DARIO RODRIGUEZ-BEJARANODepartamento de Bosques

Escuela Nacional de AgriculturaChapingo, Estado de Mexico

Mexico

Density Slicing Applied to ForestType Delineation*

Printing aerial photographs of a forested area onAgfacontour film to achieve density slicing, and subsequentlycopying the density slices on very high contrast lith film,aided in the discrimination of conifers and broadleafed trees.

INTRODUCTION

T E APPEARANCE OF tone in anaerial photograph is one of the most im­

portant factors for the photo-interpretation oftree species or forest type delineation. Thedifferences oftone between objects are regis­tered as different densities by the photo­graphic material. And it is these density dif­ferences that constitute the key to photo­interpretation.

The tone density differences are quite ob­vious between conifers and broadleaves in anaerial photograph. Their greatest tonal dif~

ferentiation, when recorded by a suitablephotographic material, is in the inli'a-red re­gion of the electromagnetic spectrum, al­though there also exists some difference inthe visible range (Figure 1), namely in thegreen (500-600 nm) and in the deep red(circa 700 nm) regions.

Density slices in photographic terminol­ogy are the points of a photographic image(i.e., negative or positive image) having thesame densi tyl.

Density slices, also referred to as equiden­sities (areas of equal density), can be ob­tained through several methods2 ;

(a) electronically, with an image scanneror TV monitoring

(b) by either solarization or the SabatierEffect;

(c) by photographic masking; or(d) by using Agfacontour film.At present the only available photographic

material capable of producing the equiden-

* Experiment conducted while following thecourse of Aerial Photography at lTC, Enschede,Holland

sities conveniently and directly from aphotographic image is Agfacontour film. Thiswas the film used in the present experimentfor the production of density slices.

The emulsion of the Agfacontour film con­tains silver chloride (with colloidal silversulphide) which makes up the negative gra­dient (green sensitive) of the characteristiccurve, and silver bromide which makes upthe positive gradient (blue sensitive) of thecharacteristic curve. The spectral sensitivitycurves for the film are shown in Figure 2.When this dual-characteristic emulsion isexposed to light the areas receiving most ex­posure produce high densities throughchemical development, producing the posi­tive part of the characteristic curve. At thesame time areas that receive very little lightor no light at all also produce high densitiesthrough physical development to make thenegative part of the characteristic curve. Theareas which receive a mid-exposure levelproduce only a slight density or are not af­fected at all, thus remaining transparent afterdevelopment. Thus the mid-exposure areascorrespond to the trough formed between thetwo characteristic curves of the material andso make up the equidensities or areas ofequal density (density slices).

This exposure range corresponding to thelow density region between the negative andpositive parts of the characteristic curve canbe adjusted to a required density interval(tllJ) by changing the illumination for print­ing (altering the light source or color filter­ing). Thus, by varying the printer light colorwe can narrow or widen our density slices.Figure 3 shows the effect of different light

1029

1030 PHOTOGRAMMETRIC E GINEERING & REMOTE SENSING, 1975

70

60

!lO..uc0

~ 40.!';cr 30

::!!0

20

10

0400 600 700 eoo

broadlltowe.

Wavelength ( l) nm.

FIG.!. Reflectance curves for broadleaves and conifers.

4~ :500 5!50

Wavelength ( ~) n m.

Spectral sensitivity of Agfacontour

2>.

>'W;c::'"01...J

o '«Xl

F,G. 2.film.

,,

----- 9'''" ••nsifiwoe(MgCIfiw)

,-./, ,,,,

,,.,

sources. In Figure 4 we can see the effect ofMagenta (M) filtering giving a widening ofequidensity and Yellow (Y) filtering giving anarrowing of equidensity compared with nofilter at all.

Thus, by increasing the Y filtering it is pos­sible to narrow the equidensity. Several testsshowed that the density slice could be nar­rowed to about 0.10 (first orderequidensity)2,3. After reaching this limit, anyfurther Y filtering will only serve to move thecharacteristic curve up the density scale(Figure 5). The relationship between a givendensity slice and the aerial negative (desireddensity interval) can be regulated by increas­ing or decreasing the exposure time. Increas­ing the exposure time will shift the charac­teristic curve to the left (corresponding to

3

=..c..0 1

I I. II I

11

11

i II

~ ,1 I. I

I III

i'. II II I

\. \

3

2

..c..o

- no filter .- ellp.

--_. H". r' ••p.

I --- .+50 Y ."."

o !:-_-----:l~---r----~-3 2 I 0

rei . log. exposure32

reI. log. exposure

o,L..---------------

FIG.3. Effect ofdifferent light sources onAgfacontour film.

