True Color View - Lecture Material - Completely Remote Sensing tutorial, GPS, and GIS -
True Color View

The next scene displays (sometimes the verb "map(s)" is used instead) the Morro Bay region in quasi-natural color, achieved using the IDRISI processing system by assigning TM Band 1 to the blue, Band 2 to the green, and Band 3 to the red color source.

The Morro Bay subscene rendered in an approximation to true color. Spurious colors, such as the pinks in the hills, indicate that color reality is somewhat compromised.

In practice, we use various color mapping algorithms to facilitate visual interpretation of an image (some types of analysis by computer work by manipulating the original DN [digital number] values of the pixels without necessarily producing direct imagery). The original DN values contain all of the information in the scene and though their range of values may make it necessary to remap them (such as by the contrast stretching described later in this Section) to create a good display, that doesn't always add much new information. In fact, although visual interpretation is easier with the remapped image, remapping potentially can lose and distort information; thus, for some kinds of analytical work, we may prefer to use the original DN values or DN values adjusted to calibrated radiances.

With this mapping, we see a pleasing and satisfying image because it depicts the world in the general color ranges with which we are naturally familiar. We can imagine how this scene would appear if we were flying over it at a high altitude.

We might comment that the ocean appears too dark in the IDRISI TM composite. This darkness is due in part to its general darkness in all of the bands, the consequence of water absorbing light from most of the visible range. (To a lesser degree, the algorithm that IDRISI uses to combine three 8-bit bands into a single 8-bit composite image also may compromise the image color display). When actually next to or above the ocean, we see blue tints partly because of some angular reflection of blue light and because of reflection of blue sky color when clouds are sparse; the ocean can in reality appear in some other colors to the eye (as for example a darkish jade green in the Caribbean). The notable exception is, of course, related to the breakers (here, with little internal structure) that are highly efficient at scattering light; there is much whitish tones in waves when standing near them. The sediment patterns as we noted vary through bands 1 to 3. Areas with higher sediment content are clearly separable (rather than gradational) in this color rendition, because areas of greater load density have some green band input (Band 3 [red] shows only the densest sediment areas). Note especially the dark brownish-green tones by the delta where the coastal marsh develops, interpreted as a heavy load dumped at the mouth of the small river (p) flowing over it.

Probably the most unrealistic color in the rendering of the IDRISI version of this "true color" scene is the bright hill slopes. In this rendition, the base color is tan with some purplish overtones, but there are small areas and patches marked by a darker brown tone which, if inspected at ground level would have some green tints. The color composite shows these surfaces in a somewhat misleading way. From an airplane, we might describe these surfaces as a light golden-brown (from the dry grass, one reason California is often called the "Golden State"), similar to the terrain shown below which matches the above true color mapping more closely.

The golden brown hills near Morro Bay, in summer.

The reason for the red-brown-purple blotches in the natural color composite is that light from surfaces at certain orientations contributes considerable red from Band 3, some blue (hence purple) from Band 1, and little green from Band 2.

The areas we presumed to be active grasslands (v) appear in "true color" as light tones that are slightly reddish brown. Areas correlated with fields and meadows are a medium brown. Some areas that were associated with medium to dark reds in the false color composite are medium green in the natural color version. This is particularly valid for the National Forest growth around (u), the stream-lining vegetation (i), and the wooded patches (l) scattered throughout the scene. However, many false color red areas do not show as green in this natural color version. The diminution of green color is explainable as follows: most of this vegetation is either California oaks, which have dark green leaves, or perhaps some evergreens which tend to be less reflective than deciduous plants, thus the dark brown color. These leaves, or needles, have generally lower reflectance in Bands 1-3 but still are somewhat proficient reflectors (from cell walls) in Band 4. But, since the natural color composite only uses Bands 1, 2, and 3, there is little color contribution from any of these in the image above.

The coastal marsh (o) appears as a dark brownish-red in this true color rendering (as it seems to do in the aerial oblique photo). At least part of this effect may link to types of vegetation which were envisaged in the photo shown on page 1-2 that indicates reddish tones in many of the plants. Another view of the marsh, with its outlet on the bay, is presented here; note the rather brownish tones over much of its expanse:

Looking southeast across part of the large marshy flatlands found south of the town of Morro Bay.

Now, lets return our attention momentarily to the mystery feature. It appears again in this high altitude aerial photo of Morro Bay (town and water) distributed by the U.S. Geological Survey:

Morro Bay aerial photo.

In the natural color Landsat subscene at the top of this page, the mystery feature displays as a small patch of bright green. We saw earlier in the black and white enlargement that the feature is arranged roughly in elongated strips. These strips do not stand out in the natural color image, partly because at the size on the screen, the resolution is insufficient to bring out the strip pattern. From the top of the page, you now know (or confirmed) that this feature is an 18-hole golf course. The aerial photo shows these public links at its bottom center. Hole Number 18 is pictured in this ground view:

Hole No. 18 of the Morro Bay Golf Course - part of the mystery feature.

Let's repeat the Landsat enlargement in false color and then show a map of the greens as printed on the official scorecard picked up at the local Pro Shop. It is rotated 90° clockwise to align with the Landsat subscene.

The Morro Bay Golf Course.
Map of the 18-hole Morro Bay Golf Course (east to the right)

We have also extracted the portion of the digitized aerial oblique photo (page 1-1) that contains this tract, known as the Morro Bay Golf Course on State Park Road. This photo shows the lighter green fairways amidst darker green trees in close-up, mainly in the center. One patch of trees (many being the Eucalyptus tree, imported from Australia, that has spread so widely that it is almost a commonplace pest), hides luxurious Condominiums built since the aerial photo was taken. The two bright patches within the photo are sand traps. They are not visible in the TM subscene. This is surprising, in that even if they are actually smaller than the 30 m resolution of a TM pixel containing one of them, the great brightness of each should have swamped the reflectance response of other classes in that (mixed) pixel, making it appear overly light. This same phenomenon was observed early in the Landsat program where small [less than 8 m {26 ft} wide] light-toned dirt roads passing through dark sage brush or grasslands could be resolved because the pixels containing these pathways were substantially brightened.)

There are several parts of the natural color image that contain distinctive orange-brown tones (as at x). This coloration is apparently due to soils that have a rich brown color. We also see this brown tone in the valley north and east of Los Osos and in part of that town. This valley appears to have its own small stream, which is not easily resolvable, or perhaps it once had a larger stream, which is now gone. Such a valley is probably underlain by alluvial deposits that typically are browner (enhanced by hydrous iron oxides similar to rust).