227 
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
.eaf layers 
modelling: 
1 studies. The use of multispectral photography in agricultural research 
J.G.P.W.Clevers 
Dept, of Landsurveying and Remote Sensing, Wageningen Agricultural University, Netherlands 
ABSTRACT: For monitoring crop growth and development, remote sensing techniques have proved to offer great 
possibilities. Narrow spectral bands in the visible and near-infrared can be very suitable for application 
in agricultural research (in this paper attention was focussed on application in agricultural field trials). 
However, to date a low temporal resolution due to high costs for repetitive recordings by a well-calibrated 
data-acquisition system has been a bottleneck for this monitoring. By using an airborne multispectral photo 
graphic (MSP) system, recordings of high spectral resolution were obtained through a judicious choice of films 
and filters (25 - 100 nm bandwidth). By using equipment which was easily available and by selecting an appro 
priate densitometer, recordings were obtained with a high temporal (fortnightly) and sufficient spatial re 
solution (in the order of 1 m 2 ). Calibrated reflectance factors of crops were obtained by applying a procedure 
for atmospheric correction and radiometric calibration using reference targets in the field. 
The system was tested in field trials at the experimental farm of the Wageningen Agricultural University. 
The main result was that quantitative information about crops was obtained with larger precision by MSP than 
by conventional field sampling methods. In this way spectral reflectance characteristics of cereals were ob 
tained during the growing season in a green, red and infrared band for several treatments (e.g. nitrogen nu 
trition, sowing date). When comparing results of successive growing seasons, results turned out to be consis 
tent. 
1 INTRODUCTION 
During the past decades knowledge about remote sen 
sing techniques and their application to areas such 
as agriculture has improved considerably. Bunnik 
(1978) described the possibilities of applying op 
tical remote sensing in agriculture, particularly 
with regard to its relation with crop characteris 
tics such as soil cover, leaf area index (LAI) and 
dry matter weight. If reflectance factors are re 
quired with high spectral, spatial and temporal re 
solution for crop monitoring, Clevers (1986a, 1986b) 
selected black and white multispectral aerial photo 
graphy . 
Systems of aerial photography provide recordings 
of the reflected visible and infrared radiation. 
The spectral resolution may be high when adequate 
films and filters are used in order to achieve a 
multispectral photographic (MSP) system with nar 
row bands. The spatial resolution will primarily 
depend on lens type, altitude, film material and 
image motion. By using an appropriate microdensito 
meter or densitometer, with small aperture, quanti 
tative measurements with high spatial resolution 
may be obtained. Moreover, the measurements on the 
photograph can be carried out at exactly determin 
able locations of an object. The camera equipment 
can be installed in small planes if small non 
metric cameras are used (e.g. 70-mm aerial photo 
graphy). Consequently, cost stays within acceptable 
limits and a high temporal resolution can be ob 
tained. 
All these considerations render black and white 
multispectral aerial photography the most promising 
remote sensing technique for application to small 
areas. The only limitation is that calibration and 
use of the sensor system must be accurate if this 
technique is to supply quantitative information, and 
this has often been a bottle-neck for its applica 
tion, in particular because of the analog data regis 
tration (e.g. Curran, 1980, 1982, 1983; Graham, 1980; 
Kannegieter, 1980; Ross, 1973; Sievers, 1976). Ways 
in which the photographic recordings have to be cali 
brated are described by Clevers (1986a, 1986b). In 
particular the calibration concerns the character 
istic curve, light fall-off, exposure time, rela 
tive aperture, transmittance of the optical system 
and atmospheric correction. This calibration is 
schematically illustrated in figure 1. 
MATERIAL AND MEASURED ADDITIONAL 
INSTRUMENT VARIABLE OPERATION 
Figure 1: Flow diagram, illustrating the calibra 
tion of aerial photography. 
One specific application of remote sensing is in 
agricultural field trials, in which it is necessary 
to carry out observations regularly during the gro 
wing season. Conventionally, the agronomist has two 
possible approaches at his disposal. The first one 
is a subjective judgement of plots and estimation of 
plant characteristics, yielding primarily qualitative 
information. The second one is based on taking sam 
ples of a few plants from each plot. This yields 
quantitative information, because plant characteris 
tics such as LAI or dry matter weight can be mea