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.

Source text:
Geodesy for the Layman