- Use non-designed surveys
- Use environmental information
- Maps

- Rescale the (flat) density and extrapolate

\[ \hat{N} = \frac{\text{study area}}{\text{covered area}}\sum_{i=1}^n \frac{s_i}{\hat{p}_i} \]

- \( s_i \) are group/cluster sizes
- \( \hat{p}_i \) is the detection probability (from detection function)

- Probability of sampling every point in the study area is equal
- Is this true? Sometimes.
- If (and only if) the design is randomised

- H-T equation above assumes even coverage
- (or you can estimate)