(b) repetitive coverage at regular intervals (limited by cloud cover). 
The ERTS-1 is equipped with two remote sensing systems: the return 
beam vidicon cameras (RBV) and the multispectral scanner (MSS). However, 
except for a short period in 1972 the RBV cameras have been switched off and 
only the MSS images are available on repetitive basis. Though the capability 
to monitor temporal changes on the earth surface over an extended period of 
time is the ERTS strongest feature, there have been relatively few reports on 
studies of multidate images. This was partly caused by difficulties to obtain 
good quality images of test areas in required time intervals and partly by the 
necessity to familiarize ourselves first with ERTS interpretation techniques 
based on a single-date multispectral imagery. 
ERTS images are available either in analog format printed on 
black-and-white or color film, or in digital format printed on computer 
compatible tapes. The latter were selected for our study for the following 
reasons: 
(a) Analysis of seasonal changes requires multidate correlation of 
several sets of ERTS multispectral images. This is very difficult 
and slow by visual observation even when multispectral color viewers 
are available. 
(b) Digital images are not subjected to image degradation due to 
photographic processing and reproduction. Once the ground data are 
digitized on board of satellite there is no further degradation of 
the digital image. 
(c) Digital images printed on computer compatible tapes form direct 
input into computerized image processing system. 
This study had three main objectives: 
(a) production of thematic map of the Larose Forest test area by 
computerized processing of the ERTS single-date multispectral 
digital images (Kalensky and Sayn-Wittgenstein, 1974); 
(b) production of multidate thematic map of the same test area by 
computerized processing of the ERTS multispectral digital images 
recorded at three different dates; 
(c) comparison of classification accuracies of the single-date and 
multidate ERTS thematic maps. 
The computer compatible tapes were recorded with density 800 bpi in 
9 tracks. Two 2,400-foot tapes were needed for each ERTS scene. Selection of 
ERTS scenes was based on the availability of tapes, quality of recorded 
digital images and ease of their registration. The following three scenes 
were selected for the single-date and multidate classification: