The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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In order to assess the area measurement accuracy, reference 
parcels with a known area and perimeter have to be selected 
from available sources or acquired using independent tools, 
here precise orthophoto. This aerial orthophoto with 0.5m of 
GSD was acquired in 14 th of May 2005 as multispectral image 
RGB using UltraCamD digital camera. Pixel level accuracy was 
determined in a separate experiment (Spruyt, pers. comm.). 
2.2 Methods 
The methodology of area measurement evaluation is based on 
statistical analysis of discrepancies between the measured and 
reference areas (Pluto-Kossakowska et al. 2007). In order to 
derive the tolerance above which an inspector will reject the 
area claimed by the farmer with a risk of a=5%, an initial 
verification that the distribution of the buffer is normal must be 
made. To obtain the final tolerance for the measurements, a 
repeatability limit of the buffer was applied. 
The scheme of validation procedure is proposed as followed: 
1. Data processing and acquiring: 
- images orthorectification - needed to make cartographic 
product to measure 
- Acquiring the reference parcels - from digital orthophoto 
- Area measurement of the selected parcels on the images 
- Buffer calculation based on measurements and reference 
data 
2. Statistical analysis of the buffer value 
- Anomalous measurements detection and elimination 
- Normality test and analysis of variance (SLS, ANOVA) 
- Determination of tolerance for the measurements as 
reproducibility limits. 
It is practical to model the maximum acceptable discrepancy 
between the measured area and the claimed area, i.e. the 
tolerance, as the parcel perimeter multiplied by a width. This 
width, also called buffer width (or simply buffer) around the 
parcel perimeter, is expected to vary as a function of the 
measurement tool, whether it is an image or a GPS-device. For 
a given parcel, the knowledge of its reference (i.e. true) area 
and reference perimeter allow the transformation of the area 
error (measured area - reference area) into this buffer width 
using: 
B = (a¡_JV) (eq . l) 
iV 
where B, = buffer width for measurement i 
aj = measured area for measurement i 
a,. e f = reference area of the parcel 
p ref = reference perimeter of the parcel 
Using the buffer values from different observations we can 
determine the tolerance between two independent 
measurements under the specified condition (the same parcels, 
same image, and independent operators). The simplest way is to 
verify whether the distribution of the buffers follows a normal 
law using different tests. 
Detection of outliers is recommended prior to verifying the 
normality of the buffer widths. According to ISO 5725 (1994), 
the detection of anomalous measurements may be made using 
different tests. Outlier detection was performed here using the 
Jacknife distance test in JMP 6.0 (SAS Institute). A Standard 
Least Square (SLS) procedure was then performed to identify 
the factors (and 2 nd /3 rd order interactions) significantly 
explaining the observed variability of the buffer. Table 1 
presents the list of factors and related modalities. “Shape” 
factor was distinguished on to three modalities: simple i.e. 
rectangular alike shape, medium i.e. rectangular shape with 
little changes and complex i.e. shapeless. The visibility depends 
on parcel itself, parcel surrounding and image properties: good 
visibility i.e. all parcel borders are easy to recognise; poor 
visibility - part of the border is difficult to recognise and must 
be deduced. “Operator” presented two different modalities 
(skilled vs. unskilled) based on the level of “experience” of 
each photointerpreter had at the beginning of the survey. 
Finally, the assumption of normal distribution of the buffer 
values leads to the derivation of a tolerance (at a=5%), above 
which an inspector would reject the area claimed by the farmer. 
For the needs of our survey and following the ISO 5725 (1994), 
the tolerance can be interpreted as reproducibility limit (eq. 2). 
Reproducibility refers to the ability of the measurement to be 
accurately reproduced by someone else working independently, 
i.e. is a value less than or equal to which the absolute difference 
between two results obtained under reproducibility conditions 
may be expected to be with a probability of x%. 
R = f*cr R * yfn (eq. 2) 
Where a R = standard deviation under reproducibility condition 
(for the method of calculation refer to ISO 5725, 1994) 
f = multiplication factor of standard deviation to 
determine the confidence interval on specified level of 
probability (here 95%) 
n = number of test results to be compared, here n=2 
For normal distribution at 95% probability level, f is 1.96 and 
f*V2 then is 2.77. The simple “rule of thumb” R=2.8o r is 
applied instead of equation (2) (ISO 5725, 1994). 
Factors 
Modalities 
Operator (n=5) 
Skilled (n=3) 
Unskilled (n=2) 
Image (n=3) 
Orthophoto 
Cartosat-1 Aft 
Cartosat-1 Fore 
Image visibility (n=4) 
Good on all images 
Good on ortho, 
poor on cartosat 
Poor on ortho, 
good on cartosat 
Poor on all images 
Parcel shape (n=3) 
Simple 
Medium 
Complex 
Parcel size (n=3) 
Small (< 2ha) 
Medium (2ha> >8ha) 
Large (>8ha) 
Land cover type (n=7) 
Bare soil 
Green cover 
Marsh 
Olive trees 
Orchard 
Pasture 
Vineyard 
Table 1. List of factors tested and related modalities