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pp_sphericalpath.pro

Routines

top source pp_sphericalpath

result = pp_sphericalpath(lons, lats [, maxlength=maxlength] [, nsegments=nsegments] [, open=open] [, _ref_extra=_ref_extra], no_fix_lon=no_fix_lon)

Given a set of lon/lat points on a sphere, this function calculates vertices of a path connecting the input vertices, placed along great circles connecting the input vertices. The path is a polygon approximation of the curved path formed by the great circles.

Return value

A 2D array [2,N] with a set of longitudes/latitudes for the N vertices of the path, for each path. The number of vertices along each edge is determined by maxlength or nsegments. If open is set, the last point is not connected to the first point. If multiple polygons were given as input, the ouput is a list, where each list element is a [2,N] array with lon/lat for each polygon (N needs not to be the same for all polygons).

Parameters

lons in required

An array of longitudes for the vertices which are to be connected by a path made of great circles. Must be in degrees, unless radians is set. Multiple polygons are supported in two different ways: 1) If all N polygons have the same number of vertices (M), lons can be given as a [M,N] array. 2) For arbitrary numbers of vertices, lons is given as a list, where each list element is an array of vertices for one polygon. With multiple polygons, the ouput is a list, where each list element has latitudes and longitudes for one polygon.

lats in required

An array of latitudes for the vertices which are to be connected by a path made of great circles. Must be in degrees, unless radians is set. Multiple polygons are supported in two different ways: 1) If all N polygons have the same number of vertices (M), lats can be given as a [M,N] array. 2) For arbitrary numbers of vertices, lats is given as a list, where each list element is an array of vertices for one polygon. With multiple polygons, the ouput is a list, where each list element has latitudes and longitudes for one polygon.

Keywords

maxlength in optional default=0.1

If set, specifies the maximum angular length of each segment of the polygon.

nsegments in optional

If set, specifies a fixed number of vertices to use in for the line connecting each pair of input points. If both maxlength and nsegments are set, nsegments takes precendence.

open in optional default=0

If set, the output path does not connect the last input point to the first input point.

_ref_extra in out optional

Anything else is passed along to map_2points, which is used to create the paths connecting the pairs of input points. One commonly used option would be the radians keyword, to use radians, instead of degrees, for lon and lat. See the IDL help for map_2points' options.

no_fix_lon

Examples

Create a rectangle in lon/lat:

 lons=[30d0,120d0,120d0,30d0]
 lats=[70d0,70d0,10d0,10d0]

Create a closed path connecting these points:

path=pp_sphericalpath(lons,lats)

Create a base map to plot the results (from IDL's imap example):

 file = FILEPATH('avhrr.png', SUBDIRECTORY=['examples','data'])
 data = READ_PNG(file, r, g, b)
 IMAP, data, LIMIT=[-90,-180,90,180], $
 MAP_PROJECTION='Mollweide', RGB_TABLE=[[r],[g],[b]], $
   IMAGE_TRANSPARENCY=50, GRID_UNITS=2, $
   IMAGE_LOCATION=[-180,-90], IMAGE_DIMENSIONS=[360,180]

Plot the input points connected by straight lines:

iplot,lons[[0,1,2,3,0]],lats[[0,1,2,3,0]],color='red',/overplot

Plot the path created by pp_sphericalpath:

iplot,/overplot,color='green',path,thick=3.

The result should look like

The green path, produced with the output of pp_sphericalpath looks like a curve, made by the 4 great circles connecting these points. In reality, the path is made of 2367 points, separated by 0.1 degrees. This can be verified by making the same plot on a gnomonic projection, which has the property that great circles are straight lines:

 imap,data,limit=[0,20,80,130],map_projection='mollweide',rgb_table=[[r],[g],[b]],$
 grid_units=2,image_transparency=50,image_location=[-180,-90],$
 image_dimensions=[360,180],center_latitude=50,center_longitude=75
 iplot,/overplot,color='green',path,thick=3.

The result should look like

Where the green path is rendered as straight lines, since great circles map into straight lines in a gnomonic projection.

Author information

Author:: Paulo Penteado (http://www.ppenteado.net), Nov/2014

Statistics

Lines:	63 lines
Cyclomatic complexity:	21
Modified cyclomatic complexity:	21

File attributes

Modification date:	Wed Mar 7 17:08:26 2018
Lines:	63
Docformat:	rst rst

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