THE SPATIAL RESOLVING POWER OF EARTH ...

(NASA-TM-82020) OF EARTd BESOU_CES (NASA)

39

p

HC

THE SPATIAL SATELLITES: AO3/Mf

a01

1_81-2 1441

_ESOLVING _OWER A REVIEW CSCL

05B G3/q3

NASA Technical

Memorandum

82020

THE SPATIAL RESOLVING POWER OF EARTH RESOURCES SATELLITES: A REVIEW

John R. G. Townshend

SEPTEMBER

National Aeronautics Space Administration

1980

and

Goddard Space Flight Center Greenbelt, Maryland 20771

Unclas 39807

BIBLIOGRAPHIC 1. Report

No.

J 2. Government

4.

Title

and

Accession

3.

No.

Subtitle

THE SPATIAL RESOURCES

RESOLVING

Catalog

No.

5.

Report

Date

6.

Performing

Organization

Code

8. Performing

Organization

Report

10.

Work

No.

11.

Contract

13.

Type

POWER OF EARTH

SATELLITES:

A REVIEW

7. Author(s)

John

Recipient's

SHEET

N81-21441

I

TM-82020

DATA

No.

R. G. Townshend

9. Performing

Organization

Name

and Address

Unit

Earth Resources Branch Code 923, GSFC

12.

Sponsoring

Agency

Name

or Grant

of

Report

No.

and

Period

Covered

and Address

TM

NASA

14. Sponsoring

15.

Supplementary

16.

Abstract

Agency

Code

Notes

The significance

cf spat:aI

resolving

power

on the ut:'lity

of current

ar, d futur '_ Earth

resource_

[

satellites is critically discussed and the relative merits of different approaches in deeming and estimating spatial resolution are outlined. It is sb.ewn that choice of a particular measure of spatial resolution depends strongly on the particular needs of the user. Several experiments have simulated

[

the capabilities

[

of these indicated

I

accuracy bcundar:¢

of future

satellite

systems

that improvements

of land cover types

in resolution

is likely

Extraction

to benefit

power:

cessing methods

that are applied

from images

these include

characteristics

as well as certain

images.

However,

upon

18.

many

in the classification where

the frequency

of

upon visual

resolutions. several

of the terrain sensor

Su."prisinNy,

Use of image.'/dependent

from higher

will depend

17. Key Words (Selected by Author(s)) Remote

methods. is found.

more consistently

of information

spatial resolving

relationship

of aircraft

may lead to a reduction

using computer-asslsted

pixels is high, the converse

interpretation

by degradation

other

factors

being sensed,

apart

the image

from pro-

characteristics.

Distribution

Statement

Sensing

Spatial Resolution Land Cover

19.

Security

C!assif.

(of this

report)

] 20.

Security

Cla_if.

(of this

page] 21. No.

J °For sale by the National Technical Information

Service, Springfield, Virginia

22151.

of Pages

22. GSFC

Price* 25-44

(10177)

TM 82020 THE SPATIAL

RESOLVING

POWER

OF EARTH

RESOURCES

A REVIEW*

John R. G. Townshend** Earth

Resources

Branch

Code 923 Goddard

Space Flight

September

*Preprint **Resident

of an article to be published National Research Council

Center

1980

in Physical Geography. Research Associate.

SATELLITES:

THESPATIALRESOLVINGPOWEROF EARTH RESOURCES SATELLITES: A REVIEW 1.

Introduction Earth

the

resources

satellites

1980's will provide

available satellite

of the Earth's

from the Landsat

during surface

series of satellites.

of this new source it wanting.

are currently

images with substantially

from civilian satellites images

suitable

Operational 1 shows

information

for remote

of a recent

sensing data in terms

resolving

perceived

survey

of spatial

capabilities

needs for higher of Landsat

son, 1978; Salomonson

been hindered

to be launched include

Mapsat

stereoscopic 1979)

in 1984 (Gaubert, (Colvocoresses,

coverage,

(Jet Propulsion

and a Synchronous

fied sarveil!ance the resul:ant

systems

imagery

1979a),

Earth

circulated.

Satellite

1979)

found

displays

poor resolution. agencies

of the Landsat

being met.

