Important photogrammetric points:
\[
\boxed{
\begin{aligned}
\text{Perspective Centre} &\rightarrow \text{Camera lens}
\text{Principal Point} &\rightarrow \text{On photograph}
\text{Nadir Point} &\rightarrow \text{Ground below camera}
\text{Isocenter} &\rightarrow \text{Midpoint between principal and nadir}
\end{aligned}
}
\]