For this example we choose bare land, forest, grass land, snow, and water body as target ground features. Now we have the 224 intensity value for every target ground feature. The spectral curves of them are shown in figure1 (color). The intensity, K value, and the divergence degree of these 5 target features in some typical channels are shown in Table 1.


Figure 1(col). Spectral curve of the 5 target ground features
Sum up the 3 correlation coefficient. Then sort all of the sum from minimum to maximum. The combination at or near the top are the best band combination. Table 2 mainfests the ten least correlative degree combination from all the channels with the divergence degree ³0.9. These are the ten best band combinations.

Table 1. The I, K and Divergence Degree Of the 5 target feature in some typical channels

CH	Ibare	Ishow	Iforest	Igrass	Iwater	Imax-Imin/2N	DD
2	1516	2889	549	562	813	234	0.9
24	2656	18279	1035	1386	4360	1724.4	0.6
60	1447	4952	1567	4689	207	474.5	0.8
71	1771	3141	1786	5275	117	515.8	0.9
175	165	.7	10	15	9	15.8	0.4

CH……..Channel DD………Divergence Degree
Table 2. The top best band combinations

CH1	CHm	CHn	DD1	DDm	DDn	rlm+rln+rnm
2	71	126	0.9	0.9	0.9	0.404684
2	70	126	0.9	0.9	0.9	0.405569
2	72	126	0.9	0.9	0.9	0.409384
2	69	126	0.9	0.9	0.9	0.420881
2	73	126	0.9	0.9	0.9	0.421018
2	71	127	0.9	0.9	0.9	0.441627
2	70	127	0.9	0.9	0.9	0.442508
2	68	126	0.9	0.9	0.9	10444903
2	72	127	0.9	0.9	0.9	0.446218
2	74	126	0.9	0.9	0.9	0.447456

CH ...Channel DD ...Divergence degree
Among the combination with sum of correlation coefficient bigger than 0.404486 and smaller than 2.0, we have chosen more than a hundred combination for comparison. The result is remarkable. The combination at or near the top of lost have very good display effect, with bright color and sharp contrast for the selected target feature. It clearly shows the following general rule: The smaller the sum of correlation coefficient is the better the image looks. The results of 2 typical combination are shown in figure 2 (color) and Figure3 (color).


Figure 2(col). CH126(R)71(G)2(B), Sum of correlation coef=0.404684

Figure 3(col). CH69(R)46(G)2(B), Sum of correlation coef=1.489137
Conclusion and Discusion

1. The method discussed above take the following two important factors, spectral curve divergence degree and channel correlation degree, into account. The result shows the RGB band combination chosen based on these principles is effective.
2. Because the method is ground feature oriented, it is applicable for different hyper spectral data user with different application needs.
3. The divergence degree threshold value is adjustable. If the candidate channels for correlative calculation are too few, the divergence degree threshold value could be set to a lower value.
4. Target ground feature can be sampled either by point or by small area. We prefer by area for its average effect.
5. Channel 1, channel 107-113, channel 154-167, and channel 221-224 of raw data area bad data. They are not considered during the calculation.

References

• Rothery, D.A., Decor relation stretching as an aid to image interpretation International Journal of Remote Sensing, Vol. 8.