$1\mathrm{.}3$ Explain why and when the E$$tv$$s correction is applied. What causes the need for this

correction and when it can be ignored?

Gravitational attraction: $(15$ marks, $5$ marks$/$question$)$

$2\mathrm{.}1$ Masses between the topographic surface and the mean sea level can be represented by a

spherical layer with an average thickness of $1800$ m and density $=2\mathrm{.}67\frac{g}{c}{m}^3.$ Calculate

the attraction of this layer in mGal experienced by an airplane flying at altitude $7,000$ m

above the topographic surface. Assume that the mean sea level is approximated by a

sphere with a radius of $6,371km.$