OVERPLOT, /FILL, NLEVELS=5 MAP_GRID, /LABEL, COLOR=255 MAP_CONTINENTS, COLOR=255
Filling contours or polygons over maps that cover more than a hemisphere will produce incorrect results. This is because of the ambiguity between polygons that enclose an area, and those that enclose the entire surface of the sphere outside the area; and because of the ambiguity of determining the clockwise-ness of polygons on a sphere that cover more than a hemisphere.