Moscow began restricting exports of inert, or "noble" gases, including neon, argon and helium to "unfriendly" countries at the end of May, according to a report by Russian state news agency TASS. All three gases are used to produce the tiny electronic chips found in a raft of consumer products, from smartphones to washing machines to cars, and which have been in critically short supply for months.

Only 23 studies actually integrated both active and passive data sources into the ITCD procedure since 2000 (Figure 2).

Another limitation is that few approaches take full advantage of the information contained within remotely sensed data, e.g., using only one band of multispectral imagery [29] or only the canopy height model derived from LiDAR data [30]. Significant amounts of information are dismissed or neglected during data preparation or processing. The integration of multispectral data and discrete LiDAR data is commonly used to improve tree species classification [10] and fusion of passive and active remotely sensed data may reduce commission and omission errors in ITCD results [31].

Comparison of methods continues to be complicated by both choice of reference data and assessment metric; it is imperative to establish a standardized two-level assessment framework to evaluate and compare ITCD algorithms in order to provide specific recommendations about suitable applications of particular algorithms.