The estimator is a function of the sample of "n" observations drawn without observational bias from the whole sample space, in this example that sample would be the set of all measurements of yesterday's rainfall in all available rain gauges within the geography of interest.
The estimator is a function of the sample of "n" observations drawn without observational bias from the whole sample space. Sample would be the set of all measurements of yesterday's rainfall in all available rain gauges within the geography of interest. This was in this example this.