3. THE SUFFICIENT AREA CONDITION
ON A SLOP
Suppose: a) the ground E, has a slope, whose
maximum € lies in the direction D: b) there is an object
on E. (figure 9(1)). What is the sufficient area condition
in this situation? It is known from geometry that the area
of the object on the ground should include an ellipse
(B,), whose long axis e, is in the direction D, and
whose short axis e, is horizontal (figure 9(2)):
OR
eme AR (7)
cos €
so that the object can be detected at least by a complete
SPOT-detector-grid. The area of the ellipse Su: is easily
calculated:
y af Re Dei?
WT =
E
= (8)
COS E
COSE
The formula (8) is identical with the formula (6), if € =0.
Therefore the formula (8) has universality.
4. CONCLUSIONS
The sufficient area condition guarantees that an object on
the ground can be detected at least by a panchromatic
(multispectral) SPOT-detector-grid and recorded as a
complete pixel in a SPOT-image.
This condition is that the object's area on the ground must
include an ellipse, whose long axis e, is in the direction
with the greatest ground slope € and equal to 2R/
cos€ , and whose short axis e, is horizontal and equal to
2 R. If the ground is smooth (€ =0), then this ellipse
becomes a circle with the radius R (R =10/2m for a
panchromatic SPOT-image; or 2042 m for a multispectral
SPOT-image).
The above conclusions may be applied to other satellite
images, if they are similar to SPOT-images in the aspects
of imagery principle and pixel form.
Here it should be particularly pointed out that an object,
which can be surely identified or which can become at
least a identifiable pixel on a SPOT-image, should meet
not only the sufficient area condition, but also an other
condition: there should be a considerable difference in
spectral responsivity between the object and the other
objects around it. The difference condition should be
studied in the future.
REFERENCES
Begni, G., 1988. SPOT image quality: Twenty months of
experience. Int. J. Remote Sensing 9(9), pp. 1409-1414.
Cheng, F. and Thiel, K.-H., 1995. Delimiting the Building
Heights in a City from the Shadow on a panchromatic
SPOT-Image: Part 1. Test of 42 Buildings. Int. J. Remote
Sensing, (16)3, pp. 409-415.
Hartl, Ph. and Cheng, F., 1995. Delimiting the Building
Heights in a City from the Shadow on a panchromatic
SPOT-Image: Part 2. Test of a complete city. Int. J.
Remote Sensing, (16)15, pp. 2829-2842.
Dowman, |.J. and Peacegood, G., 1989. Information
content of high resolution satellite imagery.
Photogrammetria (PRS), (43), pp. 295-310.
Huertas, A. and Nevatia, R., 1988. Detecting buildings in
aerial images. Computer Vision, Graphics and Image
Processing, (41), pp. 131-152.
Jensen, John R.; Narumalani, Sunil; Weatherbee, Oliver
and Mackey, Halkard E., 1993. Measurement of Seasonal
and Yearly Cattail and Waterlily Changes Using Multidate
SPOT Panchromatic Data. Photogrammetric Engineering
and Remote Sensing, (59)4, pp. 519-525.
Moore, H.D., 1989. SPOT vs Landsat TM for the
maintenance of topographical databases. ISPRS J.
Photogrammetry and Remote Sensing, (44), pp. 72-84.
Manavalan, P.; Sathyanath, P. and Rajegowda, G. L.,
1993. Digital Image Analysis Techniques to Estimate
Waterspread for Capacity Evaluations of Reservoirs.
Photogrammetric Engineering and Remote Sensing,
(59)9, pp. 1389-1395.
Figure 1. A SPOT-detector-grid
(a - side length; d - diagonal length)
y
| | | |
| | | |
sem es tjr = > = jh ze je ae ss i elf mo +
| | | |
etl jo v0 (CB
| | |
| # (8/2, a/2)
—— hmmm DCE BCE —
| tg | P ek ta |
+ | | x
| | 95-3 |
SL ZN deua
| | | |
| | | |
vis bc. i4 I= Bl
| Lobo t5
wn so nl ee Sm et fre mp
| | | |
| | | |
Figure 2. The original position of the grids
(o - object’s center; ty, ta …- eight neighbours of p)
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