Probatoire

satisfied

by the better 1976; Salomon-

d'Observation

de la Terre)

systems

that have been proposed

looking

fore and aft to produce

a large format

(Doyle,

but others

the imagery

in 1982 (CORSPERS,

with sensors

Laboratory,

1975),

resolution

will be partially

high resolution

of

the potential

U.S. government

the current

to be launched

Stereosat

such as the KH-11

is not widely

imagery

commonly

air photographs.

users are not apparently

Other

Observation

to exploit

which

in

were obtained

by their relatively

Given

and of SPOT (Syst_me

1978).

properties

of the needs of the major

resolution

than those

1973a; NASA detail

for_launching

the very large majority

were quick

(e.g. NASA

resolution.

D scheduled

et al. 1980)

scientists

with that in conventional

MSS of 79m it is clear that the needs of many

These

1970's

of terrain

by the lack of ground

compared

or designed

resolution

In the

for the study

use of such data has similarly the results

constructed spatial

the last decade.

Not least, this was caused resolution)

being better

Many Earth

of environmental

(i.e. its low spatial

Figure

which

1978).

camera

on Spacelab

Additionally

there

of the U.S. with very high resolving

powers,

(Doyle,

are classithough

U.S. GOVERNMENT AGENCY USER REQUIREMENTS FOR SPATIAL RESOLUTION OF REMOTELY SENSED DATA FROM SPACE (INTER-AGENCYTASK FORCE, 1979) 60

-

40

Z tll ILl t_

o ill

20

0-10

10-30 GROUND

Figure

1.

Survey

of spatial

hended not

by many

properly

monly

significantly as electrical of remote

systems which tion

optics,

and

have

been

3.

affect

Finally the

detail

systems,

> 800

field

and

of sensing

derived

which practical

of several

it is important of information

are

(IFOV),

often

empirical

extractable

arc poor.',y

arises

because

are quoted. which

measures

more into

the

simulation

to recognize

this

ones,

factors from

The

for many

By borrowing

other

insight

part

which

systems.

science

US government

1979)

even current

figures

of view

photographic

of major

Force,

In large

of resolution

a more

the results

Task

geographers.

capabilities

We can obtain

by examining

strongly 4).

the

systems

in Section

including

is the instantaneous

engineering,

section.

future

the significance

over-estimates

requirements

(Inter-Agency

of such

scientists

measure

sensing

next

resolution

earth

understand

available

in the

the capabilities

80-890

RESOLUTION R EQUI REMENTS (IFOV IN METRES)

resolution

agencies.

In practice

30-80

from

benefits

remote

most

such

disciplines

spatial

of future spatial

sensing

power

are reviewed

of improved

than

do

com-

resolving

these

experiments other

users

applications

of spatial

informative;

compre-

satellite

resolution

imagery

(Sec-

Our attention

is restricted

to the visible and near infrared

is the source of nearly all higher collection

of data outside

the late 1980's

or early

2.

of Spatial

Concepts Spatial

minimum

far from simple.

can values

obtained

resolution

that has been obtained

at high resolutions

since this

so far, and extensive

is unlikely

to take place

until

Resolution

size of objects

proven

Spatial

of these wave bands

refers

this apparei_tly

imagery

of the spectrum,

1990's.

resolution

forming

resolution

parts

to the fineness

on the ground

depicted

concept

necessarily

turns out to be a much

distinguished

into an operational

is no single satisfactory

from one method

in an image

that can be separately

straightforward There

of detail

measure

be readily

more complex

or measured.

quantitative

of spatial

converted

topic

that is it describes

to those

Trans-

measure

resolution

the

has

available,

derived

than our initial intuitive

nor

by another. definition

suggests.

Recently

i_:has been suggested

conveniently the ability targets

be assigned

will be reviewed

2.1

respect

as follows

=

the ability properties

merits

properties

the simplest

measure

orbit height size of the optical

resolution

of the imaging

the periodicity targets.

of repetitive

Examples

system 3

quoted

is the instantaneous

resolution-available for satellite

systems.

can

system,

of these

systems

of spatial

-.

f is the focal length

of small finite

in this category

H d

d is the detector

to measure

of imaging

since it is the most widely

H is the satellite

properties

of spatial

discussed.

