After
you clearly understood the
problem that needs to be solved,
you need to determine or choose the right imagery to be obtained for extracting the information we need. There
a large number of data types are available to suit various needs.
As Examples
1. If
we are studying thrust belts, we may wish to order 1:1,000,000
B/W single-band Landsat multispectral scanner (MSS) images with 80 m resolution and large area coverage
(185 × 185 km) to make a mosaic of
part of a continent.
2. If
we are doing a basin analysis, then a color MSS image at
1:250,000 or 1:500,000 should be
acceptable.
3. If
we are mapping details of lithologic and facies
changes, or vegetation patterns and wildlife habitats, we can use color SPOT
multispectral (XS) imagery
with 20 m resolution and 60 × 60 km (vertical view) coverage at a
scale of 1:100,000 or 1:50,000, or
Color Landsat
Thematic Mapper (TM) imagery,
with 30 m resolution, and covering 185 × 185 km at a scale of
1:100,000 would also work.
4. For
mapping alteration associated with
mineral deposits, the ideal choice is high
resolution (1–10 m) airborne hyperspectral imagery. If that is not
available or is too expensive and the area is large and remote, we may wish to
use Landsat TM for a reconnaissance look.
5. If we want to know where a well was drilled
several years ago in a poorly
mapped part of the world, we chose a SPOTP
high resolution (10 m) panchromatic
image or 5 m resolution.
6. For very fine detail, GeoEye
or WorldView images have approximately >
1 m resolution. These can be enlarged to > 1:25,000 and still appear
clear and sharp.
The following factors
should be taken into account when ordering data (Dekker, 1993).
1. Cost
One factor to keep in mind is that the smaller the area covered, the higher the cost per unit
area. Airphotos or airborne imagery will
almost always cost more per
unit area than satellite images.
2. Timing
If the imagery is needed immediately, one can screen grab free but rather low-quality imagery from internet
sources such as Google Earth or purchase off-the-shelf data from a
government agency or vendor. Large image archives exist, and data can
often be obtained quickly. Purchasing custom images from
vendors and consultants, or
processing digital data in your own shop can take
up to several weeks.
3. Coverage
A- Large
area coverage can be obtained using weather
satellites such as GOES (covers a
full hemisphere), the Advanced Very High Resolution Radiometer, which covers a 2700 km swath on the
Earth’s surface, or the SeaWIFS instrument,
with a 1502–2801 km swath width.
B- Moderate
size areas can be covered using some handheld Shuttle
(and other mission) photos (variable area coverage), as well as Landsat images (MSS and TM), which cover 185 × 185
km. Systems that cover 50 × 50 to 500 × 500 km include MK-4
photos (120–270 km), KATE-200 photos (180
× 180 km), KFA-1000 photos (68–85 km), and the SPOT systems (XS and P) that cover 60 × 60 km.
Satellite images generally
cover larger areas than airborne photos or images, and the synoptic
view is one of their greatest advantages.
C- For
field or local studies, airborne surveys or
small area satellite images will save cost and/or provide more detail. Recent
satellites, such as Worldview 2, 3 and GeoEye-1, have
very high resolution (up to 31 cm for the
panchromatic sensor and 1.65 m for the multispectral instrument) and
cover correspondingly smaller areas (16.4 and 15.2 km swaths, respectively).
4. Resolution
The scale of the final image will to
some extent be a function of the sensor system resolution, in that one cannot
enlarge, say, an image with 80 m resolution to a scale of 1:100,000 without the
image becoming “pixelated,” that is, breaking up into the individual resolution
elements that appear as an array of colored squares.
5. Nighttime Surveys
Thermal and radar
surveys can be flown effectively at night because neither
system relies on reflected sunlight: the radar instrument illuminates the surface by
providing its own energy source, and thermal energy is radiated from the
surface.
Predawn thermal imagery reveals,
among other things, lithologic
contrasts related to differing rock and soil densities or color
tones (light versus dark). Nighttime thermal imagery can
reveal shallow groundwater and moist soil
(generally warmer than background), can detect oil
spills on water, and can help map
underground coal mine fires. Because radar illuminates the ground with
microwaves, it can be flown at night to map oil
spills, for example, or during polar night to map
the movement of ice floes that could threaten an offshore oil rig or platform.
6. Seasonal/Repetitive
Coverage
Certain seasons are better for
specific surveys. For example, a geologic mapping
project in an area covered by temperate forest would see more of
the ground in spring before deciduous plant leaf-out or in
the fall, after leaves have dropped.
High sun elevation
angle (summer) provides images with the best
color saturation, which can be useful when
mapping lithologies in low contrast areas.
On the other hand, low sun angle images (flown
during the morning or in winter), especially with a light snow cover, enhance geologic features in low-relief terrain.
If repetitive coverage is needed to monitor natural (e.g., flooding, ice floes) or man-made (e.g., drilling, roads) changes, it is often most
cost effective to use satellites because of their regular repeat cycles.
Repeated aircraft surveys provide more detail but
are much more costly.
7. Relief
Low-relief terrain may require low sun angle or grazing radar imagery to enhance subtle topographic and structural features.
On the other hand, high-relief terrain poses the potential problem of
large shadowed areas that can obscure important areas or details. These areas
should be flown during mid-day or using radar with
a steep depression angle to minimize shadows.
8. Vegetation Cover
Color infrared images are very sensitive to changes in vegetation type or
vigor, since the peak reflection for vegetation is in the near infrared
region. Combinations of infrared and visible
wavelengths have been used to map
changes in vegetation related to underlying rock types and even hydrocarbon seepage
(Abrams et al., 1984).
Lidar which will
also provide an image of the top of the vegetation canopy that looks like
topography. There are some
image processing methods, called “vegetation
suppression” techniques, which appear to remove
vegetation and reveal subtle changes in the underlying soil or bedrock.
These algorithms tend to remove the reflectance attributed to vegetation and
enhance the remaining wavelengths.
9. Water-Covered Areas
Shorter wavelengths (blue and
green light) penetrate water farther than longer
wavelengths. The euphotic or
light-penetrating zone is known to extend to 30 m in clear water
(Purser, 1973).
Infrared light, which has
longer wavelengths than visible light, is absorbed
by water and does not provide information on
bottom features.
Landsat TM, with its blue band, is excellent for mapping shallow water features such as
shoals, reefs, or geologic structures. Likewise, true color and special
water penetration films such as Kodak Aerocolor SO-224 have
excellent water penetration capabilities (Reeves
et al., 1975). Side-scan sonar is available for shallow and deep water mapping, and produces images of the sea bottom reflectance using
acoustic energy, much like radar uses microwave energy to produce an image.
References
Abrams, M.J., J.E.
Conel, H.R. Lang. 1984. The Joint NASA/Geosat Test Case Project Sections 11 and
12. Tulsa: AAPG Bookstore.
Dekker, F. 1993.
What is the right remote sensing tool for oil exploration? Earth Obs. Mag. 2:
28–35.
Junge, C.E. 1963.
Air Chemistry and Radioactivity. New York: Academic Press: 382 p.
Purser, B.H. 1973.
The Persian Gulf. New York: Springer-Verlag: 1–9.
Reeves, R.G., A.
Anson, D. Landen. 1975. Manual of Remote Sensing, 1st ed, Chap. 6. Falls
Church: American Society of Photogrammetry.
U.S.
Bureau of Land Management. 1983. Aerial Photography Specifications. Denver:
U.S. Government Printing Office: 15 p.
