## 6 Deltification Function  
In a wiki environment, different versions of a same article are fully recorded with a specific version number. The editing potential of hundreds of different versions of the same article justifies the interest of being able to quickly and easily compare two versions and only display (cartographically in the case of WikiGIS) their differences. We regroup the computing, storing and transferring the differences between versions under the term \"deltification\". The deltification function, dedicated to texts (words), is based on different algorithms used to compare two files and display their differences. Some of those algorithms are paid: Araxis Merge, Beyond Compare, code compare, compare \(+\) +, Duplicate Annihilator, _etc._ Other algorithms are free: diff, diff3, Ediff, Tkdiff, WinMerge, Kompare, UCC, _etc._ Some are web-enabled: Web Diff, Diff Cheker or Pretty Diff [24]. Table 6 provides an overview of the analysis that we conducted to compare some of these free algorithms in order to choose the most suitable for implementation in the WikiGIS. The diff function allows comparison between two texts. The diff algorithm is popular because it compares two files and displays their differences, and it works with the three major operating systems (Figure 9). For this reason, we adopted this technique in WikiGIS. Figure 8: Patterns as subclasses of the Edit class (Source: [18]). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Pattern** & **Action** & **Trustworthiness** & **User’s reputation** \\ \hline \multirow{4}{*}{Confirmation} & The absence of negative feedback is & A passive confirmation & An active participation \\  & considered as a confirmation of the & that only affects a & increases a user’s reputation \\  & feature data correctness & feature’s trustworthiness & \\ \cline{2-4}  & If wrong or low-quality information & & A user’s reputation decreases \\  & added is corrected by another user & & when the edition is corrected \\ \cline{2-4}  & A rollback is defined by three & & Corrections should intuitively \\  & subsequent versions of a feature, where & It decreases because of the & decrease a user’s reputation if \\  & the first and last of the three subsequent & loss of persistence & they revert a feature to an \\  & versions of a feature are identical & & earlier state \\ \cline{2-4}  & Special cases of rollbacks where & & \\  & the user makes corrections to his & & Evaluations are needed to verify the credibility \\  & own updates & & and reputation \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Patterns as subclasses of the Edit class (Source: [18]). For the geometric component, we propose a method of comparison (spatial deltification) based on the \"base state with amendments\" method proposed by [26] and updated by [27]. The principle of this method is quite simple: a complete record of the study area at t0, termed the base state, is created. Changes that have occurred since the previous update are recorded, at appropriate intervals, in new database layers known as amendments. The intervals used to create the amendments do not have to be regular. To determine the situation at a certain date ti, all the amendments recorded between the date ti and t0 are overlaid onto the base state.