Let's work through each of the questions based on the provided figure and its details.

1. Taking A as the origin, find the coordinates of $P$:

The rectangular plot is defined with points $A$, $B$, $C$, and $D$. From the saplings marked on the boundary, the grid can be used to identify the position of point $P$.

From the figure, the distance between consecutive saplings appears to be 1 unit. By counting the positions, we can estimate the coordinates of $P$. $P$ seems to be at (4, 6) based on the alignment.

So, the answer is (4, 6).

2. What will be the coordinates of $R$, if $C$ is the origin?

In this case, when $C$ is the origin, we need to measure distances relative to point $C$. $R$ seems to be directly beneath $C$ and a little to the right. Counting from $C$, $R$ should be at (8, 6).

Thus, the answer is (8, 6).

3. What will be the coordinates of $Q$, if $C$ is the origin?

To find $Q$, we count from $C$ again. $Q$ is positioned closer to $C$ in the opposite direction of $R$. By counting the grid points, the coordinates of $Q$ are approximately (13, 6) when $C$ is the origin.

Thus, the answer is (13, 6).

4. Calculate the area of the triangle if $A$ is the origin.

The triangle formed is between $A$, $P$, and $Q$. To calculate the area of the triangle using coordinates, we can apply the formula for the area of a triangle:

$\text{Area} = \frac{1}{2} \left| x_1(y_2 - y_3) + x_2(y_3 - y_1) + x_3(y_1 - y_2) \right|$

Using the coordinates $A(0, 0)$, $P(4, 6)$, and $Q(8, 6)$: