Index
Map Pairs Analysis Tools III
Boolean operations
A-Z
To be continued


Zonal Operators

Let our interest now focus on maybe distinct zones: what is the maximum height throughout all the areas of vegetation class 16? What is its mean height? Or even more expressive - its range?

EXTRACT helps you in finding a solution to that spatially complex questions:

During several lessons I often found it a bit difficult for students to grasp what 'zones' are, when they first heard about it. No magic around, so - de-mystifying - zones are nothing but 'a set of data pertaining to one of the variations' in an image (TOMLIN, p. 10). So all the pixels with value 16 in our image together define the zone for value 16.

Areas belonging to a zone may vary in shape, they may be scattered all over our image or just be represented by one patch. Important is: the 'zone' embraces all those patches and areas in our image as a whole for the calculations.

What's the maximum summary type doing? It looks for the least value throughout every zone. How does IDRISI get to know about zones? By entering a 'Feature definition image' (look at dialogbox). Where do the values (for the maximum values search) come from? They are stored in the image we type in 'Image to be processed' (dialogbox). As a result IDRISI faces us with a table or a values file with the summary-listing for the maximum-values for each zone.

So remember the sample images from the preceding chapter about the boolean operations. Now let us pick up such a 'zone' from the TAURVEGE dataset, e.g. vegetation class 16 and try to answer the question in the first paragraph of this page. For convenience the resulting tables from 4 subsequent EXTRACT-sessions have been put together:

Maximum values
extracted from c_dem
based on c_vege

Category       maximum
----------------------
  0        0.000000
  1     3005.000000
  2     2768.000000
  3     2752.000000
  4     2585.000000
  5     2563.000000
  6     2371.000000
  7     2306.000000
  8     1597.000000
  9     1326.000000
 10     2053.000000
 11     2483.000000
 12     2316.000000
 13     2107.000000
 14     2288.000000
 16     2217.000000
 18     2210.000000
 19     2151.000000
 20     2146.000000
 22     1782.000000
 23     1493.000000
 24     1539.000000
 25     2064.000000
 26     1781.000000
 27     2248.000000
 28     1512.000000
 29     2955.000000
 30     2744.000000
 31     2827.000000
Range extracted
from c_dem
based on c_vege

range
---------------
0.000000
1456.000000
986.000000
1163.000000
648.000000
1187.000000
1054.000000
872.000000
104.000000
6.000000
251.000000
1038.000000
802.000000
76.000000
776.000000
881.000000
485.000000
513.000000
774.000000
259.000000
59.000000
40.000000
605.000000
223.000000
790.000000
18.000000
825.000000
1286.000000
1049.000000
Average values
extracted from c_dem
based on c_vege

average
--------------------
0.000000
2504.381592
2369.511963
2314.930420
2305.068604
2076.591797
1831.569946
1776.380005
1512.675293
1323.000000
1926.371460
2124.806885
1952.322754
2067.333252
1850.957520
1832.449463
1946.748413
1893.650879
1683.381470
1622.614868
1466.476196
1506.329468
1736.760986
1653.132080
1928.270752
1500.677734
2595.644043
2214.259521
2448.753174
Pop. SD extracted
from c_dem
based on c_vege
population
standard deviation
------------------
0.000000
210.497955
155.876114
146.568085
163.373474
232.300858
271.871185
245.223694
13.514922
2.449490
71.104385
135.756699
166.755127
23.584364
182.762695
147.938126
84.431808
109.676735
128.012512
57.986622
17.589550
7.656429
123.231606
42.241016
136.568451
2.839514
176.245438
275.286438
325.876984

From the results above we could say: vegetation class 16 climbs up to a height of 2217 m a.s.l.
But: perhaps the images we compared do not match exactly, perhaps there has been some error of the field team, perhaps ...


Index
Map Pairs Analysis Tools III
Boolean operations
A-Z
To be continued

last modified: | Comments to Eric J. LORUP idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis idrisi resource center idrisi gis idrisi gis