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olar zenith 
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L turbidity 
ingles of 29 
d by Koepke 
A soil surface is an imperfect diffuse reflector. 
Therefore the reflection behaviour will be a 
function of relative magnitude of diffuse and 
direct radiation and of solar zenith angle 
(Makarova et al (1973), after Kondratyev et al 
1981). Bartman (1980) and Larson and Barkstrom 
(1977) established maps in which the surface 
reflectance at a certain solar zenith angle is 
converted to standard reflectance. This formula 
implies a relatively large influence of solar 
zenith angle due to the non diffuse behaviour of 
natural surfaces. Applying this formula a standard 
surface reflectance of for instance 20.0 percent 
would give for a solzr zenith angle of 60 degrees a 
sun dependent reflectance of 22.0 and for 29 
degrees of 16.3 percent. These formulas are 
probably dependent of the type of surface and 
wavelength. 
With increasing solar zenith angle the amount of 
shadow will increase especially in rough areas and 
areas with a high drainage density. Although an 
higher amount of shadow at a certain sun elevation, 
will lead to a decrease in reflectance this effect 
will be counteracted by the difference between the 
solar zenith angle dependent reflectance of May and 
January. Sloping areas with various expositions 
will react different from relatively flat areas. 
Finally it must be noted that the reflectance of 
parts of the playa with hygroscopic salts are a 
function of the time of the day and hence of the 
time of Landsat overpass. 
4. CONVERSION OF JANUARY AND MAY DATA 
The reflectance values calculated for the top of 
the atmosphere cannot be compared directly. As is 
shown, these values can be converted to reflectance 
at the surface, if turbidity, as a function of 
wavelength and solar zenith angle, are known. But 
even in that case, reflectance values have to be 
corrected for sun elevation to get comparable data 
for both days. 
Since there is much uncertainty in these 
calculations other approaches are to be followed. 
These may be the assumption that the upper and 
lower limits or the PI and P99 of both data sets 
are equal. It can also be assumed that the shape of 
the feature space plots are comparable. Another way 
is the selection of reference surfaces which are 
constant for both days. 
All approaches have disadvantages: 
- Upper and lower limits for both dates are not 
necessarily the same. One of the days may have 
lower or higher reflectance values, for instance 
due to the fact that very wet surfaces or sealed 
surfaces are present in May or January. The 
advantage of using PI and P99 is laid in the fact 
that the influence of misregistrations is 
minimized. 
- The assumption of similar shapes of the feature 
space may be wrong. 
- Reflectance of reference objects may not be 
constant. 
In order to compare the results of calculations, 
reflectance values of May are converted in values 
comparable with the January data. 
Ultimately that approach has been adopted in 
which a reference object has been chosen, since 
this will give the most reliable results. A part of 
the footslope area with low relief intensity has 
been selected. There may be some changes in 
reflectance due to the influence of vegetation of 
this object. The fact that parts of the complete 
bare playa however have to be corrected in the same 
way in all bands leads to the assumption that this 
approach is still not too bad. Bands 1, 4 and 7 of 
May have to be multiplied by respectively 0.919, 
0.882 and 0.913 in order to get values comparable 
with January reflectance at the top of the 
5. RESULTS AND INTERPRETATION 
The reflectance values of May and January are made 
comparable with the aid of a reference surface. 
Apart from this radiometric transformation also 
geometrically the May image was converted with the 
January data with the aid of ground control points. 
A first order transformation has been applied, 
which gave an estimated standard error of less than 
a pixel in both directions according to the 
statistical analysis. In this way both the location 
and the reflectance values are comparable now. 
In order to study the changes a large range of 
images and plots can be made. The construction and 
interpretation of 1, 4, 7 combinations of both 
data, with a linear stretch between the same 
limits, is a good first approach. A compariéon of 
both images will give a good overview of spatial 
dynamics. 
It is also possible to make multitemporal 
combinations or perform division or subtraction of 
bands of the two days after the geometrical and 
radiometrical corrections. Three types of 
multitemporal images can be discriminated: a colour 
combination, a ratio and a difference product. 
Again the use of band 1, 4 and 7 is most promising. 
- colour combinations 
Useful products are combinations in which a primary 
colour is assigned to one day and a secondary 
colour to the other day. An example is a 
combination of band 1 of May in red and band 1 of 
January in cyan. If no changes occur, the grey 
tones represent the relative reflectance. 
- ratio 
A division of one band in May (corrected) by the 
same band in January will give a ratio of 1 for 
pixels with unchanged reflectance, above 1 for 
pixels with a relatively high reflectance in May 
and below 1 for a high January value of 
reflectance. Lines with a constant ratio in plots 
connect points with the same relative decrease or 
increase in reflectance. 
- difference 
An image made by subtraction of January from May 
(corrected) would also give a value of 1 in the 
case of no change, above 1 for a relatively high 
reflectance in May, and below 1 for a low May 
reflectance. Still there is an important difference 
between the ratio and difference image. In this 
latter image the same value will be found for all 
points with the same absolute decrease or increase 
in reflectance. The choice out of the two latter 
products will depend on the type and cause of 
change. For a simplified hypothetical example the 
difference will be explained (table 2). Assume the 
reflectance of surface 1 to be 0 % and the 
reflectacne of two surfaces with a different 
mineralogical composition respectively 20 % and 40 
%. If due to a special event, surface 2 and 3 
changes in such a way that both surfaces are 
covered for 50 % with surface cover type A, the 
reflectance will change to respectively 10 % and 20 
%. A ratio image will give in this case the same 
value for this identical change. A comparable 
example will be given for the difference image. 
Again the reflectance of the three surfaces are 
respectively 0 %, 20 % and 40 %. However the 
reflectance of surface 2 is now made up for 50 % of 
surface cover A and 50 % of surface B. If the 
amount of cover with A increases for both surface 2 
and 3 with 50 % the reflectance will change 
respectively to 0 % and 20 %. A difference image 
will give now the same value for this identical 
change. The second hypothetical example is also