Table Of Contents [an error occurred while processing this directive] previous page [an error occurred while processing this directive] next page
Before leaving this 7 band review, we want to comment on a peripheral item but one that illustrates a typical danger and misuse to which space imagery has been subjected. Look at any of the reflectance bands images (but well defined in bands 5 and 7) at two points marked w. You will see two thin dark linear features that are almost straight. These "linears" as some have called them are common phenomena observed in satellite images and in aerial- or astronaut-camera photographs. Peruse the rest of the image you've chosen and you will find more (although some are actually sharp boundaries between two areal features or classes). In the early days of Landsat applications, many geologists reported that the MSS (and, later, TM) sensor was especially adept at singling out such linear features. They chose to identify a significant fraction of these features as geological in nature, being either faults or fracture systems. Maps showing numerous linears of presumed faults/fractures were produced and often published but too frequently without appropriate field checking. When several exacting studies discredited this interpretation of many such features (although some and sometimes a majority were correctly verified), this use of Landsat led to widespread skepticism and negative criticism. Today, caution is the watchword and proper inspection in the field is demanded. Incidentally, the nature of the two linears at w has not been established at the time this section was written but one is probably vegetation lining a narrow gully. The fact that the two line up may be a coincidence and does not prove any commonality.

False Color View

We are now in a position to upgrade our image processing by generating first some color composite combinations and then exemplifying two special types of end product - principal components images and classified images.

Lets start by producing the now-conventional false color composite, made by assigning TM band 2 (green) to the blue electron gun in the monitor, band 3 (red) to the green gun, and 4 (the near or photographic reflective IR) to the red.

  • TM Band 4 = red
  • TM Band 3 = green
  • TM Band 2 = blue

Two color patterns dominate the land classes: reds, depicting vegetation, and medium grayish-browns, found mainly along the bright sun-facing slopes. The ocean and the bay are evinced in deep blues that, nearshore (a), become a bit lighter where thicker sediments add reflectance. The breakers are presented in mottled blue and white patterns.

The various expressions of vegetation can be placed in several categories based on their specific red tints and in most cases also from the spatial patterns they occupy. The continuous and rather deep red at g represents a segment of the forested areas in the Los Padres National Forest near the crest of the Santa Lucia Mtns. Elsewhere, as at i, thin strings of red or irregular red patches (j) can be largely attributed to trees and/or scrub vegetation (l) lining stream channels or scattered as copses and patches along the hillslopes. In the valleys bright red areas, some rectangular and others more uneven, at k and other points are primarily examples of field crops, hay meadows, or other types of cultivation. Areas believed to be barren to varying degrees, as at m and o, have darker gray-brown tones but may have faint pink overtones implying limited vegetation cover. Where vegetation is spare and scattered on the hills, particularly where well illuminated by the sun, the prevailing tan to grayish brown colors imply joint contributions from underlying soils combined with reflectances imparted from the brown grasses (with much diminished band 4 input). However, this last statement must be tempered by the fact that the image produced for this same scene by EOSAT (not reproduced here but examined by the writer) shows more pinks over most of the grassland areas than does the image on your screen. At the time of scene acquisition (mid-November), if winter rains had started early, and have been only moderate to this date, the hills would still be relatively brownish but the EOSAT rendition suggests some greening has started.

Table Of Contents [an error occurred while processing this directive] previous page [an error occurred while processing this directive] next page

Code 935, Goddard Space Flight Center, NASA
Written by: Nicholas M. Short, Sr. email: nmshort@epix.net
and
Jon Robinson email: Jon.W.Robinson.1@gsfc.nasa.gov
Webmaster: Bill Dickinson Jr. email: rstwebmaster@gsti.com
Web Production: Christiane Robinson, Terri Ho and Nannette Fekete
Updated: 1999.03.15.