Geoscience Reference
In-Depth Information
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The sample site locations on the NAPP photos were found by first plotting the sites on TM false-
color composite images then finding the same area on the photo by context.
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During the interpretation process, cover type and other related information such as site homogeneity
were recorded for later analysis.
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When there was some doubt as to the correct class or there was the possibility that two classes could
be considered correct, the interpreters selected an alternate class in addition to the primary class.
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The interpretations were based on the majority of a 3
3 pixel window (Congalton and Green, 1999).
7.2.3.2
Interpretation Procedures
The Landsat TM images were displayed using ERDAS Imagine. By plotting the site locations
on the Landsat TM false-color composite images, the interpreters precisely located each site. Then,
based on the context from the image, the interpreters located the site on the photographs as best
they could. Clearly, some error was inherent in this location process; however, this was the simplest
and most cost-effective procedure available. The use of a 3
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3 pixel window for interpretation
was intended to reduce the effect of location errors.
The interpreters examined each site's characteristics using the aerial photograph and TM image
and determined the appropriate LC label for the site according to the classification scheme, then
they entered the information into the project database. The following data were entered into the
database: site identification number (sample site), coordinates, photography acquisition date, pho-
tograph identification code, imagery identification number, primary or dominant LC class, alternate
LC class (if any), general site description, unusual observations, general comments, and any
temporal site changes between image and photo acquisition dates. The interpreters did not have
prior access to the MRLC classification values during the interpretation process.
Individual interpreters analyzed 15% (
= 75) of each of the other interpreters' sample sites to
create an overlap database to evaluate the performance of the interpreters and the agreement among
them. Selection of these 75 sites was done through random sampling. This scheme provided 225
sites that were interpreted by all three interpreters. Agreement analysis using these overlap sites
indicated an average agreement of 84% among the three interpreters (Table 7.1).
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7.2.3.3
Quality Assurance and Quality Control
Quality assurance (QA) and quality control (QC) procedures were vigorously implemented in
the study as designated in the interpretation organization chart (Table 7.2). Discussions among the
interpreters and project supervisors during the interpretation process provided an opportunity to
discuss the problems that occurred and to resolve problems on the spot.
The QA and QC plan is shown in Figure 7.3. Upon completion of training, a test was performed
to determine how similarly the interpreters would call the same sites. The initial results of the
analysis revealed that some misunderstandings about class definitions had remained after the training
process. As a result, the interpreters were retrained as a group to “calibrate” themselves. This helped
to ensure that calls were more consistent among interpreters. Upon satisfactory completion of the
retraining, the interpreters were assigned to complete interpretation of the 1500 sample sites.
7.3 RESULTS
7.3.1
Accuracy Estimates
Table 7.3 presents the error matrix for MRLC Level II classes. The numbers across the top and
sides of the matrices represent the 15 MRLC classes (Appendix A). Table 7.4 presents the error