FIG.4. Effect ofY and M filtering on Ag­facontour film.

DENSITY SLICING APPLIED TO FOREST TYPE DELINEATION 1031

>-~ 2c..a

--340Y

_. _. _.- 400 Y

-----490Y

oL----_--__---~--"2 I 0

rei. log. exposure

FIG. 5. Effect of excessive Y filtering onAgfacontour film.

higher densities in the aerial negative); de­creasing will shift it to the right, given a con­stant Y filtering. This time-density relation­ship is expressed quite well by the followingexpression:

where T z is the exposure needed to obtain agiven density D z and T 1 is the exposureneeded to obtain a given density D 1 .

Through this formula it is possible to de­termine the position of a given density sliceby calculating the exposure time required bythe particular contact pri nter-filtering systemin use .

For example, with the filtration 220Y(gOY + 80Y + SOY) and a trial exposure of4 sec­onds, the density slice at 0.54 (read on therelative log exposure axis) was obtained;then, to obtain the desired exposure time forthe density slice corresponding to the broad­leaves,

T z = (4) . (101.60-054)

= 48 seconds.

ApPLICATION OF ACFACONTOUR FILM To

FOREST TYPE DELINEATION

The task of applying density slicing to ob­ject recognition should be approached care­fully. The image of the object that the in­terpreter sees depends on quite a few vari­ables. The object on the ground will reflect aparticular intensity of a certain quality of

r--.

~,-BfA'~ ~' ,"'T'~:~~' ~'.<.~l§r: e~·'l';'~.."··

.­./".....

..~;:·;t.·

." r .;'t,t-:-'~

~-::::~~>;. .:' ";:1;;'.... , .''''

FIG. 6. Agfacontour density slice.

1032 PHOTOGRAMMETRIC E GI EERING & REMOTE SENSI G, 1975

light that depends on time of the day, season,physical condition of the object, position ofthe camera (angle of reflection), haze, filter­film combination, type ofmaterial used in theproduction of the photograph, the type ofprocessing, and the photographic systemresolution*4. Furthermore, the appearance ofthe object in the aerial photograph will beaffected by the type of processing which notonly involves chemicals, temperature, andtime but also whether a rewind spool tank oran automated processing machine is in use5 .

Within these limitations the problem of ac­curate density slicing was approached. It ispossible to produce density slices orequidensities corresponding to areas that ex­hibit tonal differences, as between conifersand broadleaves, using Agfacontour film.

Aerial photography of the SchneegatternForest District of Austria was available forthis particular experiment. The photographywas taken in June 1969 with Kodak InfraredAerographic mm 5424, using a Wild RC915/23 camera, at an approximate scale of1:10,000.

A sample area of the forest was selected,and the aerial negative of that sector was ob­tained. In this negative densities of areas ofconifers and broadleaves were measuredusing a Macbeth TD 404 Transmission Den­sitometer with a 2mm diameter aperture: 10measurements for conifers, 9 measurementsfor broadleaves. From these measurements a

* The resulting system resolution (R.) after tak­ing into account the resolution of the film, lens,image movement, and Agfacontour film (densityslice). It could be approximately expressed as l/R.= IIRf + l/R, + l/R;m + lIRA.

mean density was obtained for the conifersand for the broadleaves. The resulting meandensities in the negative were: for conifers,0.70, and for broadleaves, 1.60.

PRODUCTION OF AGFACONTOUR DENSITY

SLICES

Several tests were made to determine thebest possible printer-Y filtering combination.The printer used was a Zeiss KG 30 ContactPrinter equipped with white lamps (26000 K)and an Agfa-Gevaert filter pack. A Ycombination of 50Y +80Y +90Y was selectedfor giving the narrowest equidensity possi­ble.

The exposure times were calculated ac­cording to the formula discussed before, andthe Agfacontour density slices produced(Figure 6). The resulting characteristic curvesfor the conifers (c) and broadleaves (b)equidensities are shown in Figure 7.

COPYING OF AGFACONTOUR DENSITY SLICES

ON LITH FILM

Although the Agfacontour film has an in­herent high contrast (gamma above 7.0 for thenegative slope and about 14.0 for the positiveslope of the characteristic cUlve2), there aresome areas of medium densities which it isdesirable to eliminate. This can be done bycopying the Agfacontour density slices onvery high contrast (hard) paper or a suitablefilm. The copying has the effect ofmaking thevisual recognition a lot easier.

