Earth

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The
expression
"figure
of
the
earth"
has
various
meanings
in
geodesy
according
to
the
way
it
is
used
and
the
precision
with
which
the
earth's
size
and
shape
is
to
be
defined.
The
actual
topographic
surface
is
most
apparent
with
its
variety
of
land
forms
and
water
areas.
This
is,
in
fact,
the
surface
on
which
actual
earth
measurements
are
made.
It
is
not
suitable,
however,
for
exact
mathematical
computations
because
the
formulas
which
would
be
required
to
take
the
irregularities
into
account
would
necessitate
a
prohibitive
amount
of
computations.
The
topographic
surface
is
generally
the
concern
of
topographers
and
hydrographers.
The
Pythagorean
spherical
concept
offers
a
simple
surface
which
is
mathematically
easy
to
deal
with.
Many
astronomical
and
navigational
computations
use
it
as
a
surface
representing
the
earth.
While
the
sphere
is
a
close
approximation
of
the
true
figure
of
the
earth
and
satisfactory
for
many
purposes,
to
the
geodesists
interested
in
the
measurement
of
long
distances-spanning
continents
and
oceans-a
more
exact
figure
is
necessary.
The
idea
of
flat
earth,
however,
is
still
acceptable
for
surveys
of
small
areas.
Plane-table
surveys
are
made
for
relatively
small
areas
and
no
account
is
taken
of
the
curvature
of
the
earth.
A
survey
of
a
city
would
likely
be
computed
as
though
the
earth
were
a
plane
surface
the
size
of
the
city.
For
such
small
areas,
exact
positions
can
be
determined
relative
to
each
other
without
considering
the
size
and
shape
of
the
total
earth.