When decoding a data array that has been encoded using a tree structure representation, the encoded tree representation of the array of data elements comprising a set of tree node data representing the respective node values for the different nodes of the tree and a set of bit count data indicating the number of bits that has been used for signalling the node values for each non-root node in the tree a data value for a set of one or more data elements associated with a first node of the tree structure is determined by determining an initial data value for the first node using the stored tree node data, and modifying the initial data value using a modifier value based on the number of bits used for signalling the node values for the child nodes of the first node in at least the next level of the tree.