Index
Datatypes	A-Z	Vector Files
		Raster Documentation Files

IDRISI names raster files as images. Each image consists of a defined count of rows and columns thus forming cells. These cells are stored as a sequence of numbers (byte, integer or real) representing values (vegetation classcodes, reflectance numbers, political units, z-values in a DEM, ...).

Let us imagine a very simple image:

	0	1	2	3	4
0	22	22	18	18	18
1	15	15	18	16	16
2	11	15	15	18	16
3	11	15	12	12	12

5 columns, 4 rows. The values may represent some code for land usage. IDRISI is starting in the upper-left corner (row 0/column 0), then advances column by column and row by row. In the simplest format - ASCII - the cellvalues are stored one in each line:

22
22
18
18
18
15
15
(...)
11
15
12
12
12

Commonly the images are stored binary, one value after the other. Depending on the datatype a value occupies more or less of memory. Simple RLC (run length compression) is supported as packed binary (i.e., the cellvalue is followed by the number of occurrences in the series):

• not packed: 22 22 18 18 18 15 15 18 16 16 11 15 15 18 16 11 15 12 12 12
• packed: 22 2 18 3 15 2 18 1 16 2 11 1 15 2 18 1 16 1 11 1 15 1 12 3

The table shows IDRISI's datatypes (except compression they apply to vectorfiles as well):

	memory required	range	compression
byte	1 byte	0 to 255	yes
integer	2 bytes	-32768 to +32767	yes
real	IEEE 4 bytes	±1*10 38, 7 significant figures precision	no

Evidently the image 'does not know' about its dimensions or about the area one cell covers in real. So we need a 'header file', the documentation file, which corresponds with a image.

	Index
Datatypes	A-Z	Vector Files
		Raster Documentation Files