Two approaches can be used to derive aggregates for multiple sites: 1) a bottom-up approach that scales up the estimated value of ecosystem services and related goods from a set of individual sites to a wider set of sites, and 2) top-down approach that builds on detailed, region-wide and spatially explicit geophysical data to derive aggregate values. The bottom-up approach is the most often used, building generally on scaling up per hectare values from a set of sites for which existing data is available and extrapolarting to that wider study area. Both methods still suffer from data availability (lack of data in suitable from and / or with suitable geographic coverage, etc. ) and require a number of assumptions to be used as a basis for aggregation.