(see Fig. 2):

f

that definitions

geometrical

that ne.eds to be considered

This is probably

IFOV

where

targets,

the spectral

based on th:e'geometric

the most important,

calculated

point

to measure

The only measure view (IFOV).

between

in turn and their relative

Measures

et al., 1980)

to one of four categories:

to dJstinglaish

and the ability

(Forshaw

field of

and is in one It is usually

d DETECTOR-_

DETECTOR

f

l

OPTIC:

LAR IFOV

ANGULAR IFOV

H

POINT SPRI FUNCTION

IFOV

HALF

IFOV

GEOMETRIC

"POINT SPREAD"

INSTANTANEOUS FIELD OF VIEW.

INSTANTANEOUS FIELD OF VIEW

(IN PRACTICE f ,_ H)

Figure

2. Definitions

of instantaneous

4

field of view (tFOV).

Sineedetectorsizehasto be defined, crete detectors

such as line-scanners

son, 1979; Wight,

1979)

lite by the movement

(Figure

multispectral

scanner

per of Landsat mirror hundreds

system

track

circuits

and Cline,

In the former,

and along track of picture

6 in Table

elements

(Thompson

adopted

configuration

is being

in the U.S. operational

For the MSS of the

these

with.

thermal

channel

of Landsat

that the detector

3 which

size is reduced

which

the photons

formcr

and 76.2m

series,

the IFOV

is 237m).

to the latter.

from 880 to 940 kms, the tFOV has varied

the trackof

the satel-

of the satellite.

Thus

was used in the

the need

along

are electronically

along track

sampled

mission

of silicon,

and electronic such that the

by one resolution

SPOT

for a moving

chip

with amplifiers

Map-

element.

and probably

The will be

satellite.

is normally

Colvocoresses

quoted

(1979b)

(walls and adhesives)

pass to reach the detectors. according

(Thomp-

This approach

radiometers,

in the French

due to cladding

radiometers

On a single monolithic

detectors

earth resources

Landsat

with dis-

1, 2, and 3 and will be used in the Thematic

1). In push-broom

adopted

movement

or pixels.

can be manufactured

1979):

to sensors

or push-broom

by the forward

entire linear array is read out in the time to advance push-broom

1975)

applicable

images are built up across

is dispensed

detectors

is most

elements

(MSS) of Landsats

to over a thousand

multiplexing

3).

of a matrix

D (see footnote

for the across

(Lansing

of a mirror

the final image is comprised

this measure

This results

Moreover from

as 79m,

(except

and Slater

around

is an IFOV

since the altitude

(1979)

point

the fibre optics

of 73.4m

1980)

out

through

according

of Landsats

76m to 81 m (Gordon

for the

to the

have varied

ignoring

the effeets

of cladding.

The IFOV ject smaller

does not necessarily

give the minimum

than this size may be sufficiently

overall radiance

brighter

of the pixel, so that it is detectable.

frequently

detectable

Its chances

of detection

scan line or falls along

on Landsat

MSS images.

will depend- strongly the boundary

between

size of objects or darker Thus

than its surroundings

roads and rivers narrower

The alignment on whether

that are detectable.

of a linear

its central

two scan lines (Gurney

object

An ob-

to change

than 79m are

is also crucial.

axis falls along the centre 1980).

the

In the latter

case,

of a

I,,i,,i

.,.,_ t+,u,

2

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the likelihood be detected; Landsat they

of its being detected many

objects

MSS imagery

are 250m an object

detect

small differences

simply

be lower.

equal to or greater

it has been found

across or more.

detect

wilt clearly

depends

One immediately

in radiance.

to low contrast

obvious

is measured

the photon

and the Johnson

(or Nyquist)

of noise exist specific character

(or shot) noise noise

to the particular

of the S/N ratio.

due to random

as a result

(Baker

to or smaller will usually

Moreover

the signal from sensors

ground

(though

not for the RBV sensor

This digitization

will further

on the use of remotely hardware decrease

noise we can consider

the contrast

of an image.

will be a function scene noise, assigned

et al., 1980)

tinguished.

atmospheric

by the sensor Thus,

caused

by those

blurring

effects

size of objects

in terms

These are due to optic phenomena

which

is

other

1975).

4 illustrates

Figure

the combined

1975a).

such as aberration

Apart

which

(1978)

with further

and diffraction.

to

pixels are

multi-spectral

in Section

of resolution

size of objects

is to reduce

also refer

image degradation

the estimate

or

in a given scene

with which

may confuse

from

can increase

can be detected

is dealt

levels

The net effect

the accuracy

of the minimum

digital analysis.

