If you've ever tried to display a large (say 1000x1000 pixel) satellite image on a base map in IDL, then you've probably been frustrated at the lack of an easy-to-use tool. I spent many hours playing with TRIANGULATE, TRIGRID, POLY_2D, and POLYWARP, and never found a satisfactory solution (let me know if you do!).

Hermann Mannstein suggested a solution in response to a question I posted to the IDL newsgroup. First, MAP_SET is used to define a map projection in the current display window. CONVERT_COORD is then used to copy pixel values from the input image to the corresponding location on the map projection. Finally DILATE is used to fill any holes in the resampled image.

The results turn out quite well, considering the simplicity of the algorithm. Remember, I'm talking about a display tool here, not a robust resampling. Some examples for different sensors are shown below.

Usage Tips

• IMAGEMAP assumes that you are using an 8 bit display. To ensure that this is true for X displays, enter this command immediately after starting IDL.

device,pseudo=8,retain=2,decomposed=0

On Windows and Macintosh boxes, you will need to configure the system outside of IDL to use an 8 bit (256 color) display.This is worth doing anyway, in order to get color tables to work reasonably in IDL (most IDL programs work best on 256 color displays).

• Before you try setting any of the optional keywords, let the program do the hard work, e.g.

imagemap,image,lat,lon

Then customize the result using the keywords (e.g. RANGE, MISSING).
 
• IMAGEMAP can do a reasonable job on low resolution data like HIRS or MSU. What you need to do is resample the image, latitude, and longitude arrays to higher resolution. The trick is to resample the image using nearest-neigbour sampling, but to use bilinear interpolation for the latitude and longitude arrays, e.g.

; HIRS channel 20 reflectance (0.0-1.0) and corresponding lat/lons

help,image,lat,lon

IMAGE FLOAT = Array(56, 40)

LAT FLOAT = Array(56, 40)

LON FLOAT = Array(56, 40)

imagemap,image,lat,lon,range=[0.0,1.0],$

  title='HIRS Channel 20 - Unresampled'

map_continents,/hires ; Bad result

; Now resample the arrays to 10 times their normal size

sz=size(image) & nx=sz(1) & ny=sz(2) & mag=10

imagemap,congrid(image,mag*nx,mag*ny),$

  congrid(lat,mag*nx,mag*ny,/interp),$

  congrid(lon,mag*nx,mag*ny,/interp),$

  range=[0.0,1.0],$

  title='HIRS Channel 20 - Resampled'

map_continents,/hires ; Good result

• You can create nice Postscript output like this (assuming you have an 8 bit display):

device=!d.name & tvlct,r,g,b,/get

set_plot,'PS',/interp

device,/land,/helv,font_size=8,bits=8,/color,file='imagemap.ps' & !p.font=0

imagemap,image,lat,lon,missing=255

device,/close & set_plot,device & tvlct,r,g,b & !p.font=-1

• If you are using !P.MULTI to create multiple images on one page, make sure you reset it's value to start a new page:

;- Make images the first time

!p.multi=[0,1,2,0,0]

imagemap,image1,lat1,lon1

imagemap,image2,lat2,lon2,/noerase

;- Make images the second time

!p.multi=[0,1,2,0,0]

imagemap,image1,lat1,lon1

imagemap,image2,lat2,lon2,/noerase

• AVHRR
• HIRS GIF, Postscript
• GOES
• MODIS Airborne Simulator
• MODIS, MODIS Ancillary Data