An orthochromatic lith film was used forthis purpose. Lith films are very high contrastmaterials and are developed in a suitable lithdeveloper which encourages a phenomenon

c= conif ....

b~ ttroacleaws

I 2 3reI. log, exposure

Characteristic curve ofa lith film.

o

FIG.8.

2>-'w;c:..o

o'2 Iret. log. exposure

oL-----..,...----~----..,.....-

3

FIG. 7. Resulting density slices for conif­ers (c) and broadleaves (b).

2

..c:..o

3

oMZCfl

:::J><Cfl

t(')-Z(')

:>­"tI"tI

tMo>-lo'rj

o::cMCfl>-l>-l><"tIMoMtzM:>-joz

Conifers (c)

.~.

,~;.., '\.. ;;{d~

'>,~

~

7"':,

'D{:~'

',j;\ 'j

j.

.;. .

.. -;

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Broadleaves (b)

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jo

i...,

:.;).....\ ..,.,-V".

~

,I

-I ..• A •

~/."".r

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". .,

FIG. 9. Lith film broadleaves (b) and conifers (c) density slices, copies.

I-'"oww

(a) C' CONIFERS B' BROAOl.E AVES Y' REGENERATION

11= ".1XEO

(bJ

0.0 PEN AREAS

......ow..,.

'"0::co....,oCJ::tl;>::::::::t%1....,::tl-(")

t%1ZCJZt%1t%12:lzCJR"::tlt%1::::o....,t%1CJlt%1ZCJl

Z.CJ......CD-...lCJl

FIG. 10. Aerial photo of the forest (a) and photo-interpretation of the area (b).

DENSITY SLICING APPLIED TO FOREST TYPE DELI EATION

PLATE I. Diazo 'color

1035

DE SITY SLIeI G APPLIED TO FOREST TYPE DELINEATION 1037

known as infectious development. Except forthe first few seconds, very little agitation isemployed to provide still-bath developmentconditions. Because of its extreme contrast,lith film does not reproduce intermediatedensities, providing only very high or verylow densities. The critical problem when de­veloping is that temperature as well as de­velopment time must be watched carefully.

The lith film only shows black-and-whitedots or lines, making it well-suited for thereproduction of density slices. Figure 8shows the characteristic curve ofa typical lithfilm.

The density slices were then reproducedon lith film using a Zeiss KG 30 ContactPrinter equipped with argon (violet and blue)light source. The argon light source wasnecessary since the lith film is an ortho­chromatic material. Figure 9 shows the result­ing density slices on lith film for both conifers(c) and broadleaves (b).

CO 'CLUSIO A 0 DISCUSSION

The resulting lith film copies of the densityslices correspond to the conifer and broadleaftypes. Figure 10 shows the forest type de­lineation and the aerial photograph of thearea. Their similarity can be readily seenwhen compared with the copies on lith film.

The process can be enhanced further byproducing a diazo color composite of the lithcopies. Plate 1 shows the resulting colorcomposite of the area.

From this experiment it seems feasible toproduce a delineation between conifers andbroadleaves. However, it should be notedthat there was a large t:J) between conifersand broadleaves to begin with, which meantthere existed a visible differentiation be-

tween them. Nonetheless, the rough delinea­tion obtained could be used for preliminarysampling for inventory purposes. A more de­tailed delineation (old and young stands sub­divisions) does not seem feasible due to thesmall density differences involved. How­ever, the problem was not explored further. Itmay be possible to do this delineation usinga larger scale photography than the one usedin this experiment.

It seems evident from the results obtainedthat density slicing can be used for this typeofdelineation. The main advantage is thatthesystem is quick once the densities are ob­tained and can yield a map (color compositeor lith film copies) of the area, ready for ob­servation and assessment. The method israther simple once some experience has beenacquired in handling the material. Also, itshould be noted that the color composite ren­ders the added information of the areas ofmixed forest, which by itself constitutes aseparate type.

REFERENCES

1. AGFA/GEVAERT - Agfacontour Profes­sional in Photographics. E57-7110/14304, W.Germany

2. Nielsen, U. Agfacontour Film for Interpreta-tion Photogrammetric Engineering(1099-1105), 1972

3. Ranz, E. Agfacontour: A ew Film for Sim­plified Production of Equidensities. NASATechnical Translation F-15. 112, Sept. 1973

4. Thomson, G. H. Photographic Limitations inDensity Slicing. ITC Journal (512-514) 1973-3

5. Thomson, G. H. Aspects of Aerial Film Proc­essing for Photo-interpretation in ResourcesSurveying. ITC Journal (515-520) 1973-3

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