(1979).

and Landgrebe

the

to the

quantization

phenomon

types

noise level will be

for transmission

of different

which

between

in all detec-

Various

and lead to further

to the disparity

ratio (S/N),

in the sensor.

soil variation

of this point

will also be present

resolution

Wiersma

For example

The significance

to

by Tucker

1974; Slater,

of a scene reducing

class.

ability

the detector

as a noise-like

conditions.

components

and thus will contribute

and the realizable

(Fraser,

to

to the sensor's

3), and for subsequent

been examined

effects

the minimum

land cover

of crop types.

Various

has recently

of local atmospheric

to the correct

classification

the IFOV

data

only if

striking

than

The impact

in

of photons

be quantized

of Landsat

add noise to the signal.

sensed

the signal received

and Scott,

Signals comparable

undetectable. surface

effects

are detectable

of noise are present

fluctuations

of thermal

sensor

in relation

Two types

Commonly

for this is that our ability

by the signal-to-noise

the ratio of the signal to the total noise present.

tors, namely

tess than 79m across may

objects

explanation

with its surroundings

This

objects

than this value will not be detected.

that medium

on its contrast

Whereas

3.

(Forshaw based on

that can be disThe former

is

muchmoreimportantthanthe latter at thewavelengths in the visibleandnearinfraredbut at longerwavelengths in themicrowave,the reverseis the case.Blurringwill alsobecausedby motion duringimagingdueboth to the forwardmovementof the satelliteandto theacross-track movement of the mirror in scanningsystems.If a multispectralimageis produced,the extent to which the imagesarenot geometricallyregisteredwill alsoproducea blurringeffect. It is usualto equatepixel sizein imagerywith the IFOV, but this neednot bethe case.For Landsats1and2 MSSdatatheIFOV andthepixel sizeareindeedessentiallythe same.In the alongtrackdirection,the pixel sizeequalsthegroundtrackvelocitydividedby the mirror scan frequencybut acrosstrack thecontinuoussignalcould In fact it is chosen between

adjacent

× 56m (General

to give a pixel width pixels of 23m.

Electric,

a cubic convolution in which pixels

algorithm

will not improve

The previous solution.

In the following

into account of IFOV point

in estimating

has been proposed,

spread

is imaged. simple point

function

spatial

sections, spatial

we discuss various

based on the point the distribution

it describes

quoted

for the Theomatic

Mapper

values (figure

of Landsat-D

attempt

2).

using 1978) smaller

of spatial

to take these

of an imaging plane,

source.

and imaging

IFOV

produces

than an alternative

in the image

function

at its half amplitude

which

image of a point

which

as a measure

(PSF)

The point

point

function

function

of energy

spread

as 79m

it slightly.

first, we note

of the spacecraft

influences.

quoted

of 57 × 57m (Holkenbrink,

of the IFOV

as the motions

as well as atmospheric spread

spread

the resultant

MSS is sometimes

this resampling

measures

However,

rate.

pixel size but with an overlap

and may decrease

the limitations

resolution.

due to such factors

However

resolution

at any arbitrary

3 digital tapes have been resampled

pixels with dimensions

demonstrates

describes

In other words

The data on Landsat

are dissimilar.

the actual

discussion

pixel size for the Landsat

to produce

case pixel size and IFOV

be sampled

the same as the along track

Hence

undated).

in theory

is defined

The IFOV

is based on this measure;

refactors

definition

system.

when a point

The source

This image is never a mirror

lens' properties

as the width

of 30 metres

of the

normally

the corresponding

point

spreadIFOV, for the LandsatMSSis somewhatgreaterthanthat of the geometricIFOV namely 90mratherthan 79m(Landgrebeet al., 1977).The point spreadinstantaneous fieldof viewisin fact closelyrelatedto measures dealtwith in the followingsection. 2.2

Measures

based on the ability

The most commonly criterion

(Perrin,

used definition

1966; Slater,

image of a point-source,

to distinguish

1975a).

This pattern

diffraction

describes

2.1).

The Rayleigh

point

sources

targets

criterion

imaged

ring of the second.

is known

for distinguishing

if the central

The angular

X is the wavelength f is the usual

From ground

(0) is simply

derived

sensing. showed for point

and arises because

spread

function

of

(see section

is based on two equal intensity aperture.

of one source

calculated

bright disk surround-

It states

that the two

lies on the first dark

as (Slater,

1975a):

1.22 Xf 1000

f-number

of the lens.

of the sensor

above the ground,

a measure

of resolution

in terms

of

can be derived.

