Geoscience Reference
In-Depth Information
Sample Points
Photo Center
N
0
0.6
1.2
Miles
Figure 7.2
Sample sites clustered around
the photograph center.
within the extent of all of the PSUs only, of 100 sample sites per LC class. The selected sites were
referred to as secondary sampling units (SSU). The number of sites per photograph ranged from
1 to approximately 70 (Figure 7.2). The total number of sample sites in the study was 1500 (100
per cover classes), although only 1473 sites were interpreted due to missing NAPP photos. This
sampling approach was chosen by the Eros Data Center (EDC) over a standard random sample to
reduce the cost of purchasing the NAPP photography (Zhu et al., 2000).
7.2.2
Training
Before the NAPP photo interpretation for the sample sites could begin, photo interpreters were
trained to accomplish the goals of the study. To provide consistency among the interpreters, a
comprehensive training program was devised. The program consisted of a full-day training session
and subsequent on-the-job training. Two experienced aerial photo interpretation and photogram-
metry instructors led the formal classroom training sessions. The training sessions included the
following topics: (1) discussion of color theory and photo interpretation techniques, (2) understand-
ing of the class definitions, (3) interpretation of over 100 sample sites of different classes during
the training sessions followed by interactive discussions about potential discrepancies, (4) creation
of sample sites for later reference, and (5) repetition of interpretation practice after the sessions.
The focus was on real-world situations that the interpreters would encounter during the project.
Each participant was presented with over 100 preselected sites and was asked to provide his or her
interpretation of the land cover for these sites. Their interpretations were analyzed and subsequently
discussed to minimize any misconceptions. During the on-the-job portion of the training, each
interpreter was assigned approximately 500 sites to examine. Their progress was monitored daily
for accuracy and proper methodology. The interpreters kept logs of their decisions and the sites for
which they were uncertain about the LC classes. On a weekly basis, their questions were addressed
by the project photo interpretation supervisor. The problem sites (approximately 400) were discussed
until all team members felt comfortable with the class definitions and their consistency in interpre-
tation. Agreement analysis between the three interpreters resulted in an average agreement of 84%.
7.2.3
Photographic Interpretation
7.2.3.1
Interpretation Protocol
The standard protocol used by the photo interpreters was as follows:
