# Quality control

The following quality control and assurance checks have been carried out on all data. These can be categorised into topological and attribute based checks (QAT and QAD respectively). It is recommended that these be performed whenever data is modified. (see Responsibilities):

QAT1: All polygons should be checked for area versus polygon perimeter to identify extremely long thin polygons. A long thin polygon is indicative of an error known as a sliver polygon usually between two large polygons.

QAT2: All polygons with only three vertices should be checked checked. Three vertices indicates a triangular polygon which is unlikely to occur in the database.

QAT3: Correct polygon topology must be checked for by searching automatically for any unlabelled or duplicate labelled polygons. Correct arc topology must be checked to ensure that they form closed loops, and that no arcs dangle.

QAD1: Ensure that every polygon has its own ISO 3 letter country code. This allows selections of polygons from the database for a particular country without the need for a spatial cookie cut that often results in sliver polygons.

QAD2: Ensure that every polygon has a vegn2 CIFOR code greater than 0.