A Gallery of Plots

The IDL code needed to create each plot is shown. The color table used is obtained by
loadct, 4
when in 256 color mode. In some cases the color has been modified using the S1R library routine ctool. The example code is written so that it may be swept up with a mouse and entered into an IDL session to duplicate the plot.

A textured sphere

Using nothing more than the two routines sphlat and sphlng a number of interesting plots may be made.

  window, xs=300, ys=300
  !p.position=[0,0,1,1]
  set_isoxy, -1.1, 1.1, -1.1, 1.1

  erase, 80 
  sphinit, long=-30, lat=40, rad=1, fill=140, pa=30 

  for lng= 0, 355, 5 do $ 
  for lat= -90, 90, 5 do $ 
  sphlat, lat+(lng/5 mod 2)*2.5, 1, lng, lng+5 

  for lat= -85, 85, 5 do $ 
  for lng= 0, 355, 5 do $ 
  sphlng, lng+(lat/5 mod 2)*2.5, 1, lat, lat+5

Shape preserved texture mapping

Here is an algorithm that preserves the shape of a texture element at any latitude.

  window, xs=300, ys=300
  !p.position=[0,0,1,1]
  set_isoxy, -1.1, 1.1, -1.1, 1.1

  erase, 30 
  sphinit, long=-30, lat=40, rad=1, fill=50, pa=30 

  lat1 = 0.
  step = 10.
  c = 210
  i = 0

  while lat1 le 85 do begin &$
    d = (i mod 2)*step/2. &$
    lat2 = lat1 + cos(lat1/!radeg)*step &$
    for lng = 0, 355, step do sphlng, lng+d, 1, lat1, lat2, col=c &$ 
    sphlat, lat1, 1, col=c &$
    d = ((i+1) mod 2)*step/2. &$
    for lng = 0, 355, step do sphlng, lng+d, 1, -lat1, -lat2, col=c &$ 
    sphlat, -lat1, 1, col=c &$
    lat1 = lat2 &$
    i = i + 1 &$
  endwhile

  sphinit, long=-30, lat=40, rad=1, pa=30, color=c

For a given step in longitude, the equal sized step in latitude is given by
cos(lat)*step
where lat is the latitude in degrees.

A solid angle in the sky

  window, xs=300, ys=300
  !p.position=[0,0,1,1]
  set_isoxy, -1.1, 1.1, -1.1, 1.1

  erase, 30 

  sphinit, long=0, lat=40, rad=1, fill=140, pa=30
  for lng=-70,70,5 do sphrad,lng,60,0,1

  sphinit, long=5, lat=45, rad=1, pa=-30
  for lng=0, 360, 10 do sphlng,lng,1,color=100, thick=2
  for lat = -80, 80, 10 do sphlat, lat, 1, color=100, thick=2

  sphinit, long=0, lat=40, rad=1, pa=30, color=100
  for lat=60,90,3 do sphlat,lat,1, thick=2

The geometry of SAR satellite images

This figure is not drawn to scale but was created to show the geometry involved in Synthetic Aperature Radar (SAR) imaging from a satellite. The satellite itself is marked by * and is moving in its orbit downward toward an equator crossing. Its radar beam is pointed to its right and scans out the highlighted strip of images. Each image line is seen to actually be one of a number of great circles that converge at the point 90 degrees from the suborbital track. This point is fixed relative to the orbit but the earth moves beneath it giving it a westward motion on the earth's surface. An IDL routine was used to generate this figure.