Most remote (1969)

circular

the resultant

in micrometers.

0 and the height

measures

of a central

two targets

peak of the image

is the Rayleigh

aberration-free,

type of point

aberration-free,

separation

measures

as the Airy pattern,

between

0-

where

of resolution

but will consist

one particular

by an unobstructed

will be just resolved

targets

Even if a lens is completely

will not itself be a point,

this pattern

point

in this category

ed by faint dark and light rings. effects:

between

sensing

targets

a resolution

Estimates

for square

that the diffraction

are of course

measure

for extended

or rectangular limited

not point

resolution

circular

objects

would

sources, sources

probably

for such sources

sources.

10

and with this in mind Otterman which

is more

relevant

not be very different.

is almost

six times coarser

to remote He than that

In practiceof course,lensesarenot aberration-free andasdiscussed in the previoussection therearemanyotherpropertieswhichwill degradethe imageandhenceincreasethe minimum separationthat is detectable.Measures of this categoryconsequentlygivesanindicationof the absoluteresolutionthat is achievable by a lens. 2.3

Measures

based on the ability

to measure

Measures

based this property

arose principally

Brock,

1973 ; Shaw,

1979) though

(e.g. Lavin and Quick, images sets of parallel to be lower linear objects pressed

pressed

Since

frequency

measures

the linear targets

in cycles/ram.

Somewhat

against

by all (Slater,

Modulation

them

when

derived

standard

practice

the contrast

and

will appear

is so small that the

in this way are consequently confusingly

NASA

of half-cycles/ram,

(Scott

that if one

and their background

as line pairs/mm

or in terms

images

derived from other sensors

used often have a sinusoidally-varying

values in te,.'-ms of single lines/ram favour

between

of resolution

such

on photographic

to images

until a step is reached Values

targets

They are based on the observation

the contrast

decreases,

are indistinguishable.

by spatial

lines/m_.

linear objects,

1974).

of repetitive

from work

they have been applied

1974; Buchtemann,

as their spacing

the periodicity

often

abbreviated

tone, values (1973b)

which

to

are also ex-

has expressed

an approach

ex-

such

has not found

1975b).

(M) is the measure

of contrast

most frequently

used in this context.

M - Emax - Emin Emax + Emin E is variously

defined

exposure

vaiues

mittance

or intensity

as object

determined (Scott

and Brock,

MTF curve is derived

transfer

1973).

M I to the object

If we plot the transfer

called the modulation

(Welch,

1977;Smith,

from microdensitometer

ratio of the image modulation fer factor.

radiance

factor

functign

against (MTF)

only for high con.trast

1978),

readings

(Perrin,

M consequently modulation spatial (Steiner

sinusoidal

wave targets. 11

M o is known

1971),

trans-

value of 1.0. The

as the modulation

the resultant

and Salerno,

or photographic

1966; Welch,

has a maximum

frequency

targets

luminance

curve

1975) (Figure

but can also be derived

trans-

obtained 5).

is

Usually

for, square-

the

T SIGNAL

I NO,SE Figure Various ,,,_e the spatial The effective which

measures

v, ,esom_lv,,

frequency

at which

1975b;

NASA

value as a result 1973b)

can be derived the modulation

of the modulation

(L) displayed

66m,

which is rather

used,

the value is 132m.

Electric

(undated),

smaller

and Weinstein, is approximately

Thus

when plotted

From

45 meters.

called the threshold inaage modulation

EIFOV

data quoted An alternative

modulation required

for

cf ..,,_.,,,,,.,.'_A_ .... ha; ,.,ore,.,, "_.... _ ._,,_.,,,,_r_"

function

of the imaging

G in metres

is derived

system from

(Slatcr

the estimate

images by this equation:

of the Landsat

of 79m (Welch

approach

on a graph of the modulation

The IFOV

of 0.35 for a square

in Morgenstern

a response

If the full cycle definition

beam vidicon

is obtained.

factor

MSS (using the 0.5 MTF value) is

1977).

et. al. (1976)

curve)

in a sensor transfer

12

it appears

which

is

given in General

wave response

as a function

factor

camera

of the Thematic

is to derive a demand

or aerial image modulation

to produce

value.

