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is currently the most commonly used elliptical model used for the
globe.
Above, we have defined a physical surface, the Geoid, as a
reference surface for heights. We also need a reference surface for the

description of the horizontal coordinates (i.e. geographic coordinates)
of points of interest. The reference surface for horizontal coordinates
requires a mathematical definition and description. The most

convenient geometric reference is the oblate ellipsoid. It provides a
relatively simple figure which fits the Geoid to a first order
approximation, though for small scale mapping purposes a sphere may
be used. An ellipsoid is formed when an ellipse is rotated about its

minor axis. This ellipse which defines an ellipsoid or spheroid is
called a meridian ellipse (notice that ellipsoid and spheroid are used
here as equivalent and interchangeable words).

Task 3. Answer the questions, using the active vocabulary.

1. What is the Geoid?
2. What is the direction of gravity dependent on?

3. How is the geoid surface described?
4. What do you know about reference and rotational ellipsoid?

5. How is called the ellipse, which defines an ellipsoid?

Task 4. Define whether the following statements are true or

false. Correct the false ones according to the text.

1. The first approximation to shape of the Earth is a sphere.
2. The plumb line through any surface point is always vertical to
the Geoid.

3. The geoid surface is described by geoid heights that refer to a
suitable Earth reference ellipsoid.
4. The geoid is regularly-shaped.
5. An ellipsoidal surface can also be approximated by a sphere.

6. An ellipsoid is a smooth elliptical model of the Earth’s surface.
7. The most convenient geometric reference for horizontal

coordinates is the oblate ellipsoid.

II. LEXICAL EXERCISES

Task 1. Complete the sentence according to the text:

1. The Earth is the home planet, the site of all life, and the

place______.
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