IDRISI INTRODUCTION -ver 3.2 
Raster GIS programs do especialy well with surface data such as that which can be collected with remote sensing instruments. A popular raster program, IDRISI, is very good at this. We will be using just a few parts of an early version of IDRISI, but it will give you a feel for the raster approach. IDRISI converts between rasters and vectors, but does its best work on pixels.

You will be combining maps of soils and vegetation in Africa to arrive at a soil map of high-vegetation areas.

This version of IDRISI is set up with a main menu on the left side of the screen, and submenus that appear to the right. You move from the main menu to the sub menus with [ENTER]. Go from sub back to the main with [ESCAPE]. The submenus are organized by groups of related procedures. DISPLAY MODULES, for example, offers many different ways to display data.

You will be using two raster files: SOIL -- a map of Africa derived from the FAO 1:5,000,000 soil map of the world digitized by the UN Environment Programme Global Resources Information agency. There are 135 classes of soils in this database.

VEG -- this is a vegetation layer from the NDVI (normalized difference vegetation index) from AVHRR satellite. Brighter colors = higher vegetation. Colors range from 1 to 16, with the lower numbers indicating less vegetation.

And two vector files: COUNTRY -- a map of inland country boundaries (Africa) COASTS -- a map of coastal boundaries (Africa)

  1. In Windows either use windows explorer (or file manager in 3.x) and start the file c:\idris\idris.exe.
  2. Display the raster maps.
      3. Use DISPLAY MODULES, COLOR, specify VEG, and then take the default values for all other options ( just ENTER past them). This will give you a map of low- to high-veg. areas.

      When finished viewing a map, press the return key to get back to the main menu.

      Also look at the soils map (SOIL) with COLOR. Notice that you will have to page down to see the various soils. Soil types not displayed will appear as green.

      Now look at SOIL with COLOR A. The "A" stands for AUTOSCALE. COLOR A will compress 135 different soils into 15 equal-range categories for display only. Notice that you only get one map with COLOR A: No paging down. But, several real soil types are assigned the same color.

  3. Isolate one soil type
      4. Go to DISPLAY MODULES, HISTO (histogram), and specify SOIL, class width of 1, graphic output. You will see the same data you saw in map form except that now it is in a bar graph showing numbers of pixels with each value. Soils in many pixels are displayed with taller bars. Arbitrarily find a soil you are interested in. You may need to use the histogram again and narrow your view by changing the maximum and minimum values. For my example, I'll use soil type 47, which is apparently common in Africa.

      Now go back to the main menu and choose GENERAL ANALYSIS, RECLASS. Reclass is a way to reassign values from any raster image to a new file. Specify the file SOIL to reclassify and name the output file "xxsoil46," with the being your initials. Choose the "USER DEFINED CLASSIFICATION" and read the menu. You are going to make a new file with 0s and 1s only (the reason will become clear in a moment). I'll give all soils with 47 a new value of 1, and all other soils 0. To do this I'll assign a new value of 0 to soils 0-just less than 47, a value of 1 to those 47 to just less than 48, and a value of 0 to old values 48 to just less than 140. I'll end the routine with "-1". Then display the the histogram (with HISTO) and the map (with COLOR). Do you know what to expect?

  4. Multipy two maps together.
      5. Back in the GENERAL ANALYSIS submenu, choose OVERLAY. What we want to do is show vegetation intensity on only one soil type. To do this we will remove the vegetation from all other soils and keep the vegetation information for soil type 47 only. Because 0 times anything is 0 and 1 times any number is that number, we'll multiply xxsoil47 (0s and 1s) by the vegetation map "veg." Name the new file xxvg_s47 with your initials and the soil's number. Give it a descriptive title.
  5. Display the new map
      6. Now display xxsoil47 with COLOR with the legend. Do you know what to expect?
  6. Add political boundaries
      7. Still in the COLOR with your map showing xxsoil47, add the vector outlines COUNTRY and COASTS to the map. Do this by pressing "v" and naming these vector files. Assign colors (from 2 to 15) to them, referring to the palette on the right side of your screen.

      SCREEN CAPTURE! You will publish this map in your web page.

  7. Look at the data for the new map
      8. Also look at xxsoil47 with HISTO graphic output. To be more precise, use the numeric output for HISTO.

      SCREEN CAPTURE! You will publish this map in your web page.

EXAMPLE

This map of vegetation intensities on soil type 47 in Africa was created with Clark University's IDRISI ver 3.2. First, soil type 47 was isolated with the reclassification routine, and the resulting data (0s and 1s) were multiplied by data indicating intensity of vegetation. The resulting map shows intensity of vegetation only over soil type 47 in Africa. Black areas indicate not soil type 47, not Africa, or not land mass.

Vegetation in Soil Type 47
The histogram shows the same data in graphical format.

Histogram of Soil47


© 1997-99 Erick Howenstine