1000S L

of 38.8m

transfer

cf its maximum

1

The EIFOV

to a modulation

1980).

transfer

Using the MTF curve for the return

an estimated

of 30m corresponds

distribution

on the actual

than the IFOV

w_ ,.an calcu-

is ha,,I,_ +,he .._1 _,,,ue of the _,4-" _r,,,,,a, 1 frequency

measurement

G-

S is the scale of the imagery.

For example,

,,,,_ to a set prc, portion _""

with a sinusoidal

(fig. 5). The ground

of line pairs per millimetre

where

from M_TF curves.

"_!,-_ fi,.,,, of view (EIFOV)

instantaneous

the ,,,,,,.,.,,,,_,,,'^A"'_'_^_ of an object

of its original

4. Signal to noise ratio (S/N).

Mapper

(Blanchard

that the EIFOV

modulation

curve (also

is a plot of the minimum of spatial

against spatial

frequency.

frequency

(Figure

0 O0

o .._-,_ .-_ ._ ,_._

_0

_

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LL

I I

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r-

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HOL3V-! EF]:ISNVHJ.NOIIV'II'IQOI/9

]3

5), the curve

plots upwards

gives the limiting photographic

resolution

film (Welch

tion (TM) curves. misleading. obtained

of the system. 1972; Smith,

Since MTF curves

A more

The intersection

This approach

1978)

estimate

resolution,

with reference

though

has been applied

invariably

are usually

non-linear,

is to calculate

the MTF curve by a rectangle

as the value of limiting

of this curve with the system MTF

when the curves

are almost

comprehensive

by replacing

refinement

from left to fight.

is to calculate

modula-

use of a single value can be

the equivalent

of equivalent

to

called threshold

bandwidth,

which

area and giving the upper

even this will be a simplification

to visual interpretation

most commonly

(Smith,

the modulation

bound

1978).

transfer

is

A further function

area (MTFA) MTFA = f[M(k)-D(k)] where

M(k) is the imagery

visual system

(Schindler,

1979).

for assessing

the usefulness

to have been

considered.

MTF curves preferably cient

derived

aircraft

Where a sensor

imagery

by imaging

1974)

flying under

the orbiting

does not possess

photographic

products

beam vidicon

(RBV)

camera

bar targets

of varying

Since standard

detectors

the IFOV

such as the Earth Landsats.

The latter

by an electron

beam and an analogue

signal derived

1970; Baker

resolution

of suffi-

and Scott,

1975).

14

from

be directly

calculated

and for the return

to television

surface the depleted

In Landsat

targets

from edge (Corbett,

of Skylab

are similar

on a photo-conducting

(Eastman,

but are

This is the case both for

Camera

the images are stored

tube

in the laboratory,

1976).

cannot

appropriate. Terrain

does not appear

and by using intermediate

(e.g. Schowengerdt,

is shuttered

the image

width

such as coast lines or roads, sensor

of such measures

this approach

MTF curves have been derived

discrete

on board

curve for the human

relevance

for photo-interpreters

based on MTF are especially

from sensors

threshold

self-evident

on the ground.

(e.g. Welch,

of resolution

initially

the apparently

for space imagery,

and estimates

the camera

Despite

from large bar targets

or line targets

images from

MTF and D(k) is the detection

of space

can be derived

size are unavailable

1974)

system

dk

which

cameras:

is then scanned

reflection

3, the RBV

when

within

cameras

were

configuredto givemonochromaticimagerywith a narrowertotal field of view andhencewith muchbetterresolutionthan that of EIFOV

of the latter

39m.

derived

the RBV cameras

from laboratory

derived

and MSS on board curves (General

The pixel size of Landsat 3 RBV images is rather

(RCA,

1977).

As already

of the actual

indicated

in Section

smaller

Landsats

Electric,

1 and 2. The

undated)

than this, namely

is about

21.8m

× 23.8m

2.1, such pixel sizes may well give an-over-estimate

resolution.

