While conducting a detail survey using the GRID system, explain the following:

a$)$ Suppose you are surveying a block with dimension $50$ x $30$ metres. Draw the grid lines to conduct this survey.

b$)$ In the following diagram, identify the points on the grid through which the contour $100\mathrm{.}800$ passes.

c$)$ Using the following data, calculate the spot height:

$($i$)$ Higher contour $=100\mathrm{.}800$

$($ii$)$ Lower contour $=100\mathrm{.}600$

$($iii$)$ Distance from higher contour to grid $=18$ mm

$($iv$)$ Distance between contours $=29$ mm