10.7 Operations between Images

10.7 Operations between Images

Operations between multi-spectral images or multi-date images are very useful for image enhancement and feature extraction.

Operations between images include two techniques; arithmetic operation and logical operation.

a. Arithmetic Operations
Addition, subtraction, multiplication, division and their combinations, can be applied for many purposes, including noise elimination. As the results of the operation can sometimes negative or small values between 0 and 1, they should be adjusted to a range, usually in eight bits or 0 to 255 for image display.

Typical operations are ratioing, for geological feature extraction, and normalized difference vegetation index, for vegetation monitoring with NOAA AVHRR data or other visible near infrared sensors.

(1) Ratioing
Ratio = Xi / Xj

Ratioing may be useful for geological feature extraction. Such ratioing can be applied to multi-temporal thermal infrared data for extraction of thermal inertia.

(2) Normalized Difference Vegetation Index(NDVI)

where ch.1 : red band
ch.2 : infrared band

NDVI shows as a high value for denser vegetation, while the NDVI is very low in desert, or non-vegetation regions.

Figure 10.7.1 shows two examples of arithmetic operations.

b. Logical Operation
Logical addition (OR set ), logical multiplication (AND set), true and false operations etc. can be applied to multi-date images or a combination of remote sensing images and thematic map images.

For example a remote sensing image or the classified result can be overlaid on map data, such as political boundaries.

Figure 10.7.2 shows an example of forest land change by overlaying a remote sensing image on the forest land which has been mapped from the old map. Such an overlay will be very useful for change detection.