Although

the MTF approach

provides

a much

more

comprehensive

description

of system 0

resolving point

power

targets

than measures

based on geometric

it has its limitations.

with their width,

Ability

and most ground

to measure

The automated Townshend,

targets

measurements the minimum are potentially effective (value) resenting Landsat

size of targets of great value. element

can be assigned the actual

3 RBV cameras, Strome

Colvocoresses (ERE)

assurance

measures

importance

dependent

(Swain

suggested

a measure

larger homogeneous

instrument.

which

a single radiance

indicate

called the response

5% of the value rep-

of this idea and defined

by a much

of the spectral

of resolution

is within

1978;

to a given of accuracy

value of 35m for the Thematic

15

from

and Davis,

measures

can be recorded

that the response

of the measuring

derived

on the fidelity

resolution

properties

a refinement

area surrounded

is 30% of the full-scale

are long compared

He derived values of 86m for the Landsat

suggested

between

targets

Consequently

(1979b)

which

Hence

based on the size of area for which

radiance.

(1979)

highly

the spectral

and gives an estimated

based on a sinNe homogeneous sured radiation

is usually

to distinguish

to the unwary.

of small finite

by the sensor.

with reasonable

relative

performance

of images is of increasing

for which

objects

do not have this form.

properties

Such classification

that are recorded

resolution

D. Subsequently

the spectral

classification

1981).

or the ability

It is based on high contrast

MTF curves may imply an overly-optimistic

2.4

properties

MSS, 30m for the Mapper

of Landsat-

a modified

ERE

one, whose

mea-

The ERE is defined

as the

minimumareafor which spectralpropertiesof the centrecanbeassigned with at least95%confidencethat the valuesdiffer from the actualparametervaluesby no morethan5%of the full scale of the measuring instrument(Strome,1979). Estimationof this areademandswe knowthe probability distributionof the observedsignal whichis estimatedfrom the point spreadfunctionandsystemnoise.Derivationof suchdistributions for varioussensorsareprovidedin Forshawet. al. (1980) but asyet no direct quantitative estimatesof this potentia!!yveryusefulmeasureof spatialresolutionusingthis methodhavebeen made.Wecan

gain an indication

wood

(1974),

who modelled

noise

and accuracy

that for typical channels points

agricultural

accuracy

2.5

area occupied

with

Very different

those

land cover On the other

(Norwood,

125m and 200m

is classification

on noise

by a particular

further

be useful

in Section

5) indicate

Norwood

MSS,

(1974)

by the MTF error,

and calibration

cover type that can be classified

This is discussed

system

values for the Landsat

respectively.

of images then it would

of Nor-

using MTF,

1974, Table

will tend to be dominated

depen_lent

work

error.

to know

to a certain

what is the degree

of

3.

for users no single definition

different estimates

be seen in Table

results

to earlier

scanners

will be a 5% error in radiance

to asymptote

with a given probability.

It is clear that

Thus

there

by reference

multispectral

Graphical

for small field dimensions

concern

Implications

concerned

calibration.

scenes

fields diminishes

If our major

of Landsat

field size is approximately

out that the error

minimum

the performance

of radiometric

4 & 6, when

and for larger

of the size of such estimates

2 which

particularly

image

of spatial

properties

of the resolution summarizes concerned

by computer-assisted hand if analysis

resolution

and these

properties

should

is based primarily

as is often

16

measures

users are of resolution.

for the same sensor

of the resolution

find measures

on traditional

since different

alternative

be obtained

estimates

with spectral methods

demand

may therefore

the various

is possible,

as can

of the Landsat

the case for those

like the ERE most

visual photo-interpretation

MSS.

mapping

appropriate. methods

Table2. Estimatesof the resolvingpowerof the LandsatMSS. •

ResolutionMeasure

Resolution (meters)

Source

1. IFOV

- geometric

NASA

1972

79

2. IFOV

- geometric

Slater,

1979

76.2

3. IFOV

- geometric

Colvocoresses,

4. IFOV

- geometric

(min. altitude)

Gordon

1980

76

5. IFOV

- geometric

(max altitude)

Gordon

1980

81

General

Electric

6. Pixel size 7. Pixel size - resampled (Landsat 8. IFOV

Holkenbrink,

1979

(undated)

