MAP_PLOTS Procedure
Plots vectors or points (specified as longitude/latitude data) on the current map projection.
Usage
MAP_PLOTS, x, y[, outx, outy]
Input Parameters
x—A scalar or vector providing the longitude coordinates of the points to be connected.
y—A scalar or vector providing the latitude coordinates of the points to be connected.
Output Parameters
note | If NoCircle or Psym is set then outx and outy are (1, n_elements(x)) arrays defining the input points in data coordinates (expressed in radians). If NoCircle and Psym are not set then outx and outy are (1, 201) arrays defining the data coordinates (expressed in radians) of the great circle segment between the last two input points. Note that outx and outy used with Nodata allow MAP_PLOTS to be used for a purely computational (rather than graphical) purpose. |
outx—(optional) Returns an array containing the projected longitude data in data coordinates (radians).
outy—(optional) Returns an array containing the projected latitude data in data coordinates (radians).
Keywords
Cylinder—If present and nonzero, specifies that lines be drawn treating longitude and latitude as Cartesian coordinates. This results in a straight line on a cylindrical projection. This keyword is useful if you want to draw your own latitude lines, since they will appear to conform to the current projection, rather than as straight lines or great circle lines.
Distance—Returns in a named variable the distance between the points provided as input to MAP_PLOTS. If two points are provided, Distance returns a scalar result. If multiple points are provided as input, Distance returns an array of distance values.
Km—If present and nonzero, specifies that the distance returned by the Distance keyword be measured in kilometers. This is the default.
Miles—If present and nonzero, specifies that the distance returned by the Distance keyword be measured in miles. Default is kilometers.
NoCircle—If present and nonzero, specifies that straight lines be plotted between points. If set equal to zero (the default), MAP_PLOTS draws great circle lines between any two specified points.
Standard Plotting Keywords
The following standard plotting keywords are used with MAP_PLOTS. See
Chapter 21: Graphics and Plotting Keywords, for more information.
Discussion
A valid data coordinate system (i.e., projection) must be established before MAP_PLOTS is called. (A call to MAP can be used to establish this coordinate system.) Also note that a PV-WAVE graphics window must be open and selected when the call to MAP_PLOTS is made for the procedure to work correctly.
The coordinates for MAP_PLOTS must be given in longitude and latitude form.
The MAP_PLOTS routine can plot lines as straight lines or as great circle lines, which appear on most projections as curved lines and represent the minimum distance between two points on the globe.
note | The great circle lines depend on the accuracy of trigonometric functions, which for some very small longitude and latitude values or values close to 90 degrees can result in errors that can cause the great circle lines to be incorrectly drawn. |
Example
This example plots a map, then draws a great circle line between two points on the map.
TEK_COLOR ; set up a predefined color table called tek_color
MAP, Range = [-150, 30, 30, 70]
MAP_PLOTS, [-105.3, -0.1], [40.0, 51.5], Distance=d, /Miles, $
Color=WoColorConvert(5), Psym=-2, Thick=2
See Also
