Upkeep - Area difference
Contents
Background
A technical tolerance is an expression of uncertainty or precision. Such uncertainty comes from two main component:
- the nature of the object measured, which is stable and independent from the actual measurement. It can be calculated (semi-)automatically in the LPIS for each parcel
- the effects of the actual measurement, in case a delineation on digital orthoimagery. This component is measurement dependent and has to be entered on a case by case basis.
Assuming the second component is duly selected and controlled as one would expect for a reference parcel, only the first component is considered. The remaining uncertainty is expressed as the expected area error amplitude (EAEA)
Procedure
The procedure for the calculation of significance of reference area difference:
- input manifest changes by type
- proceed with types I and III
- (for type II changes, make Anew the new reference area and finish the procedure)
- determine the reference area for types I and III
- use the EAEA formula that takes parcel properties into account
- calculate the difference |Aold - Anew|
- if |Aold - Anew| >= EAEA than the difference is significant - replace Aold with Anew
- if |Aold - Anew| < EAEA than the difference is not significant - keep Aold
- finish the procedure
Please follow the sequence on the diagram bellow
figure 3dii.1: activity diagram for assessing significant area difference
Formulas
It's scientifically possible, but rather impractical to establish the probability for each individual image, land cover feature and landscape. Rather a simplified formula is proposed.
The Expected Area Error Amplitude is then calculated as
- EAEA = C(RMSp) * B [m2]
RMSp - resolution
RMSp accounts for the map scale represented by the pixel size of the map (note that the guidelines impose a relation between GSD and pixel size, dependent on the sensor type. For CAPI, the RMSp equals the pixel size in meter
C - shape and size
The C-value depends on the shape of the parcel (regular, elongated, irregular…). It is calculated from the Gaussian formula and is a function of the pixel size RMSp:
B - border properties
B relates to the nature of the parcel boundary perimeter, it is estimated by the following table, based on the abundance of border quality:
Perimeter composition | >60% precise borders | >25% vague borders | all other conditions |
B value | 1.96 | 2.94 | 2.45 |
where:
- precise borders: those composed of ground level features that have an identification precision better than 50 cm. Examples are: crop boundaries, base of fences, base of walls, road sides,....
- vague borders: are composed of features NOT at ground level (raised or sunk for 2 meters or more), that block visibility to the ground level and that are subject to cyclical dynamics. Examples are: Lines of threes, high hedges, deep ditches, terraces....
- intermediate borders can be identified for border qualities which do not qualify as either precise or vague: e.g. low hedges, degraded walls.
- Note: Any third party boundary (e.g. cadastral boundary, topographic map line) is considered a precise border IF it represents a physical (fence, boundary marker) rather than a virtual (legal) feature.
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