1978

73.4

79 × 56 57 X 57

3 CCT's)

- point

Landgrebe,

spread

et al., 1977

90

9. EIFOV

- half cycle

Welch,

1977

66

10. EIFOV

- full cycle

Welch,

1977

135

11. ERE

Colvocoresses,

12. Modified ERE - estimate for Cilannel 4

Norwood,

13. Minimum

Shay et al., 1975 General Electric, 1975

classifiable

area

17

1979b

1974

87 125

500 X 350 320 X 220

asis often the casefor thosemakinginferencesaboutsub-surface conditionssuchashydrological or geologicalones,thenmeasures basedon the MTF shouldbepreferable.In practicea userof remotelysenseddatamay only havean IFOVvalueavailableasa measureof spatialresolution.As a comparativemeasurethis canbeuseful,solong asits limitationsareclearlyunderstood.Thus the smallerIFOV of the ThematicMappercomparedwith the MSSshouldleadto an approximately ninefoldreductionin the areaof detectabletargetswith the samespectralproperties,spatialcharacteristicsandcontrastwith their surroundings.It shouldbeapparenthoweverthat the IFOVs cannotsimply be translatedinto the groundmeasurements of the smallestdetectableobjects. The latter will dependon manyfactors,not leastof whicharetheterrainpropertiesthemselves whicharebeingobserved.Thesewill affect the spectralresponse of objectsandtheir surroundings andhencelargelycontributeto the contrastof objectswith their surroundings andthusto the ability of a sensorto resolvethem. 3.

Benefits

of improvements

It is important stood.

Firstly,

launched

resolution

on the accuracy

Automated

simulations

inal aircraft

the spatial

satellite

resolution

resolution

of future

are eventually

classification

systems of satellite

be aware of the capabilities

the designers

of automated

benefits

of better

using data obtained

have carried

of decreasing

systems

of systems require

launched.

images are underdue to be

this information

The consequences

and on visual interpretation

resolution

resolution

the simulated

simple

imagery

been

assessed

The resultant

on the usefulness

simply

them to create Such

have

in aircraft.

out such degradation

et al., 1974).

between

spatial

from sensors

pixels and averaging

1975; Thompson differences

spatial

of future

so of finer

are discussed.

classification

to assess the effects

experiments

Secondly

with appropriate

power

of improving

the users of such data should

The potential

graded

the effects

in the near future.

that sensors

3.1

that

in the resolving

by taking

in several

imagery

is progressively

of the images.

regular square

does not

and the actual

18

imagery

fully take

de-

Most of the

blocks

new pixels (e.g. Clark and Bryant,

averaging

empirical

of the orig-

1977; Kan et al.,

into account

from satellite-borne

possible sensors

in

termsof propertiessuchastheirpoint spreadfunctionsandsignal-to-noise ratios. Moresophisticatedalgorithmshavethereforebeenappliedby someworkersto the originalaircraftimageryto simulatesatelliteimagerymoreclosely(e.g.SadowskiandSarno,1976;Morgenstern et.al.,1976). The principalobjectiveof nearlyall theseexperimentshasbeento assess the effectsof changing spatialresolutionon the classificationaccuracyof landcovertypesidentifiedby computer-assisted automatedmethods. The latter basicallyrely on the applicationof multivariatestatisticalprocedures,the variablesusuallybeingthe valuesof the separatespectralbands. Identificationof classes is eithela priori

or a posteriori,

vised in remote

sensing parlance

usually

according

assessed

a. The effects

(Swain

accuracy

over the range or resolutions in terms

an area of woodland homogeneous

Some

areas of crowns

cover

may

tissue

will vary between

surfaces,

errors

of the internal

trees,

cover

classified.

residential",

below;

separate

we might expect

is coarsened

for this paradoxical within

individual

will normally

conclusion cover

be found

and Landgrebe

will be strongly the amount

example,

individual

types.

illuminated;

openings

residential

cover sub-types,

(Figure

6)

has usually For example

term scene noise.

leaf matter

a suburban

since

to be far from spec-

(1978)

of reflective

consider

of the previously

heterogeneities

For example,

classification

as resolution

of what Wiersma others

of what

including

in the crown and woody area where roofs,

road

others.

of either

component

errors.

Explanation

As a second

will reveal many

classification

to improve

at high resolution

grass cover, trees and many

!icial classification

and nonsuper-

of the images is then

is correctly

been the converse

heterogeneities

will be in shadow,

imagery

found

conside'r_'d.

due to the presence

if their separate

"suburban

has been

reveal the herbaceous

Automated

The usefulness

of an area which

have often

when viewed

trally

high resolution

supervised

of scene noise

c!:-,:-