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Fractional 
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1 Square error 
stigation 
In case the percentage error referring to the phenomena under 
investigation should be of interest (relative error), the 
following formula can be used: 
dlls pn [2] 
ny is the number of sampling points within a certain 
phenomena. These formulas should only be appiied if the land 
use units under consideration have small surfaces compared 
with the width of the sample grid and are randomly 
distributed. In case adjoining sample points are likely to 
fall within the same land use parcel, the error will be 
overestimated. This will happen to non-scattered land use 
catecories with extensive surfaces. In this case formula 2 
with the exponent 2/3 instead of 1/2 will allow a better 
error estimation (8, 9). 
These formulas can also be used as a basis for an estimation 
of the number of sample points needed for obtaining a certain 
accuracy. 
3. Different possibilities for sampling on aerial photographs 
In case land use data have to be collected by sampling on 
aerial photographs, the geometrical characteristics of the 
pictures (central perspective view) have to be taken in 
consideration. Variations in scale and displacements due to 
elevation differences would lead to systematic errors if a 
regular sample grid would be placed directly on the photos. 
There are several possibilities for eliminating these errors: 
- The air photos serve only as a source for land use informa- 
tion but not as a basis for sampling. The land use 
boundaries are first transferred to a topographic map which 
is then superimposed by the sample grid. 
- Instead of transferring the land use information on a map, 
the sample grid can be transferred to the photos. The land 
use occuring under each dot can be interpreted directly in 
the photo. 
- Sampling can be done with the aid of a stereoplotter. In 
this case preference should be given to line sampling 
(traverses) instead of point sampling. 
- The air photos can be used for producing orthophotos. Since 
these are free from variations in scale they can be directly 
used as a basis for sampling.